takahashi

I have seen a few ads in the recent few Japan magazine. In the tyre ads they (ALL 3 different companies with BIG ADS) suggest tyre and wheels size for RX-8.
Front 18x8 (8.5) and Rear 18x9 (9.5)
Tyre 235/40-18 front and 265/35-18 at the rear

I use the tyre calculator to check the difference to stock!
Specification Sidewall Radius Diameter Circumference Revs/km Difference
225/45-18 101mm 330mm 660mm 2073mm 483 0.0%
235/40-18 94mm 323mm 645mm 2027mm 493 -2.2%
265/35-18 93mm 321mm 643mm 2019mm 495 -2.6%
245/40-18 98mm 327mm 653mm 2052mm 487 -1.0%
255/40-18 102mm 331mm 661mm 2077mm 481 0.2%

The closest it actually is 255/40-18. 245/40 that many of you are using is not bad but the ones that Japan suggest is a bit off :p

See what I found from tyrerack.com of the tyre available for 255/40-18:
Bridgestone S-02 S-03, Michellin Pilot Sport, Goodyear Eagle F1, Dunlop SP 9000, Pilleri PZero Rosso.... not a bad mix

The problem is the wheel that I that I like Volks CE28 N. Although I allows many size widths, but I don't know if 8.5 will take 255/40. It should.

After all this bullshit, THE QUESTIONS I WANT TO ASK ARE:

How much importance is to keep in to same ratio?
How much of right/wrong to have difference size front/rear... coz that will change the cornering characterics.
What do you think of the size by the Japanese?

Thanks Taka

Ole Spiff

Well something interesting I learned while talking to Jim Mederer at Racing Beat, is that the RX8 does NOT use the traditional speedometer system of a cable connected to the transmission. Instead it uses the computer which averages the rotational speed of both rear wheels and calculates your speed accordingly. That's what you see on your digital readout as your mph or kph.

I found this very interesting as it's not dependant on the engine or drivetrain speed at all! Unless I'm mistaken, the computer will be able to adjust to the different tire sizes because it's tracking the actual rotation speed of the wheels, not the mechanical speed of the transmission. The ratio of the transmission speed versus wheel rotations is not an issue.

If anyone who has put non-stock size wheels/tires on could do a test by driving next to or behind another RX8 owner who has a stock 8 and see if speedo readings match, that would prove it.

tintagel

Indeed interesting:

However you have missed an important ingrediant. The computer can measure the angular velocity of the wheel, but it has to calculate the linear velocity based on the diameter of the wheel. Obviously you have to tell the computer what the diameter is. The calulation is simple:

Pi (3.14159whatever) * diameter = the distance traveled in one rotation of the wheel.

So if you change your diameter by putting on non stock wheels/tires you need to have the computer adjusted to compensate.

Having said all of that, consider also that we are dealing with pretty small differences here. The difference in tread depth (and thus radius) of a new -vs- worn tire can exceed 5mm, so clearly a 1.5% variance is expected anyway.

takahashi

Very true... but consider in my state (Melbourne) that have tolerance of 3km/h over the speed limit - correct speedo is very important.

Anyhow, no one answer or commented about Japan tyre shop's size choices?

Genom

I think their recomendations are OK, but it's really a matter of driving preference I think. That being said, also remember that the cars speedo has been shown to be a little off on the safe side (about 2MPH over actual speed at 70MPH) so putting on smaller tires will increase your margin of safety. Just dont put on larger ones and you wont have to worry about the speedo not being right.