After getting ticketed with a new to me Chevy S 10 with 235 75 R 15 tires ... stock are 205 75 R 15 ... I checked one of the tires sites and found that the 235 tires have a circumference 6.02% larger .... I then bought a TomTom and found that when traveling an indicated (speedometer) reading of 100 KPH the TOMTOM indicated 106 KPH ... I am currently home to Nova Scotia for the summer and am using that TomTom on my Bertone .... it is telling me that at 100 kph the speedo reading is actually about 103 kph.. which bears out your point about the factory having the speedo reading slightly high. I am planning on replacing the tires on the S 10 with 235 65 R 15 tires .... but apparently this size is only manufactured by GOODYEAR and was stock on the DAKOTA SPORT pickup. However, they are the same circumference as the stock 205 75 R 15 tires that came from the factory .... the circumference of the 65 series tire is approximately 6% smaller than the 75 series ... which makes sense and should bring the speedo back to what it was when it left the factory.

Ole Reliable is wearing 205 75 R 14 tires and the circumference worked out to within 2 revolutions per mile of the stock 6.00 15 tires.
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Brett Sutherland & the 1.5 million mile 122 CANADIAN --- WINDSOR, Nova Scotia the birthplace of HOCKEY www.ecvintagevolvo.com