View Full Version : Wheel/Tyre Question
Wul
25th March 2005, 03:30 AM
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
25th March 2005, 04:32 AM
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
25th March 2005, 04:36 AM
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
25th March 2005, 04:38 AM
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
25th March 2005, 04:53 AM
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
25th March 2005, 05:15 AM
http://www.miata.net/garage/tirecalc.html - gives a value of 3.3%. Useful site for checking though
Wul
25th March 2005, 04:33 PM
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
29th April 2005, 07:32 PM
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
29th April 2005, 08:20 PM
coughgetalifecough :D
Mini Me
1st May 2005, 04:41 AM
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 !
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