Best size tyres?

[Steve Burnett]

Okay then, its going to be:

235 x 40 x 17 or
235 x 45 x 17

What reason can you give me for going for one above the other?

Why not 235 front and 245 rear?

[Steve Burnett]

[Shark]

Originally posted by Steve Burnett
Okay then, its going to be:

235 x 40 x 17 or
235 x 45 x 17

What reason can you give me for going for one above the other?

Why not 235 front and 245 rear?

Sorry steve just spotted this.

Original 16" wheels and tyres are 225/50 16
Overall diameter is 24.86inches or 631mm

235/45 17
Overall diameter is 25.33inches or 643mm  
A difference of +1.90% from stock

235/40 17
Overall diameter is 24.44inches or 620mm
A difference of -1.74% from stock

Remember school maths?  
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Circumference (i.e. rolling distance) = Pi x diameter

One full revolution of stock wheels = 1982mm
235/45 17 = 2020mm
235/40 17 = 1947mm

I went for 235/45 simply because Mazda used it and there willbe less tyre wear - marginal, but I\'m a tight git and 2% more miles from the rubber seems beter than 2% less.  

The back wheels are where the speedo and odometer readings are taken from I think, but to use the same numbers for example

The 235/45 will travel further per revolution so the speedo will read lower than you are doing

The 235/40 will travel less distance so will read more than you are doing.

In both case the speedo inaccuracy will be approx 2% so not a significant error.  Your choice really.  The speedo can be recalibrated but will be a hassle especially so far from any specialists.

Again Mazda used 255/40 17 on the cars so I went with that.  

Hope this helps - sorry for the maths lesson.

[Shark]

Oh yeah, and 235 on an 8" wheel provides a little rubber beyond the rim giving a little protection for your expensive wheels.
255 on a 9" rim gives the same deal.

[Steve Burnett]

Mine are 8.5" wide front and rear.

[clive]

Hi Steve,

I know a few have answered this already but l found that by having

235x40x17 on the front it stops the tyres possibly rubbing the inner arches ( car felt better than when l had 45 profile on the front as well.

For the back you need 255x40 at least as this is the stock width and that way you will keep your spedo reading properly as well.

As John says 98+ RX-7\'s with 17\'s came with 235x45 front and 255x40 Rear from the Mazda factory, and although these cars were on 8" rims the extra 1/2" you have wont present a problem with these sizes.

So my recommendation is


235x40 Front and 255x40 Rear.

Hope this helps.

[MikeLMR]

better to have a narrower tyre on a wide rim than visa versa (within reason that is)