Tyre OD size and their effect on the speedo

[Sharkie]

It is time to upgrade my tires and am looking at 215-45-17 or 215-40-70 based on info that 215s are about as wide as you can go on without rubbing.

205-45-16 (standard) is 591mm Overall diameter (OD)
215-40-17 is 604mm OD
215-45-17 is 625mm OD

No, as we all know the Polos all overread by about 8kmh at true 100. Will larger diameter tyres cancel out the overread or make it worse ?

Any info would help.

Thinking of either Toyo Proxes R888 or Federal 595RS semi-slicks.

[Ben_GTI]

larger diameter will make the speedo more accurate at 100..... to a point.

[Eddy]

Hoyhoy.
I had my speedo checked by a professional & he came up with a 9% over read at different speeds. So if you add 9% to 591mm it comes to 644mm to get a true reading.

[MACCAA]

Be careful of fitting larger diameter tyres to correct speedo error.You'd be surprised how much it can knock performance.
Had slightly larger tyres on a previous car,really noticed the difference every time I put the stockers back on...

[PeterHBne]

As MACCAA said, larger (or smaller) diameter tyres/wheels really knock things about.

Even a small difference either way is VERY noticeable. Particularly off the line.

Go smaller and your gearing gets rooted. Go larger and it feels sluggish off the line and noticeably slower to accelerate.

Don't do it.

[PoloGTi]

All depends on your off set and width i guess.
I am running a 205/40/17 595.8mm Diameter.
I dont think a 215 would fit very well my rims have an offset of 32 which pushes them right out to the gaurds. looks the good but yeah u get a little stretched look.
This is a set of TT comps on a Polo GTI in the uk.



If the offset was 35 i think a 215/40/17 would be a good way to go.

[PeterHBne]

Nice wheels!

[dammfool]

I have 15 inch vw rimms with 195 tyres . Can i go to 205 or 215 tyres without upsetting things

[ralphk]

215/40/17

This seems to now give correct speed...at least according to the GPS

[Old_Fart]

So what tyre size did you finally decide on the 205x40x17 or the 215x40x17?

If you fitted the 215x40x17 - did they fit without any tyre scrubbing on either the suspension or inner guards.

Reason for the question is that I have picked up a very good set of the optional VW "Siata" 17 x 7 wheels for my Polo GTI - and I am keen to know what works best.

Obviously the 215 section tyre will give a meatier look - and given the 6% speedo error, I can live with the 2.2% variance to the gearing off the original 205x45x16 tyres

Appreciate your views

[gareth_oau]

heres a good tyre/wheel calculator - shows how much changes can affect yr speedo

http://www.carmodifier.com/default.asp?Action=tireTechCentreCalculator

[Old_Fart]

Thanks Gareth - but I already have access to a "converter" that determines the effect on the speedo of changing wheel size and tyre section.

The question is: Does the 215x40x70 tyre size on a 17 inch set of VW Siata accessory rims with the correct offset suit a 2008 Polo GTI - without any tyre rubbing on suspension or body parts?

[Timbo]

My understanding is that 215's can be fitted satisfactorily. I'm assuming you haven't dumped the car to its bump stops

[Cygnus]

As MACCAA said, larger (or smaller) diameter tyres/wheels really knock things about.

Even a small difference either way is VERY noticeable. Particularly off the line.

Go smaller and your gearing gets rooted. Go larger and it feels sluggish off the line and noticeably slower to accelerate.

Don't do it.

Hmm... is this just because of the weight? Or am I missing something? If its just the weight, can't you just get some lighter rims?

[Grant]

Hmm... is this just because of the weight? Or am I missing something? If its just the weight, can't you just get some lighter rims?

larger diameter tyre's are the equivalent of having longer gearing. so slower acceleration and lower rpm at given speed. combine that with the extra unsprung weight and more friction/rolling resistance of a larger tyre.