Motorcycle Questions? Ask a Mechanic.
you have 2 routes
either lower the rear axle gear ratio
or tune the engine up or fit something more powerfule
by far the easiest method is to reduce the rear axle ratio
The diff will most likely only be either 4.7:1 Series diff or a standard 3.54:1 and you should be able to tell the difference between them using the propshaft revolutions as 10 wheel revs will either produce 35 or 47 prop revs.Counting the pinion and crownwheel teeth is of course the "correct" way but unless you have a "special" there are only two factory diff ratios and the vast majority of diffs out there will be the 3.54:1 which is what has been fitted in all 90/110/Defender, RR and Discovery vehicles.
I saw the 'defender' title and assumed it was a land rover
I'll opt out so another expert can help
you have to request the refund yourself