changing speed-o on my suzuki sierra due 2 bigger tyers.?

[emjme]

i have a 1992 suzuki sierra with 30" mud's on it and was wondering if any1 could tell me how 2 make my speed-o run as normal as it is about 15 km/p slower than it should be.

 

[Angus L]

it can be calibrated by a suzuki dealer.