Which speedo gear?

Hi,
anyone knows what speedo gear (how many teeth) I need to correct my speedometer when swapping to 30" or 31" tyres? My axles got 1:3.55 gears and my cherokee is a 1989 Laredo 4.0litre automatic with selectrac (NP242).

A link to a chart with different axle gears and tyre sizes would be great, in case I change something later.

-Simen

http://go.jeep-xj.info/HowtoSpeedoGears.htm

I used his chart when I went to a 245/75/R16 (30.5). I got the 32 tooth speedo gear (part number 52067632), as my next tire will be a 31 x 10.5. I haven't checked it against a GPS yet. This is on a 2000 XJ with the 242 transfer case.