Change tyre size/speedo error

Why not just use the one published on ExplorOz: Tyre Size Calculator

Just use a GPS to give you your speed, If you have one !

60, 80, and 100 km/hr, and check your speedo, I have done this to several of my mates vehicles, and the problem is solved

Cheers
Bucky

Definitely using a GPS is the way to go.

The problem with ALL the tyre size calculators out there is this - not all tyre manufacturers make tyres that match the specifications printed on the sidewall.

For example, a 285/75R16 'should' have a diameter of 833.9mm if you calculate it using the tyre section, sidewall and rim diameter figures. Unfortunately, you will find that most manufacturers do not stick to this, and most 285/75R16s (with the exceptions of Coopers and Mickey T - and maybe a couple of others) have their 285 tyres usually with a diameter of 840+mm, with BFG having their AT at 852mm!!! (I.e. not even close....)

That results in a sigfnificantly different speed reading compared to what the tyre calculator suggests. This is the case for most tyre sizes, and when the diameter is out, the circumference, etc. are all out as well (as are the comparisons between tyre sizes).

Use a GPS or get the manufaturers specifications for the specific tyre you're using (or wanting to buy) and do your own calculations from there if you want to be sure you've got it right.