http://www.tyresizecalculator.com/tyre- ... dimensionsTrainman wrote:OK the tyres fitted to the car when I bought it were 195/65R15 the current wheels are 205/50 17

For the first tyre, rolling circumference 1994mm and 502 revolutions per km, for the second one 2000mm, with 500 revolutions per km. That's a 0.4% difference which is completely negligible, as up to a few percent is allowable.