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Sat Nav Speed Check


Richwood
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I've not posted for a while and maybe this is a the wrong time to start this thread when we are all trying to do more than 20mph in the snow but has anybody checked the speedometer against a Sat Nav?

I have no idea whether the in built Sat Nav has a car speed function but I have 2 sat Navs and Tom Tom on my PDA and a Sony. Both have this function. I would say with 8 sats reading the speed would be pretty accurate on a level road.

I have always tested my cars and my previous car (Saab93 TDi) would have 80mph on the speedometer and 77mph on the sat nav same with my wifes Merc C200. I tested my IS250d and at 80mph on the speedometer the Sat Nav read 73mph.

Firstly has anybody else "tested" their speedos and if so what readings have you got and do you think its because I have the 220d with 16" wheels and Lexus haven't bothered to recalibrate the speedo. I doubt this but it is a worry as it will make the odoemeter and mpg readings wrong as well. If nobody else has noticed then I will take it in to my garage and have larger wheels fitted under warranty!

However the car is great and hardly believe it cost me less than my IS200 7 years ago which I kept for 4 1/2 years. No rattles........yet. :tomato:

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I've not posted for a while and maybe this is a the wrong time to start this thread when we are all trying to do more than 20mph in the snow but has anybody checked the speedometer against a Sat Nav?

I have no idea whether the in built Sat Nav has a car speed function but I have 2 sat Navs and Tom Tom on my PDA and a Sony. Both have this function. I would say with 8 sats reading the speed would be pretty accurate on a level road.

I have always tested my cars and my previous car (Saab93 TDi) would have 80mph on the speedometer and 77mph on the sat nav same with my wifes Merc C200. I tested my IS250d and at 80mph on the speedometer the Sat Nav read 73mph.

Firstly has anybody else "tested" their speedos and if so what readings have you got and do you think its because I have the 220d with 16" wheels and Lexus haven't bothered to recalibrate the speedo. I doubt this but it is a worry as it will make the odoemeter and mpg readings wrong as well. If nobody else has noticed then I will take it in to my garage and have larger wheels fitted under warranty!

However the car is great and hardly believe it cost me less than my IS200 7 years ago which I kept for 4 1/2 years. No rattles........yet. :tomato:

The speedometer is usually 5-10% over reading, my IS200 is about 5% according to my Road Angel. The GPS on a straight road is going to be more accurate as it doesn't have the variation you can have in the rolling radius of the tyres that affect the accuracy of the speedo. Changing the wheels and tyres can make a difference and it's calculable..there are tables on the internet that makes it easier to do. My 250 speedometer should be pretty accurate with it's new boots as the fronts are marginally larger diameter than standard (and the rears marginally smaller than originals). :whistling:

Your 16" rims will not make a difference apart from the tiny variation I've mentioned, as the 16" rims will have more tyre to keep the diameter the same...the 17" have a lower profile tyre, the 18" lower still and my 19" has elastic bands :D...but the diameter of them all is the same (within a very small margin).

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As far as I know the nav on is220 can be calibrated just for tyre size, and you can do it yourself... take a look at the nav manual :)

You must be talking about the built in lexus Sat Nav, but if you can calibrate the Sat Nav just for tyres I don't see what relationship the tyres have with the Sat Nav speed unless it uses them to wheel revolutions and not the satellites.

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You must be talking about the built in lexus Sat Nav, but if you can calibrate the Sat Nav just for tyres I don't see what relationship the tyres have with the Sat Nav speed unless it uses them to wheel revolutions and not the satellites.

Yes the built in...that nav has an odometer cable and the real velocity is recalculate in the nav with this program... in fact under gallery, or whenever ther's no gps signal, it carry on moving your position... and real strange, I believe it has two reciever, cause if you are not parallel with the street the pointer is ALWAYS in the right place

hope you understand my faulty english

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As far as I know the nav on is220 can be calibrated just for tyre size, and you can do it yourself... take a look at the nav manual :)

You must be talking about the built in lexus Sat Nav, but if you can calibrate the Sat Nav just for tyres I don't see what relationship the tyres have with the Sat Nav speed unless it uses them to wheel revolutions and not the satellites.

It is used so the computer can calculate your position when no satellite signal is recieved, as in driving in a forest or through a tunnel.

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http://www.lexusownersclub.co.uk/forum/ind...speedo+accuracy

I did this a while back - mine was 4mph shy at 70. Apparently the law allows for Over reading by 10% and under reading by 1MPH - but I've never seen that written anywhere

Built-in car systems rarely show the MPH, as everyone would ignore the speedo

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http://www.lexusownersclub.co.uk/forum/ind...speedo+accuracy

I did this a while back - mine was 4mph shy at 70. Apparently the law allows for Over reading by 10% and under reading by 1MPH - but I've never seen that written anywhere

Built-in car systems rarely show the MPH, as everyone would ignore the speedo

Thanks jamboo I knew I could rely on you for a great answer and even an old thread that started last year. By the way your famous last words about not having a ticket in 9 years, is it still holding up?

I might look at replacing my 16" with 18" if there is so much margin, what do you think?

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18"s make the car look so much better

Stick with Tyres for the 18" alloys as recommended by Lexus - Your speedo accuracy is determined by the rolling circumference of the whole wheel. The current 16" wheels will have higher sidewalls.

Still not had a ticket he says holding his head

I still think nearly 10% differentuial is too large and wonder if lexus could recalibrate the speedo, Will ask at the first service. I am getting some 18" wheels as they look so much better.

it's not who you know, but what you know about who you know.

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According to my TomTom my last car used to over report the speed by 4-5mph at (a clock indicated) 70mph with the inaccuracy increasing the faster you went. I've tested other cars and found that they too over report by approximately the same margin. In fact it's so consistant I've begun to wonder whether there's some legislation mandating this behaviour.

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According to my TomTom my last car used to over report the speed by 4-5mph at (a clock indicated) 70mph with the inaccuracy increasing the faster you went. I've tested other cars and found that they too over report by approximately the same margin. In fact it's so consistant I've begun to wonder whether there's some legislation mandating this behaviour.

I have tested it on my IS200 and my wifes 318i. In the IS at 133km/h or so on the clock I am actually doing about 120km/h. With the BM though it is more accurate. 128km/h is 120km/h.

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Built-in car systems rarely show the MPH, as everyone would ignore the speedo

I'm puzzled by the logic of that. Why should it be of any concern to the manufacturer? As long as they meet the law's requirement for a speed indicating device, their job is done.

Every aftermarket satnav I have owned (four) has indicated speed, why would the manufacturer of an aftermarket device think differently on this matter than the maker of a built-in device?

A speedo that depends upon measuring the distance covered by a rolling tyre is bound to have inaccuracies, which is why they are built to indicate speed "optimistically". If a satnav device offers a more accurate measurement of speed then it offers drivers a better basis upon which to make judgements and would appear to be a useful feature.

How many of us drive a bit faster than the indicated "70" on the speedo, because we "know" that it has a tolerance inbuilt?

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Built-in car systems rarely show the MPH, as everyone would ignore the speedo

I'm puzzled by the logic of that. Why should it be of any concern to the manufacturer? As long as they meet the law's requirement for a speed indicating device, their job is done.

Every aftermarket satnav I have owned (four) has indicated speed, why would the manufacturer of an aftermarket device think differently on this matter than the maker of a built-in device?

A speedo that depends upon measuring the distance covered by a rolling tyre is bound to have inaccuracies, which is why they are built to indicate speed "optimistically". If a satnav device offers a more accurate measurement of speed then it offers drivers a better basis upon which to make judgements and would appear to be a useful feature.

How many of us drive a bit faster than the indicated "70" on the speedo, because we "know" that it has a tolerance inbuilt?

I use my PDA GPS for my speed always but I have to admit that there sometimes can be errors. Once while I was driving through Malaga in Spain I noticed I was doing just over 625 km/h.

I would say it is because of flaws like this that they will not put the speed up. If you are in cities or a forest for example you might not get enough satellites in view to give a correct reading.

I know also that when in cities you can get satellite signals bouncing off buildings. This means that your receiver is getting the signal slightly delayed so again another inaccuracy.

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  • 2 weeks later...

I have my new wheels now 18" sport wheels, look amazing. The speed indicated by the sat nav is now "normal" I would say i.e. at 80 I am doing 77 mph. So the extra 4 inches has sorted the speedo differential which was nearly 10%.

My car is now 3 months old with 4500 miles on the clock averaged 40 mpg and no rattles yet. I am v. v. pleased with it. My only gripe was really the 16" wheels which even though an improvement on the IS200 base model still don't really suit the car. I didn't want leather seats basically if you're wondering why I didn't go for the SE.

I would think the mpg may go down a bit now but to be honest the 5th gear has now got longer legs so I don't use 6th as much which should improve the mpg as it can labour a bit especially with cruise on.

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I have my new wheels now 18" sport wheels, look amazing. The speed indicated by the sat nav is now "normal" I would say i.e. at 80 I am doing 77 mph. So the extra 4 inches has sorted the speedo differential which was nearly 10%.

My car is now 3 months old with 4500 miles on the clock averaged 40 mpg and no rattles yet. I am v. v. pleased with it. My only gripe was really the 16" wheels which even though an improvement on the IS200 base model still don't really suit the car. I didn't want leather seats basically if you're wondering why I didn't go for the SE.

I would think the mpg may go down a bit now but to be honest the 5th gear has now got longer legs so I don't use 6th as much which should improve the mpg as it can labour a bit especially with cruise on.

So you are saying that as well as putting on larger wheels you also put on larger tires. I.E. increasing your overall diameter / circumference. Am I not right in saying that this could lead to your tires rubbing and hitting the underside of the wheel well on larger bumps?

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Built-in car systems rarely show the MPH, as everyone would ignore the speedo

I'm puzzled by the logic of that. Why should it be of any concern to the manufacturer? As long as they meet the law's requirement for a speed indicating device, their job is done.

Every aftermarket satnav I have owned (four) has indicated speed, why would the manufacturer of an aftermarket device think differently on this matter than the maker of a built-in device?

A speedo that depends upon measuring the distance covered by a rolling tyre is bound to have inaccuracies, which is why they are built to indicate speed "optimistically". If a satnav device offers a more accurate measurement of speed then it offers drivers a better basis upon which to make judgements and would appear to be a useful feature.

How many of us drive a bit faster than the indicated "70" on the speedo, because we "know" that it has a tolerance inbuilt?

@ Sagitar - I know what you mean :blush: but can you imagine how many people world wide would complain about their speedo and the satnav, both integrated into the car and provided by the same car manufacturer, showing 2 different readings for the same thing? Can of worms comes to mind...

I am sure that Manufacturers play within legislation for speedo accuracy - else there could be loads of people suing the manufacturer for speeding fines.

Also, each country will no doubt have a different legislation for speedo accuracy, and it would become a nightmare.

I asked the company that installed the Sony/Becker Satnav in my Audi about this - I understand that Automobile Satnav, whilst accurate, is not 100% accurate in itself, as it can only be relied to track accuracy down to 5 meters - so the lower the speed, the more likely for it to give you wayward readings, especially where the signal is poor and you don't have full 3d mapping accuracy (you need 5 sats to be tracked for 3d mapping on most systems, and in really built up areas I have known my Tom Tom to track only 4 Sat's.)

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I have my new wheels now 18" sport wheels, look amazing. The speed indicated by the sat nav is now "normal" I would say i.e. at 80 I am doing 77 mph. So the extra 4 inches has sorted the speedo differential which was nearly 10%.

My car is now 3 months old with 4500 miles on the clock averaged 40 mpg and no rattles yet. I am v. v. pleased with it. My only gripe was really the 16" wheels which even though an improvement on the IS200 base model still don't really suit the car. I didn't want leather seats basically if you're wondering why I didn't go for the SE.

I would think the mpg may go down a bit now but to be honest the 5th gear has now got longer legs so I don't use 6th as much which should improve the mpg as it can labour a bit especially with cruise on.

So you are saying that as well as putting on larger wheels you also put on larger tires. I.E. increasing your overall diameter / circumference. Am I not right in saying that this could lead to your tires rubbing and hitting the underside of the wheel well on larger bumps?

Tyrrp

Yes that does sound a bit daft 4". The new wheels are 2" bigger and the tyres are standard Bridgestone so low profile. I would imagine the wheel complete is about 1" bigger. The wheel and tyre combination is off a Lexus IS Sport that opted for upgraded wheels so standard. Anyway all works well looks, good and speedo accurate.

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I have my new wheels now 18" sport wheels, look amazing. The speed indicated by the sat nav is now "normal" I would say i.e. at 80 I am doing 77 mph. So the extra 4 inches has sorted the speedo differential which was nearly 10%.

My car is now 3 months old with 4500 miles on the clock averaged 40 mpg and no rattles yet. I am v. v. pleased with it. My only gripe was really the 16" wheels which even though an improvement on the IS200 base model still don't really suit the car. I didn't want leather seats basically if you're wondering why I didn't go for the SE.

I would think the mpg may go down a bit now but to be honest the 5th gear has now got longer legs so I don't use 6th as much which should improve the mpg as it can labour a bit especially with cruise on.

So you are saying that as well as putting on larger wheels you also put on larger tires. I.E. increasing your overall diameter / circumference. Am I not right in saying that this could lead to your tires rubbing and hitting the underside of the wheel well on larger bumps?

Tyrrp

Yes that does sound a bit daft 4". The new wheels are 2" bigger and the tyres are standard Bridgestone so low profile. I would imagine the wheel complete is about 1" bigger. The wheel and tyre combination is off a Lexus IS Sport that opted for upgraded wheels so standard. Anyway all works well looks, good and speedo accurate.

Eh :unsure: ? I assume you mean off an IS250 Sport, not an IS200 or the rolling radius would be smaller. Even so the fitting of the 18" wheels from an IS250 with standard Bridgestone tyres as fitted from new would have the same overall diameter as the 220D, not bigger...or am I missing something here? Does the 220D have a smaller rolling radius than the 250? :unsure:

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I have my new wheels now 18" sport wheels, look amazing. The speed indicated by the sat nav is now "normal" I would say i.e. at 80 I am doing 77 mph. So the extra 4 inches has sorted the speedo differential which was nearly 10%.

My car is now 3 months old with 4500 miles on the clock averaged 40 mpg and no rattles yet. I am v. v. pleased with it. My only gripe was really the 16" wheels which even though an improvement on the IS200 base model still don't really suit the car. I didn't want leather seats basically if you're wondering why I didn't go for the SE.

I would think the mpg may go down a bit now but to be honest the 5th gear has now got longer legs so I don't use 6th as much which should improve the mpg as it can labour a bit especially with cruise on.

So you are saying that as well as putting on larger wheels you also put on larger tires. I.E. increasing your overall diameter / circumference. Am I not right in saying that this could lead to your tires rubbing and hitting the underside of the wheel well on larger bumps?

Tyrrp

Yes that does sound a bit daft 4". The new wheels are 2" bigger and the tyres are standard Bridgestone so low profile. I would imagine the wheel complete is about 1" bigger. The wheel and tyre combination is off a Lexus IS Sport that opted for upgraded wheels so standard. Anyway all works well looks, good and speedo accurate.

Eh :unsure: ? I assume you mean off an IS250 Sport, not an IS200 or the rolling radius would be smaller. Even so the fitting of the 18" wheels from an IS250 with standard Bridgestone tyres as fitted from new would have the same overall diameter as the 220D, not bigger...or am I missing something here? Does the 220D have a smaller rolling radius than the 250? :unsure:

Short of measuring it Tango, which I think I will now you think it should be the same, I think they must be bigger as the speed is now reading slightly higher which means the rolling radius is greater. Doesn't it?

There is an increase in speed showing on the speedo from 73 to 77 mph at 80 mph so I'm sure there is a calculation that will work out the increase in diameter before I measure it.

Another thought if they are the samne size is with more sidewall the compression at speed may be more than with a higher pressured low profile tyre that stays in shape, just a thought.

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I have my new wheels now 18" sport wheels, look amazing. The speed indicated by the sat nav is now "normal" I would say i.e. at 80 I am doing 77 mph. So the extra 4 inches has sorted the speedo differential which was nearly 10%.

My car is now 3 months old with 4500 miles on the clock averaged 40 mpg and no rattles yet. I am v. v. pleased with it. My only gripe was really the 16" wheels which even though an improvement on the IS200 base model still don't really suit the car. I didn't want leather seats basically if you're wondering why I didn't go for the SE.

I would think the mpg may go down a bit now but to be honest the 5th gear has now got longer legs so I don't use 6th as much which should improve the mpg as it can labour a bit especially with cruise on.

So you are saying that as well as putting on larger wheels you also put on larger tires. I.E. increasing your overall diameter / circumference. Am I not right in saying that this could lead to your tires rubbing and hitting the underside of the wheel well on larger bumps?

Tyrrp

Yes that does sound a bit daft 4". The new wheels are 2" bigger and the tyres are standard Bridgestone so low profile. I would imagine the wheel complete is about 1" bigger. The wheel and tyre combination is off a Lexus IS Sport that opted for upgraded wheels so standard. Anyway all works well looks, good and speedo accurate.

Eh :unsure: ? I assume you mean off an IS250 Sport, not an IS200 or the rolling radius would be smaller. Even so the fitting of the 18" wheels from an IS250 with standard Bridgestone tyres as fitted from new would have the same overall diameter as the 220D, not bigger...or am I missing something here? Does the 220D have a smaller rolling radius than the 250? :unsure:

Short of measuring it Tango, which I think I will now you think it should be the same, I think they must be bigger as the speed is now reading slightly higher which means the rolling radius is greater. Doesn't it?

There is an increase in speed showing on the speedo from 73 to 77 mph at 80 mph so I'm sure there is a calculation that will work out the increase in diameter before I measure it.

A bigger rolling radius will give you a lower speedo reading not higher as your wheels will be rotating slightly less for the same distance travelled due to the increase in circumference.e.g If your GPS says you're doing 80 and your speedo now says 77 instead of a reading before of 73 then your wheels are rotating at a higher RPM and are therefore smaller in diameter than before.

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I have my new wheels now 18" sport wheels, look amazing. The speed indicated by the sat nav is now "normal" I would say i.e. at 80 I am doing 77 mph. So the extra 4 inches has sorted the speedo differential which was nearly 10%.

My car is now 3 months old with 4500 miles on the clock averaged 40 mpg and no rattles yet. I am v. v. pleased with it. My only gripe was really the 16" wheels which even though an improvement on the IS200 base model still don't really suit the car. I didn't want leather seats basically if you're wondering why I didn't go for the SE.

I would think the mpg may go down a bit now but to be honest the 5th gear has now got longer legs so I don't use 6th as much which should improve the mpg as it can labour a bit especially with cruise on.

So you are saying that as well as putting on larger wheels you also put on larger tires. I.E. increasing your overall diameter / circumference. Am I not right in saying that this could lead to your tires rubbing and hitting the underside of the wheel well on larger bumps?

Tyrrp

Yes that does sound a bit daft 4". The new wheels are 2" bigger and the tyres are standard Bridgestone so low profile. I would imagine the wheel complete is about 1" bigger. The wheel and tyre combination is off a Lexus IS Sport that opted for upgraded wheels so standard. Anyway all works well looks, good and speedo accurate.

Eh :unsure: ? I assume you mean off an IS250 Sport, not an IS200 or the rolling radius would be smaller. Even so the fitting of the 18" wheels from an IS250 with standard Bridgestone tyres as fitted from new would have the same overall diameter as the 220D, not bigger...or am I missing something here? Does the 220D have a smaller rolling radius than the 250? :unsure:

Short of measuring it Tango, which I think I will now you think it should be the same, I think they must be bigger as the speed is now reading slightly higher which means the rolling radius is greater. Doesn't it?

There is an increase in speed showing on the speedo from 73 to 77 mph at 80 mph so I'm sure there is a calculation that will work out the increase in diameter before I measure it.

A bigger rolling radius will give you a lower speedo reading not higher as your wheels will be rotating slightly less for the same distance travelled due to the increase in circumference.e.g If your GPS says you're doing 80 and your speedo now says 77 instead of a reading before of 73 then your wheels are rotating at a higher RPM and are therefore smaller in diameter than before.

I hear what you're saying but imagine the wheels were half the size which means the speedo thinks it doing 80m mph because the wheels are revolving twice as fast when in fact the car is only going at 40 mph. So the bigger the wheel the lower the RPM of the wheel which woudl translate to faster the speeed per revolution of the wheel. I think?

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You lot are doing my head in.

The speedo reading is a function of the rotational speed of the axle.

The actual speed over the ground is a function of the rotational speed of the axle times the circumference of the tyre.

If you keep the speedo reading (i.e. the axle speed) constant but increase the diameter of the wheel then the actual speed of the vehicle over the ground will increase.

If you keep the speedo reading (i.e. the axle speed) constant but decrease the diameter of the wheel then the actual speed of the vehicle over the ground will decrease.

So, for a constant speedo reading of 80 m.p.h. and a change in satnav reading from 73 to 77 mph the effective wheel diameter must have increased by about 5.5%

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