9j's and tyre stretching

[323iNathan]

I'm looking into going 9" wide at the rear on 16's, but was wondering...

does running tyres with small external diameters on your car throw the speedo out by much?

Some of you are rocking 205/40 16's which have an external diameter of 571mm, and the stock 195/65 14's have an external diameter of 610mm. So going by that fact means that at an indicated 60mph you're actually going 56mph, and the speedos aren't accurate from the factory, they always read a bit on the safe side, so you're more than likely actually going about 52mph. 8mph is a huge difference, does stretching the tyre somehow counter this (even though I would've thought it would only exaggerate it)?

Btw I used this tyre size calculator for the measurements:
http://www.miata.net/garage/tirecalc.html

Also, how does a stretched tyre handle compared with a tyre that is the 'correct' size for the rim its going on?

Thanks

[suchy]

I'm running 9j's on the rear but only stretching 225/45's onto them- handling is fine & I think the rolling radius is pretty much correct (hence choice of tyre!)

[R4N_S_S]

sorry but stretched tyres look shit

[mr_dink]

You're quite right about the speedo being out.

A popular guesstimate is that they are around 10% under from the factory.

I'm running 195/50 15s where the stock size is 205/55/15.

This gives a diameter of 576mm instead of 606.5.

The calculator you linked to says this makes my speedo overread by 5%.

It's more than that though.

I've "calibrated" my speedo using some of those signs that flash up your speed in a 30 limit.

When the sign says I'm doing 30, speedo reads 35.

HTH

[HairyScreech]

a better choice for 15" wheels is 205/50/15, this makes you over read by an extra 3.46% (and lowers the car by 10mm so be cairful on sump height) meaning an extra 1.96mph under every 50mph.

assuming 10% over read anyway that leaves you doing 43mph at 50. although i think they are better that 10% accuracy.

as for 205/50/16s tyre size calculator actually says they are 4mm larger than standard and will cause 0.6% under read, obviouly with regard to current accuracy.

edit, just seen it was 205/40 not 50.

205/40 is an extra 6% over on the speedo, thats a bit too much.
go for a 225/40 at 3.4% over
a 215/45 would be best at 1.1% over
or 225/45 at 0.33% under
if such tyres exist for sensible prices.

[Felix79]

I would rather it show me doing a higher mph than I am actually doing to be ultra coutious.