Speedometer ratio

[Big Dutch]

How do I calculate the correct ratio off for a ratio adaptor using the speedometer and a gps without knowing the ratio or tire size?

Where can find a calculator to do this?

I shouldn't need any more info., but no one has this.

Let me give you my problem. The speedometer reads 96 and I am going 60 using 2 gps units to check the speed. What percentage ratio adaptor do I need?

I know that the driven gears do not give me enough play and it would be cheaper to buy a ratio adaptor than play with the gears and gear housings, which would also have to be changed.

Total for driven gears and gear housings playing around would be $120 and probably not correct. Ratio adaptor is $70.

I'm not a junkyard person and would like to buy online.

Thanks, Big Dutch