speedo calibration calculator?

I'm sure there's got to be one somewhere outthere - I'm hoping someone has it bookmarked.

I recently swapped from 3.55 to 4.10's in my XJ and have some relatively small tires for diagnosing vibes and trying different things (removing 1 shaft at a time, etc).

Anyway, the federales around here are on a tear these days ticketing everyone for anything and I don't want to get nicked for speeding cuz I don't know what my actual speed is.

Seat of the pants tells me I'm going way slower than the speedo indicates but I'd like to have a better idea than seat of the pants. Can anyone point me to a good calculator where I can put in my old gear ratios and new gear ratios and tire size?

Thanks

www.go.jeep-xj.info/ Scroll through he has a speedo calibrator thign on there.

tag for reference

Old tire diameter/ New tire diameter X New gear ratio/ Old gear ratio X old tooth count = New tooth count.

I think factory tooth is 34 with 3.55 gears