Precisely. I run larger diameter tyres on 17" wheels and the speedo is accurate (vs. @GPS) to 1-2mph up to 90. OE wheel/tyre makes speedo read too high, at least on early Coops, maybe later speedos were changed?

All speedos are set slightly too high, on all cars of all brands. Don't remember exactly why but that's how it is.


No, it's 225/35-18.
These are 0.5% bigger diameter than the original 20VT 205/50-16, which is the closest you get with 18".

no 215/35/18 is the same rolling radius as a 205/50/16 or 225/45/16 standard coupe wheel and tyre - check for yourself - http://www.rtecshop.com/?p=calculator

Stock Tyre: 205/50x16 New Tyre: 215/35x18

Sidewall: 4.0 in .... Sidewall: 3.0 in
Radius: 12.0 in .... Radius: 12.0 in
Diameter: 24.0 in .... Diameter: 24.0 in
Circumference: 75.4 in .... Circumference: 75.4 in
Revs/mile: 840.3 .... Revs/mile: 840.3
--------------------------------------------------------------------------------
Your speedo will be 100% accurate!


as you say yourself its 0.5% bigger so it must be wrong

I'm not sure how well speedo's are calibrated to be honest so all this tyre talk may not be that important, I know with the tyres I have the speedo matches the sat nav spot on.

Mine fairly well matches the sat nav so don't see it as any issue really.

Calculating the circumference will not give you a figure of the percentage difference. It's all about the amount the tyre deforms when it contacts the ground.

Measure from the ground to the wheel centre and note the measurement. Try it again with different tyre pressures and you will get different figures. These are the numbers you need to use for calculating your effective rolling radius so you can compare.

In short it's suck it and see. These tyre calculators are nonsense. There are far too many variables to get anything more than a guess at the percentage difference.

so maths is wrong then i'v seen and read it all now

Your maths is fine. I think you misunderstand. If you have a calculated radius of 12 inches ,when the tyre is unloaded, it will change as soon as you put the load of a car on it. Like I said measure it. Measure from the top of the tyre to the wheel centre and then measure from the ground to the wheel centre. You will produce two different radius measurements. The later of the two is the one you use to calculate the effective rolling radius.

Remember the above mentioned phenomena varies based on tyre, pressure, relative conditions and rotational speed. It is also more pronounced on a higher aspect ratio tyre, like the stock wheels. So once again tyre calculators are nonsense.

Well the 215/35-18 is 0.6% smaller than 205/50-16 so it must be more wrong then..?
Check for yourself:
http://www.algonet.se/~leffmans/dack/dack.htm

Also, most 18" rims are 7.5-8.5" wide which makes a 215mm wide tire a little narrow.
Also #2, on your 18" wheels you'd better off with a little more tirewall/comfort than less...
Also #3, 225mm looks better, offers more grip and is what 20VT Plus has.

what about 19 inch rims

I think the 225 35s are best option

My Speedo matches gps pretty close (maybe a mile out)

So they must help make it more Accurate

19" would need such a small side wall tyre that the ride would be shocking!

I went from 17" to 18" on the GT, main difference I noticed was acceleration, really affected it badly. My new wheels are lighter thank god!

If it affected acceleration it must be due to different gearing (bigger wheels). The extra weight wont be noticable that way.

You're kidding...

Heavier wheels are a brilliant way to slow down a car - rotational inertia is a huge drain on acceleration

Increasing the wheel diameter almost always ends up with a corresponding increase in weight, which has an immediate adverse effect on acceleration.

Think of your wheels as a pair of extra flywheels - we all know that a lightened flywheel helps throttle response and acceleration, so it follows that heavier rotational items in the drivetrain will have the opposite effect.

Why do you think F1 cars have such small wheels? I know that they use tyre deflection as part of the suspension, but if 20" wheels and ultra-low profile tyres gave more grip, they would use them (and then find a way of making the mechanical suspension work around it). Ultimately, a small wheel and tall tyre is easier to accelerate and brake

The primary reason for me having 17" wheels on my car is that they are lighter than the stock wheel and thus aid performance. The secondary reason is better handling due to a lower sidewall and less unsprung weight and the final reason was aesthetics.

Absolutely! And as we all know, unsprung weight increases are bad ideas...

My Car and Nismos are pretty spot on to each other on acceleration - we both run the same wheels - But even with my stock wheels (which I stuck on for autoitalia)- we were equal - I agree that the wheels are heavier but its not alot nothing to make a drastic difference - and the rolling radius is virtually the same

Yeah, my 18" wheels are MUCH heavier than the std 16" wheels but when I swap over I can't say I feel any difference in acceleration. What I do feel is the nimbler feel and better handling.. Which also could be down to the Toyo R888 on the std wheels!

Doesn't matter how much the tyre deforms, the circumference does change. Rolling radius will change but the RPM of the wheel does not.

Not quite. The circumference of the tyre remains constant. What I'm saying is the deformed tyre will produce a smaller radius (effectively) and therefore produces a smaller effective rolling radius and thus cover less distance per rpm .

I think I may produce a how to guide with some diagrams to help explain this.