Correct tyre sizes...?

[just7]

I\'ve finally got my wheels back from re-furb.
All the 4 wheels are 18" x 8.5" size. I have 2 Eagle F1 235 x 40 tyres, but i need to buy 2 new tyres. My question is;  is it best to try & fit the same 235 x 40s all the way round, or could i just put the tyres i have at the moment on the front & get wider tyres for the rear?  + what size would be best? Someone else has even suggested that i use the 235 x 40s on the rear & buy 225 x 40s to put on the fronts to really make sure i don\'t have any clearance problems..??    I\'m also confused as to what effects running different sizes will do to the speedo reading etc. Any advice is appreciated.
BTW my FD is sitting on Racing Beat lowered springs.

[just7]

What..... is no-one running tyres on their wheels......?
Anyone have any suggestions......?

[chrisBoid]

With regards to your speedo query, depending on the year/model of your FD, it would have come from the factory with 16" or 17" wheels, don\'t know the factory tyre specification though.

Now you have 18\'s you can bet that your speedo is out by probably at least single digit % accuracy.

For example, I went from: -

FRONT 17"x9" rim with 235mmx45% tyre
REAR 17"x10" rim with 255mmx40% tyre

to

FRONT 18"x9" rim with 245mmx35% tyre
REAR 18"x9" rim with 265mmx35% tyre

So as I increased the rim size by 1 inch I tried to reduce the tyre wall height as much as possible so the overall diameter of the wheel stayed the same.  I think I managed to get within 2% of the original setup.  Note that I am also running wide arches hence the wider tyre widths in my new setup.

As for clearence, that depends as much on the offsets of your rims as the ride height.

The ideal setup is 235mm on the front and 255 on the rear.
Depends how much you are botherd about speedo inaccuracy, if not then stick your existing F1\'s on the front and get some 255\'s for the rears.
If you are bothered about accuracy, plus your lowered height, you might want to look at lower profile tyres - but that increases ride harshness.  When I went to lower profile, I ended up buying better shock absorbers as well.

Chris.

[just7]

Thanks Chris,

At present i\'ve got the standard 16" mazda wheels on.
225 x 50 R16 on all 4 wheels. It\'s a UK spec FD but, i\'m thinking it would be best to go with the best tyre set-up rather than accuracy. So i think i\'m gonna go with what you recommended
235 x 40s on the front 18s & 255 x 40s on the rear 18s (as all 4 wheels are 18"x 8.5" wide does it matter having wider rear tyres on the same width wheels)?

[chrisBoid]

Did you mean you\'d go with: -

FRONT 18x8.5 with 235 x 45 <-- you mentioned 40 here.
REAR 18x8.5 with 255 x 40.

That recommended tyre setup is for 17" wheels.

With regards to what tyre widths you can fit on what rims, there is certainly a window of tyres sizes that will fit a particular width rim.  I\'m running 9" width rims all round and have 245mm on the front and 265mm width tyres on the rear.

Just found this great online calculator: -

http://www.rochfordtyres.co.uk/TyreCalc.asp

This states that a 255mm width tyre is safe to fit to a 8.5" through to 10" rim.

Wouldn\'t rely 100% on this but it is probably a fair calculator.

If you talk to the drift boys, they\'ll be stretching these limits to there maximum, so there is always leeway.

Check this thread: -

http://www.mazdarotaryclub.com/forums/showthread.php?s=&threadid=279

Has some suggested tyre specs for 18\'s about half way down the first page.