Tyre changes & speedo inaccuracies - I understand that if i change the tyres and their profile on my car there will be a change in the accuracy of the speedo....
In message , MrJolly writes
Yes.
Well done - I couldn't have answered that without at least a spot of sarcasm.
In message , DougP writes
I can do sarcasm when the mood takes me but I've just put away a bottle of Muscadet and at the moment I'm concentrating on spelling all the words correctly and putting them in the correct order ;)
Thanks Paul and Doug for the straight replies - at least it proves that i still have one or two brain cells left working! Happy Easter!
Yeah. Distance overall as well as distance per unit time will vary by the same percentage. You'll also affect:
(larger wheels)
- Gear ratios will fall, meaning worse acceleration coupled with better mpg in top gear
- larger rolling radius will lift the car higher off the ground, increasing the likelihood of it flipping over backwards at motorway speeds, or toppling over when going round corners on the limit / striking kerbs sideways.
- if you lower and stiffen the springs to compensate, the various suspension arms will be pointing up at the end of it, rather than horizontally, which reduces their ability to supervise bumps cleanly.
(smaller wheels)
The reverse of the above, except that you wouldn't need to change the suspension geometry because you would sit lower, which carries with it the fact that you'll have less ground clearance.
In either case, you can leave the ground clearance the same and also have the suspension arms horizontal, if you modify the floorpan appropriately.
Most of this will be fairly small if the change in rolling radius is small, e.g. 2.4% is nothing to worry about as the tyres will wear that sort of amount through their life, and pumping up or down a couple of PSI would be in the same ballpark, I think.
10% or 20% would be rather less irrelevant.
I find tequila most conducive to huge sarcasm. :)
With sufficient tequila it's not a worry...
A
Doesn't really matter - you're speedo isn't exactly "accurate" anyway. Putting some bigger, lower profiled wheels on will more than likely make it /more/ accurate.
My mate got done for speeding due to larger rolling radius! Mentioned it in court to no avail (no supprise there!) It is interesting though, as there is no set test for the accuracy of the speedo at the mot, except for the sva test which is for kit cars and imports.
No he didn't, but I'm sure it sounds good down the pub.
No surprise that a court isn't stupid ? Must admit actually, yea, that is a bit surprising.
You'd have to change the wheels by a *huge* margin and keep the same profile tyre to get more than a couple of percent change.
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