Wheel/Tyre Question

[Wul]

Ok folks - get your thinking caps on please!

I've gone from 205/45 17 tyres to 205/40 17. This means my rolling diameter has reduced, meaning the circumference has reduced meaning the acceleration has got more responsive.

However - this also means my speedo now reads a faster speed than i'm actually travelling at!

So what percentage is my speedo now out by? My calcs equated to 3.3% but it seems more to me!

[mgemie]

i've no clue to be honest..i suppose i'd test it with a highly accurate gps device with surface speed... does this work in reverse - i.e. if one has 18" meaning a larger circumference does ur speedo read slower than ur actual?

[mgemie]

in otherwords the bigger the wheels (thus tyres) u have, the slower ur speedo shows so the more chance there is of going over speed limits meaning more probability of having a chat with mr plod?

[Neil and Lorna]

Sorry folks I was of school the day we did physics.:p

But i would be very interested in any findings.:)

I have 18" jcw wheels.Hmmmmmmmmm.:cool:

Cheers Neil.

[Scottie]

ok done some number crunching and get it to work out @

3.42176% increase @ 60 KPH which = 2.053 KPH faster @ 60 KPH.

to work out the rolling diameter to make it easy convert inch to mm
take the radius r=(pye)r2 to give the rolling diameter you do this for both the 40 and 45 walled tyres once calculations are complete you can then work the % of increase.

[MartinSullivan]

http://www.miata.net/garage/tirecalc.html - gives a value of 3.3%. Useful site for checking though

[Wul]

Wahey - so my maths wasn't wrong afterall!! :D

Mgemie - you are getting confused mate - the key figure is the sidewall aspect number ie 205/45

The original wheels were 205/45/17 and now they are 205/40/17 if you have gone up to an 18" wheel you will prob have 225/35/18 tyres - thus the rolling diameter of your wheel goes unchanged!

[red_cooper]

the speedo will be out by up to 10%, so the excuse my speedo readys out, doesnt actually work when plod pulls u over the manufacturers set them to read about 65 66 mph when truley doing 60 mph, read auto car testings of cars they show using a fancy 10grand system. i know mine reads over 130 flat out and im pretty sure she's not doing 130. wasnt on public highway ull understand all testing was tried on a two mile runway!

but top whack is quoted as 124 if im not mistaken, my last car had a claimed topspeed of 116 but i had it reading 130 a few times! it did more than 116 in 4th gear!! so wouldnt worry bout ur speedo reading!!

[Big Col]

coughgetalifecough :D

[Mini Me]

It's times like these i'm glad we have a service dept, cos if a customer asked me that, he'd get swiftly slapped with a rolled up(damp)brochure for asking.....:eek::blackeye::p:evil:

<from behind of course, in fact i'd get Clare to do it>:eek::blackeye:

ffaaaaarr to technical for a sales monkey !