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Hi i have a 1.2 16v clio 02 plate and was wondering if i put larger alloys on my car having the 14" standard alloys at the moment will this affect the way the speedo works by giving false readings
It's all to do with rolling radius. You need to pay attention to the diameter of the wheel and the aspect ratio of the tyre. (The small number after the width of the tyre)
As an example. Say you have 195/60/14s. That means the tyre is 195mm wide, with the sidewall being 60% of the width, running on a 14 inch rim. Usually, if you go up 1 inch on wheel diameter, you go up 10mm on with and come down 5% on the aspect ratio. So, you'd end up with 205/55/15 or 215/50/16 and so on.
Using old school mathematics, that should make the circumference of the tyres about 1.5 to 2 inches more at most.
So they'll go from being about 53 to about 55 inches in circumference. (I think, but I have been drinking!)
So I wouldn't worry too much about speeding. I think most cars' speedo's under-indicate the speed you're doing anyway, so as long as you're sat right on the speed limit or less when passing a camera / sleeping policeman you shouldn't have any issues.
If you're that concerned about the speed you could always get a sat nav that indicates your speed measured by GPS. I think that's a bit more accurate than your car's speedo, although it's still not 100% perfect.