13" wheels and ratio effects

Post by **polo2k** » Mon Jul 07, 2008 10:32 am

Hi all, On Mona I have 13" A40 wheels. Ive not driven her much (not at all with a decent bottom end) but the A40 wheels are smaller diameter so give closer gearing. I want Mona to be fairly capeable of modern motorway speeds but the lower ratio is going to make this less achiveable. The speedo is a different one suited to the A40 wheels.

Is there a Diff available to restore/improve the ratio? 4.22? what would I need to do speedo wise?

If it helps, im told that mona is "healthy" on power for a 1098cc.

Ive asked before about overdrive and it seems to be more hastle than its worth. Is there a modern alternative? i.e. a modern coupling (viscous possibly?) that could be mounted to the input to the diff and offer reduction/increce in ratios. kind of an inline gearbox I suppose. Would need to be lightweight so as not to increce unsprung axle weight.

Post by **Willie** » Mon Jul 07, 2008 11:17 am

The revs per mile for the standard crossply 5.20-14 = 865.
.................................... usual radials 145--14 = 892
I don't know which size of radial you could safely fit to your wheels but
these are the differences.

Revs per mile for 145X13 =934,155X13=913,165X13=892.

Post by **bmcecosse** » Tue Jul 08, 2008 11:21 am

The 165's would be very wide on the 13" wheels - better to look for decent 14" or even better 15" wheels. Diffs at 3.9 and 3.7 are available - but often expensive and hard to justify with the supposed fuel saving. However - they could be considered 'investment' - as long as they don't go wrong the value will only increase with time !