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IS250 2006 - Speedo adds about 10-15% to readout


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Hey all, 

Long time listener, second time caller! 

My 2006 IS250 with 110k miles on it (spotless engine though!) came with your typical 220d 16" rims, the nasty silver ones you can find on eBay for £30 a rim. 

The speedo shows way too much compared to GPS. It goes something like this: 

 

GPS - 30mph, car - 35mph

GPS - 40mph, car - 47mph

GPS - 70mph, car - 82mph

 

I initially thought that the car must've had shot rims and the seller bought the cheapest fitting set (16s) and assuming the 250 comes standard with 17s I bought a nice set of Lexus 17 inchers and fitted them but the speedo readout is still the same (not sure if identical but still way too much). 

 

Does anyone have any experience with this kind of issue? I'm emailing specialists all over the place with mixed results, some saying the tires are too small some saying my GPS is wrong and my car is correct?? 

I tested this against multiple cars, two GPSs and one of those digital speed signs that measure your speed. The margin of error between these controls were the standard 1-2mph between them. 

 

Appreciate any insight or ideas, thanks! 

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15 minutes ago, Morbid Valve said:

Does anyone have any experience with this kind of issue? I'm emailing specialists all over the place with mixed results, some saying the tires are too small some saying my GPS is wrong and my car is correct?? 

I think GPS is unlikely to be wrong.

First step is to establish the correct wheel & tyre size that should be fitted and then go from there.

If you find you have the incorrect wheel & tyre size then its fairly simple maths to work out the rolling circumference and whether that is the cause of the error.

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10 minutes ago, Spock66 said:

I think GPS is unlikely to be wrong.

First step is to establish the correct wheel & tyre size that should be fitted and then go from there.

If you find you have the incorrect wheel & tyre size then its fairly simple maths to work out the rolling circumference and whether that is the cause of the error.

Is there any specific way to go about finding out the correct sizes? 

Is it car specific or range specific? 

Apologies for being a block head but I only recently started looking into rim, wheel and tire sizes and I'm struggling to absorb it all! 

Thanks 

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Vehicle mechanical speedometers usually over read actual speed by about 10%, this is normal as manufactures have to ensure that after 10+ year and 100,000 miles it should never Under Read actual speed, as the parts ( as they are mechanical) wear over time the manufacture would be in a world of s**T if Speedo was to under read.

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Could it be the speedo is calibrated for 17 or 18" wheels?

A 205/55R16 has a considerably smaller rolling radius than the rear tyre size when 17/18" wheels are fitted.

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6 hours ago, Morbid Valve said:

Hey all, 

Long time listener, second time caller! 

My 2006 IS250 with 110k miles on it (spotless engine though!) came with your typical 220d 16" rims, the nasty silver ones you can find on eBay for £30 a rim. 

The speedo shows way too much compared to GPS. It goes something like this: 

 

GPS - 30mph, car - 35mph

GPS - 40mph, car - 47mph

GPS - 70mph, car - 82mph

 

I initially thought that the car must've had shot rims and the seller bought the cheapest fitting set (16s) and assuming the 250 comes standard with 17s I bought a nice set of Lexus 17 inchers and fitted them but the speedo readout is still the same (not sure if identical but still way too much). 

 

Does anyone have any experience with this kind of issue? I'm emailing specialists all over the place with mixed results, some saying the tires are too small some saying my GPS is wrong and my car is correct?? 

I tested this against multiple cars, two GPSs and one of those digital speed signs that measure your speed. The margin of error between these controls were the standard 1-2mph between them. 

 

Appreciate any insight or ideas, thanks! 

This is spot on as it is on all modern cars. I find Lexus on the higher side, most competitors are ~5-10% range, Lexus is most of the time spot on 10% for me, when the car is on standard 17" wheels, 16" being even smaller means it is even more off. I would not worry too much about it, because this is legal requirement and every car has to always show more then the actual speed (or at least they can't show less).

You can play around with numbers here to see what size would you need to use to make it spot on, but it is quite unlikely that wheels of such size would even fit within the fender. I reckon around 245/55R18 is where you would get +/- 1mile and that is the size which probably would fit on the rear (as far as I know speedo reflects on the size of rear wheels). 

https://www.wheel-size.com/calc/?wheel1=205-55-16X6.5ET45&wheel2=245-45-18X7.5ET40&fcl=50mm&wcl=30mm&scl=50mm&sr=0mm

But other thing to consider - even if you find the sizes which would make it perfect, it does not mean you could get the tyres in that size of that they would be economical to purchase. Like fronts would have to be something like 225/60R18, which is not very common size and costs like £160+ per tyre

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Mine's a 55plate and find it behaves the same. I just live with it now but used to annoy the sh#t out of me. The previous owner managed to set the speedometer light to come up at 70mph so at least I have that...

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2 hours ago, zoricib said:

Mine's a 55plate and find it behaves the same. I just live with it now but used to annoy the sh#t out of me. The previous owner managed to set the speedometer light to come up at 70mph so at least I have that...

You do know you can set the LED alert level on the speedo & Rev counter to whatever to want?

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6B4EDA96-81B7-41CA-A50B-9E4C3C0A1C93.png

A0F46C2D-7ECF-4512-9CFA-71DE1642A153.png

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On 4/16/2022 at 12:51 AM, Texas said:

You do know you can set the LED alert level on the speedo & Rev counter to whatever to want?

AE0302E8-3A26-42AF-96FC-D3DC51417BD9.png

6B4EDA96-81B7-41CA-A50B-9E4C3C0A1C93.png

A0F46C2D-7ECF-4512-9CFA-71DE1642A153.png

Yup, was aware. Meant that the previous owner set it at actually 70mph as per google maps, not 70 on the dial. 

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On 4/15/2022 at 12:32 PM, Morbid Valve said:

GPS - 30mph, car - 35mph

GPS - 40mph, car - 47mph

GPS - 70mph, car - 82mph

I tested this against multiple cars, two GPSs and one of those digital speed signs that measure your speed. The margin of error between these controls were the standard 1-2mph between them. 

My local speed sign shows i'm doing 20MPH but the car is showing 25MPH. Same with my GPS on both my dashcam and phone. Remember GPS has to travel from point A to space and back to point B which then calculates how far you travelled in that time and gives you a good indication of the speed you're doing but it's not 100% accurate. a 5mph difference is well within that tolerance.

The local speed signs i wouldn't be happy with since who calibrated it? When was it calibrated etc...
If you really want to get dicey you could find a cop with a speed gun and ask them to check your speed as you pass them again. You will more than likely be asked to visit a garage but if you get a good one they might be willing to check as this shows the gun and your car match up and all tickets are issued correctly. But yeah! That runs the risk of a fine so... 

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On 4/15/2022 at 6:44 PM, Linas.P said:

But other thing to consider - even if you find the sizes which would make it perfect, it does not mean you could get the tyres in that size of that they would be economical to purchase. Like fronts would have to be something like 225/60R18, which is not very common size and costs like £160+ per tyre

No point messing about with the tyres. Chances are it will still be wrong. I can't imagine the look of 225/60R18s on IS250😂 That's a Volvo XC60 tyre! (Well technically 235/60R18) I don't think it would fit lol

But yeah, like others have said. The car is over reading. I notice that a lot when I occasionally use Waze instead of Google Maps as Waze shows your speed.. I'm doing ~78mph when Waze says I went over 70.. so 10% makes sense. Still, I got flashed for less before, so I'm sticking to speedo reading because I cba to appeal this at court if another fine comes..

and following this logic, since you can go over by 10% + 3 legally, your speedo will be reading ~88mph and that's your legal motorway speed.. technically.. I'm not testing it, that's for sure.

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3 minutes ago, H3XME said:

No point messing about with the tyres. Chances are it will still be wrong. I can't imagine the look of 225/60R18s on IS250😂 That's a Volvo XC60 tyre! (Well technically 235/60R18) I don't think it would fit lol

But yeah, like others have said. The car is over reading. I notice that a lot when I occasionally use Waze instead of Google Maps as Waze shows your speed.. I'm doing ~78mph when Waze says I went over 70.. so 10% makes sense. Still, I got flashed for less before, so I'm sticking to speedo reading because I cba to appeal this at court if another fine comes..

I don't say that is "viable" way. But technically one can fit such tyre which would make speedo more or less correct. As that was the question I answered it, but I don't think such tyres would fit the car.

Other thing - many people actually don't know that speedos always under-reads and drives in outside lane at 66MPH and get annoyed when I overtake them. Or drive in the city consistently at exactly 26MPH causing unnecessary delays.

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Bob ..........  it might be sadly that your speedo is simply showing signs of old age and miscalibrating itself ........ at a certain age my elderly Ls400 has been doing this from time to time too .. and others with older cars experience this phenomenon

Often used to read 10mph less than others were doing on the same strtetch of 70mph road .....  then mioraculously remedied itself too .....  happens over several years now and there's nowt to be done it seems ...  it's possibly just an old age syndrome for many " modern " cars these days

so fear not, the solution you're thinking is there just might not be there at all

Malc

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4 hours ago, H3XME said:

and following this logic, since you can go over by 10% + 3 legally, your speedo will be reading ~88mph and that's your legal motorway speed.. technically.. I'm not testing it, that's for sure.

Sorry but that is not correct, the speed limit is the speed limit.

The Association of Chief Police Officers (ACPO) have provided guidance of enforcement at 10% + 2mph, but that is only guidance and different forces may or may not follow this guidance.

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2 hours ago, Spock66 said:

Sorry but that is not correct, the speed limit is the speed limit.

The Association of Chief Police Officers (ACPO) have provided guidance of enforcement at 10% + 2mph, but that is only guidance and different forces may or may not follow this guidance.

Yes, that is correct. As well this guidance only applies to stationary "spot" cameras, whereas some police forces allows discretion for the officer if they have like "speed trap" and it does not apply to average speed. I have been done for 2MPH over on motorway in the middle of the night, because there were "roadworks" and speed was limited to 50MPH, there were no cones, no workers, just the sign at the beginning of it... and I didn't brake just let the car to slow down... thus did 52MPH in the first zone of average speed on literally empty 4 lane 70MPH motorway and got fine. Lucky for me it was eligible for SAP.

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30 minutes ago, Linas.P said:

I have been done for 2MPH on motorway

One can also be done for travelling too slowly 😉

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24 minutes ago, NemesisUK said:

One can also be done for travelling too slowly 😉

It would be very funny, if not for reality in UK where such think could ruin lives... If not for SAP 3 points on license can mean not being able to afford insurance or even losing job... and why - just to protect "road workers" who are not even there!

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11 hours ago, Linas.P said:

just to protect "road workers" who are not even there!

are they ever there ? anywhere in fact ?   ......  discuss :wink3:

Malc

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38 minutes ago, Malc said:

are they ever there ? anywhere in fact ?   ......  discuss :wink3:

Malc

Yes.. they've been doing up a new roundabout by my work, near the NEC and I always see about 10 guys just standing there watching 1 that actually does work. 😂

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hahahahaha ....  universally i meant really ........  anywhere in the UK 

often see masses of " work " disruption cones and even trenches etc, idle pieces of kit too BUT so so rarely actually see any real people working away at it .......  anywhere

Malc

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I drive buses through Edinburgh city centre every day i believe there are around 60 sets of roadworks ongoing at present,if there is any activity the only person working is the JCB operator while the other 8 people are shooting the breeze.Last weekend,the easter bank holiday one lane of the main route west out of town was dug up on Thursday leaving one remaining lane,with the inevitable chaos and no further activity till the following Tuesday.

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28 minutes ago, McRattlehead said:

I drive buses through Edinburgh city centre every day i believe there are around 60 sets of roadworks ongoing at present,if there is any activity the only person working is the JCB operator while the other 8 people are shooting the breeze.Last weekend,the easter bank holiday one lane of the main route west out of town was dug up on Thursday leaving one remaining lane,with the inevitable chaos and no further activity till the following Tuesday.

At least they actually did some work and a lane was actually out of use. I love the cones placed out blocking lanes, for work due to start two weeks later, very usual practice this way.

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3 hours ago, Malc said:

are they ever there ? anywhere in fact ?   ......  discuss :wink3:

Malc

No - it was like 4 or 5 AM at night, and there were no roadworks. My assumption - they planned to start roadworks in that stretch of the road at some point, somebody came and installed average speed cameras before works actually started and turned them on despite there being no road works and no workers. 

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