bigbadbob76 wrote:Getting back to true rolling radius vs calculated tyre diameter...
This would give me slightly lower gearing than the maths says.
Which might account for my tacho appearing to read slightly high at 70MPH. (4kRPM vs 3800RPM.)
I'm still not going to worry about it but just thought I'd throw it in there.
Vehicle gauges are not laboratory instruments.
I was wondering about this. I'm not sure that the rolling radius, which is smaller because the vehicle weight compresses the sidewall, actually changes the circumference, i.e. the distance all the way round the tread pattern. The tread pattern would have to compress or stretch for the circumference to change, and I'm not sure it does. Imagine that tread detached completely from the sidewall - its circumference would stay the same regardless of the shape. I could be quite wrong of course.