Sorry Szkom. It was in response to Dave G.

According to kwikfit i should be running 205/50/15's as standard.

I am not sure I would ever trust KwikFit to be correct.

No, I think I understand you perfectly.

AFAIK, the 20v na and 20vt have speedo's that are "calibrated" differently.

Yes, but both ours and your "educated guess" is based on the "misleading" unloaded circumference/radius. My point is that it's not exactly correct, but it's certainly not misleading.


petep was referring to the 16v na (195/55-15) and 16vt (205/50-16) where the larger tyre is larger by closer to 3% (2.7%). The OP has a 20v na and is presumably fitting 20vt wheels, where the difference in unloaded circumference is 4%.

On the 20v na/ 20vt, that 4% difference translates to a difference in OD (or “height” if you will) of 611 – 586 = 25mm (1”), so that the same car would sit 12.5mm (½”) higher on the 20vt wheels/tyres. Now I concede that the 20vt engine is heavier than the 20v na, if only because of the turbo and extra radiator fan, but at the same time the recommended tyre pressure increases from 2.5 to 2.7 bar. So I can’t really see how the effective rolling radius on the same 50 profile tyres could be the same. Similar maybe, in the same way that unloaded circumferences are similar, but not the same. But without all this special tyre data we’ll never know, so we’ll just have to agree to disagree.

Just as a footnote, according to the workshop manual, the 16v na could come with 205/55-15 as an option which is larger than the 195/55-15 by 1.8%. Likewise, the 16vt could come with 205/50-15 instead, which is 4% smaller different. And maybe these will all have the same effective rolling radius, maybe not.

At the end of the day, we can’t care too much about something we can’t estimate, but we can care about something that we can, however misleading you think it is.

Oh and finally

Compared to 205/50-16, your 225/40-18 is around an inch “bigger” (OD) or ½” bigger (radius) and my 225/50-17 are indeed almost an inch bigger in radius.

I chose that size deliberately to reduce the speedo error on my car with 205/50-16 (an indicated 74.9mph at a true 70.0mph from GPS, YMMV) to precisely zero, whilst at the same time filling out the arches as I mentioned, perhaps raising the car a little to overcome the “a bit too low?” drop on Eibachs, and also introducing a degree of “comfort” by maintaining the 50 profile instead of going to 45 or even 40. Similar reasons that other people have used in going to 205/55-16, which can be significantly cheaper than 205/50-16.

And with a width of 225, the same as I have used in the past (on 16” wheels) I thought I would be OK on clearance, and in general I am, but there is some rubbing on the front “air grid” part, and on the small cover over the aux belt tensioner or whatever it is. I’m no handling expert (nor, clearly, an expert in effective rolling radius) but it seems to handle pretty good to me.

Dave, hopefully no offence caused. Likewise for Petep.

I think there's a lot we will have to agree to disagree on. But equally I think we're mostly on the same page.

I didn't calculate the circumference/radius/diameter of my tyres. I simply deduced that I'd likely need a smaller tyre profile with 2 more inches of diameter on the rim. I then went for the one that would likely fit (eye-balled), but not behave like it would be painted on. As it turns out a reasonable guess as my speedo tends to read just under.

Where I disagree is that it's not the same 50 profile between the two tyres except in a perfect cross section. ones a 16 inch, the other 15. There's simply more rubber per radian on the 16 inch. Therefore able to deform more under load.

I'm not suggesting we need blinding accuracy; why when a stock car under reads by 7% (using your figures)? But this thread has lots of suggestions of alternative sizes based on an unloaded, static dimension that are comparable to the OP's unloaded, static tyre. I'd like to think you'd agree that as the effective radius varies when in use it's difficult to state that tyre A will be comparable to tyre B without using the "special" tyre data.

Hopefully you can see why I consider the unloaded static measurement of a tyre misleading in context of the OP's question.

I hadn't considered the speedo error!
Like you I wanted to fill the arches a little more than they are at present and give a little more clearance over the many speed humps around here.
That's why I opted for the 40 profile instead of the 35.

Hmm seems to be 2 topics running here....

The kwikfit site is accurate for std fit wheels, as the coupe didn't have a million choices of wheel then it's right.

In simple terms for every one inch increase in rim drop 5 on the tyre wall, same for every 20 in width.

Sheep 45 16 on a 205 are as close as you'll get, 195 will fit on the vt rim also and be closer. Check your speedo to sat navigation on the flat if being a bit out worries you, you can always compensate once you know.

Thanks for the replys guys, im going for some 205/45/16"s. Everyone has assumed i picked up some cheap 20vt rims, infact i picked up some TSW evo 6 spoke alloys for a bargin price of £26 on ebay. These are a temp replacement as i picked them up so cheap. Thinking of getting some spacers for them do i need hubcentric? The et on these wheels are 35 so only looking at 5mm or maybe 10mm at the very most.

Actually I have 225/45-17 fitted and not 50 profile, so not quite as big a deliberate oversize

Decided not to go for the 16's as they still look to small, so now on the look out for 17's and 18's qustion is what size tyre on the 17's. 18's i will run 225/40/18.

I'd go for 215 40 17 or 215 45 17 as they are easy to find sizes.
Mine have 205 40 17 on at the mo and they look very small in the arches.
It also depends on whether you want comfort or sharp handling.

Fancy a set of dark grey Rota Circuit 10's?

Can you post up a picture of the wheels, discriptipn of tyre and wheel condition and then pm me a price.
Cheers

Tyres are all budget and 3 are at least half worn with the 1 other being quite new (6mm left)

Wheels are coming off this weekend and I will be refurbing the lips as they have some minor kerbing.
The rest of the wheels are virtually unmarked.

Not sure on the width and ET until I get them off the car but I am running spacers with them.

Here's a pic of them on the car:

How much for the wheels.