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[Lesopc]

Me again, bet you are all bored silly by now?
Would the Ronal alloy wheels i mentioned earlier (R44 in size 7.0x17" and the R39 in size 8.0x18") fit straight onto a miggy or would i need to make some adjustments?
Also would there be any effect on speedo reading?

[2woody]

speedo reading depends on the tyre size.

[Lesopc]

speedo reading depends on the tyre size.

225x45x18

[Kevin Wood]

Be aware that you need to make sure the stud pattern and offset are compatible too.

Kevin

[Lesopc]

Be aware that you need to make sure the stud pattern and offset are compatible too.

Kevin

Already ahead of you on this one.

[Kevin Wood]

Be aware that you need to make sure the stud pattern and offset are compatible too.

Kevin

Already ahead of you on this one.

Just making sure.

kevin

[Liam]

Standard sizes are 205/65 15 and 225/55 16 for Elite. Working it out, that means Elite has 1% bigger diameter so the speedo would be that amount different between standard and elite - not sure if Vx would have bothered gearing the elite speedos differently to compensate for this.

Your suggested 225/45 18 would be slightly bigger still - 0.9% bigger than Elite or 1.8% bigger than standard. Could also consider 225/40 18 which would be a little smaller than the standard sizes - 1.6% smaller than standard (so actually closer to standard than the 45's would be), but 2.6% smaller than Elite size.

Wouldn't you want something fatter, though, for your racer replica/lotus tribute? 245/40 18 would be spot on Elite size. I think the Lotus Carlton even has 265 on the rear!

[Lesopc]

Standard sizes are 205/65 15 and 225/55 16 for Elite. Working it out, that means Elite has 1% bigger diameter so the speedo would be that amount different between standard and elite - not sure if Vx would have bothered gearing the elite speedos differently to compensate for this.

Your suggested 225/45 18 would be slightly bigger still - 0.9% bigger than Elite or 1.8% bigger than standard. Could also consider 225/40 18 which would be a little smaller than the standard sizes - 1.6% smaller than standard (so actually closer to standard than the 45's would be), but 2.6% smaller than Elite size.

Wouldn't you want something fatter, though, for your racer replica/lotus tribute? 245/40 18 would be spot on Elite size. I think the Lotus Carlton even has 265 on the rear!

Yeah, still a lot of homework to do on this.

[2woody]

225/45 x 18 are quite big (diameter)

compared to 205/65 x 15, there's 6.1% difference
compared to 225/55 x 16, there's 5.1 % difference

these figures are very large, so I wouldn't advise fitting these - you may not even get front tyre to strut clearance on these.

for reference,

205/65 x 15 = 817 wrpm
225/55 x 16 = 811 wrpm
225/45 x 18 = 770 wrpm

[Liam]

Maths error somewhere there, 2woody. Unless I made one - but that's imposible . 225/45 would be a good choice for 18's to replace the standard wheels - see my calcs above.

[aaronjb]

Yeah, the miata.net tyre calculator comes out with different numbers: http://www.miata.net/garage/tirecalc.html

[2woody]

using these tyre calculators and trying to do it with geometry causes WRONG results.

the automotive industry uses wheel revs per mile, which is a measured property of a tyre, taking into account the static sag in the laden radius and the forward motion slip.

it's a property that can only be measured and not calculated.

you need to get the figures from a tyre company - as I did. Check out bridgestonetyre website for example.

[2woody]

Maths error somewhere there, 2woody. Unless I made one - but that's imposible . 225/45 would be a good choice for 18's to replace the standard wheels - see my calcs above.

just out of interest - how have you done this ? - have you used (225mm x 45%) + (225mm x 45%) + 18inch

have just been back to work out the error.

205/65 x 15 calculated wheel revs per mile = 791. Actual wheel revs per mile = 817
225/55 x 16 calculated wheel revs per mile = 783. Actual wheel revs per mile = 811
245/45 x 18 calculated wheel revs per mile = 776. Actual wheel revs per mile = 770

you're right in that the difference between, say 205/65 x 15 and 225/55 x 16 is 1% in diameter just considering the tyre on the floor on its own, but adding wheel weight and slip gives a different answer. Precisely why the industry uses wrpm.

[Liam]

Oh - you've done it properly!:) I just worked out diameters and am surprised the 18" is so much further away from the 15" and 16" when worked out 'properly' than when worked out the simplified way! What info did you use out if interest - couldn't find anything on the bridgestone site? I guess it's due to the increased rigidity of the 18" wheel/tyre? Interesting.

[2woody]

oddly enough, I've never really considered this,I suppose that the narrower sidewall flexes less and therefore causes less tyre slip.