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		 Idiots - the speedo will effectively be reading wrong with bigger tyres. 
 
The speedo calibration is based on the circumference of the tyres - bigger tyres go further for each revolution. 
 
Therefore, with a slightly bigger tyre (say 5% bigger), with the speedo registering 50 mph, you'll actually be doing 52.5 mph.  Which isn't a big difference, but I know that some of the police forces (farces?) in the UK will nail you for speeding with that sort of difference .... and possibly do you for driving with a car thats dangerous/not legal/something else. 
 
DEI37 - thanks about the 75 being the percentage, I was 99% certain of that. 
		
		
		
		
		
		
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