Rolling Diameters for Various Sized Wheels

[WhiteGTB]

Does anyone know what the rolling diameters are for these wheels below? Different from each other or pretty similar?

Does it affect the speedo much?

215/45 17"

225/40 18"

225/35 19"

[WhiteGTB]

Ahh.. Ric, can you move this post to Suspension, Brakes, Tyres and Wheels please. Thanks.

[Kevlar GT]

not sure what the rolling diameter is but I did notice that the 18's I got with 215 45 make the speedo almost spot on according to my GPS. 120 on the speedo was 118 on the GPS, prior to fitting it was 115 with stockies.

Cheers.

[UGSTI]

You may want to look at this site as it has some good information and tools for calculating such.

http://www.carbibles.com/tyre_bible.html

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[tangcla]

www.miata.net/garage/tirecalc.html

Answers all your questions

[WhiteGTB]

You may want to look at this site as it has some good information and tools for calculating such.

http://www.carbibles.com/tyre_bible.html

.

Excellent and very userful website, thanks!!!!

[tangcla]

You're right, that site is a good read! I just spent half an hour at work reading it

[adam]

You may want to look at this site as it has some good information and tools for calculating such.

http://www.carbibles.com/tyre_bible.html

.

i read it for so long it gave me a headache..
the dude is funny(most of the way down the first page, tyre-black polish bit)

[mianos]

I wasted a good hour on that, including popping down to the carpark to get the numbers of the sidewall of mine

[tangcla]

I wasted a good hour on that, including popping down to the carpark to get the numbers of the sidewall of mine

haha... nice work

[WhiteGTB]

Now I know what ALL the numbers on the side wall of the tyre mean

[Ric]

You may want to look at this site as it has some good information and tools for calculating such.

http://www.carbibles.com/tyre_bible.html

.

There's an interesting quote on page two of this site:

For the first two years of our new life in America, I'd take our Subaru for its service, and it would come back with the tyres pumped up to 40psi. Each time, I'd check the door pillar sticker which informed me that they should be 32psi front and 28psi rear, and let the air out to get to those values. Eventually, seeing odd tyre wear and getting fed up of doing this, I asked one of the mechanics "why do you always over-inflate the tyres?" I got a very long and technical response which basically indicated that Subaru are one of the manufacturers who've never really adjusted their recommended tyre pressures in line with new technology. It seems that the numbers they put in their manuals and door stickers are a little out of date. I'm a bit of a skeptic so I researched this on the Internet in some of the Impreza forums and chat rooms and it turns out to be true. So I pumped up the tyres to 40psi front and rear, as the garage had been doing, and as my research indicated. The result, of course, is a much stiffer ride. But the odd tyre wear has gone, and my gas-mileage has changed from a meagre 15.7mpg (U.S) to a slightly more respectable 20.32 mpg (U.S). That's with mostly stop-start in-town driving. Compare that to the official quoted Subaru figures of 21mpg (city) and 27mpg (freeway) and you'll see that by changing the tyre pressures to not match the manual and door sticker, I've basically achieved their quoted figures.

So what does this prove? Well for one it proves that tyre pressure is absolutely linked to your car's economy. I can get an extra 50 miles between fill-ups now. It also proves that it's worth researching things if you think something is a little odd. It does also add weight to the above motto about not trusting forecourt pressure gauges. Imagine if you're underfilling your tyres because of a dodgy pressure gauge - not only is it dangerous, but it's costing you at the pump too.

[adam]

absolutly ric. after purchasing (as some nerds on this site reccommended) a digital tyre gauge i've found that every servo pump is inaccurate. usually over the mark which means that you're under-inflating your tyres leading to more friction and fuel consumption.

It's a conspiracy i tell's ya