adair69

Ok technical people

If I increase the overall diameter of my tyres from:

634mm TO 647mm

What difference would this make to the speed, and obviously the mileage?
My guess and common sense dictates bigger rotation less distance and speed, probably -2%?? But is this accurate i.e. my interpretation not the fact that the speedo is right in the first place.

Your response appreciated
Thanks

Devil's Refugee

I think this affects both Speedo readout and gearing too.
Why would you want to increase the diameter ?

adair69

Unfortunately a consequence of going from:
225.45.17 To 235.35.19 increases the overall diameter from 634mm to 647mm.

But they'll still lose 20kg off the unsprung weight of the car!!

Glenn Coombs

I think that your speedo will now underread by 2% compared to how it was previously. Same for odometer. But a 2% error is not worth worrying about. If your speedo was totally accurate before then you you would see an indicated speed of 70mph when you were actually doing 71.5 mph. And for the odometer, an annual mileage of 20,000 miles instead of 21,400 miles.

tinvek

your corect about the effect on your speedo but on the other hand most speedo's over read as the band of tolerance allowed for the munufactures gives them 2.5% over read but 0% under read so they all aim for an over read. logically you might find your speedo is actually more acurate

the next bit isn't too serious

while your worrying about it add this in. as you go faster your tyres expand due to cetrifugal force so your gearing actually rises at very high speed any way its quite posible that your new 19 inch rubber bands will "grow" less at high speed and so in fact at high speed your gearing could be unchanged

dragsters use tyre growth as a way of raising the gearing at high speed and when porsche were racing the 917 at le mans in 1970 a driver was asked what revs he was pulling down the straight. after the driver gave his answer the team manager got his slide rule out and after adding the tyre growth at speed to the gearing whislted and turned away saying the driver didn't want to know. when pushed he said the speed was 245 mph which was 15mph higher than the theoretical maximum in top gear at the red line. admiadly that was on crossplys(!) but the principal still holds with radials

adair69

mmmmmm That really makes food for thought! Maybe I should take out the arch lining and stick in some seriously huge Bands, or maybe not eh.
Great Story

Thanks

Andy McCord

So having swopped my 205x50 R16 to a 205x45 R16 what affect have i now got as my speedo over reads badly now i.e passing hardly no one on the m6 showing 100mph?

tinvek

quick and dirty mental calc - you've reduced the diameter and there for the circumfrence of the tyres by about 4%. therefore you've put a 4% overread in your speedo

Andy W

http://www.miata.net/garage/tirecalc.html

redwrx

A while ago, i remember trying to work this lot out ..... what happens if you change wheel and tyre..etc?

I was going from 195/60/14 to 205/40/17 ...

inshort i think if you increase the rolling diameter, you will travel further per revolution...but at the same time recording less distance, on a downside you will lose some acceleration.

On the other hand reducing the tyre wall height increases stabilty and therefore improves handling.



I've stuck with the standard 16's

Frank

Andy W

Your speedo should be reading 2% too slow, so if your speedo was 100% accurate to begin with at an indicated 60mph you would actually be travelling at 61mph, given your speedo probably overreads anyway so no big deal!

harvey

iTrader: (48)

The important thing is the tyre circumference.
Go to a deserted supermarket car park or similar,chalk the bottom of the tyre and road surface, move forward in a straight line perhaps along a painted line for say 10 or 15 wheel revolutions , measure distance and divide to get accurate rolling circumference with no tyre growth. Adirect comparison of circumferences will give an accurate %age change.
Now 634mm dia =1.991m and647mm =2.032m Bigger wheels cause the speedo to under read current values and it is likely to exagerate by 6-11% at present.
Going as described above will result in a lesser reading of 2.o6%.
In other words the speedo is slightly more accurate.

harvey

iTrader: (48)

P.S. Double check your calculated weight saving. A standard 225x45x17" is 18kg. When I went from standard 16" to 17" Superleggeras the overall saving was from memory 3kg total on new tyres v/v part worn 16"

adair69

Thanks everyone

In respect of the weight saving, The standard alloys with RE040 weighed in at an incredible 18kg!! a piece. The Superlegs' at 2" Diameter more, with wider thinner rubber bands on come in at a combined 12kg!! Thats with more metal and more rubber, I haven't parted the standard Alloys from the tyres yet but a rough guess is they are about 15kilos or more on there own,probably nearer 16kg's.
Now maths was never my strong point but that makes about 72kg-48kg = a saving of 24kg, approx' a third unsprung weight saving. Its not a huge saving, of maybe as much as I could have got from pure forged or magnesium, but lets remember these are very sexy 19" Gold Italian Alloys... mmmm .... can't wait to see them on, but just have to sort out a power steering gripe.
Anyway Thanks Again all for your feedback