Tyre Size

[Williboy]

Hi, i have a set of 18x8s. The recommended tyre size is 225/40. I was wondering if 235/40s fit without affecting the speedometer? I was thnking more rubber more grip!

[billybandit]

they wont effect the speedo, your not changing the wheel and tyre diameter just the width. as long as you stay with 40series tyres you should be fine. not sure if you will have any rubbing problems

[Williboy]

thanx! :D

[andyhart21]

There is a very useful convertor on the Toyo website that tells you the rolling diameter for all widths and diameters of tyre. As long as the rolling diameter is nearly the same your speedo will read about right.

Toyo Tyre Convertor

[ColinBarber]

they wont effect the speedo, your not changing the wheel and tyre diameter just the width. as long as you stay with 40series tyres you should be fine. not sure if you will have any rubbing problems

This isn't true because the profile of the tyre is percentage of the width. Therefore if you increase the width and keep the same profile the total diameter of the wheel/tyre will increase.

[p.k.blaauw]

TRy this one, calculates spedo difference.

http://www.miata.net/garage/tirecalc.html

Speedometer reading whit non-stock tire is 1.3% slow.

60mph on speedo = 60.8 mph on the road.

[Dodgy]

I never knew this. If i get caught by a speed trap, i think I'll get my speedo checked before paying the ticket :D

[Williboy]

ok will have a try. :D

[Rob]

a bit of info for u the IS speedo reads 4 - 5 mph high eg speedo says ur doing 30 but in reality ur doing 25 ish, have checked this in different IS's with different gps systems and they all say the same

[billybandit]

they wont effect the speedo, your not changing the wheel and tyre diameter just the width. as long as you stay with 40series tyres you should be fine. not sure if you will have any rubbing problems

This isn't true because the profile of the tyre is percentage of the width. Therefore if you increase the width and keep the same profile the total diameter of the wheel/tyre will increase.

you learn somthing new every day

[LennyLexus]

a bit of info for u the IS speedo reads 4 - 5 mph high eg speedo says ur doing 30 but in reality ur doing 25 ish

No wonder people say i'm going too slow

[maneesh]

a bit of info for u the IS speedo reads 4 - 5 mph high eg speedo says ur doing 30 but in reality ur doing 25 ish

No wonder people say i'm going too slow

Under EU law, all new cars' speedos can be up to 10% optimistic. Some car magazines include info on true mph versus indicated mph.

So in the worst case, you could be driving at an indicated 77mph, but only be doing 70mph.

This affects all cars, not just more mainstream brands.

[Chris Skelton]

:P ....So I waz only doing 115, NOT 125 the other day...phew! :zee:

[maneesh]

Yes, I have always driven at an indicated 45 in a 40 zone past speed cameras and never been flashed. Just shows you doesn't it, can't rely on everything to be accurate...

[ColinBarber]

Just shows you doesn't it, can't rely on everything to be accurate...

It's made to be that way though, it's not poor design that makes it inaccurate. If you look at a workshop manual for a UK model the test procedure for a speedo gives the allowed range for a given speed. Basically if your speedo reads 70mph at a real 70mph it should be replaced as it is faulty (it should be reading something like 72-79)

[RX-Men-8]

That's correct. It also prevents these complaints:

- I have gotten a speeding ticket while my speedo indicated legal speed

- My car doesn't reach the max speed as specified

- My car has poorer mpg than specified (people are driving slower than they think and the odometer is also a bit on the + side)

;)

RX-Men-8

[ColinBarber]

- My car has poorer mpg than specified (people are driving slower than they think and the odometer is also a bit on the + side)

The odometer should be accurate.

[maneesh]

Just shows you doesn't it, can't rely on everything to be accurate...

It's made to be that way though, it's not poor design that makes it inaccurate. If you look at a workshop manual for a UK model the test procedure for a speedo gives the allowed range for a given speed. Basically if your speedo reads 70mph at a real 70mph it should be replaced as it is faulty (it should be reading something like 72-79)

Thanks for the info. More people should know these facts, so they don't slow down abruptly to an indicated 35 when approaching cameras in a 40 zone!