that stop watch method is pretty acurate,i checked my speedometer that way last weekend it showed that i was going faster than the speedometer showed 4-5 mph, my daughter has a garmin gps it showed 5 mph to. recalebrated the swith in the speedometer head now it and the old ometer is right on with the gps.it should gain 10 miles in 120 miles if i figured the oldometer right, better mpg.the speed limit on interstate 45 is 75 ,2 weeks ago i was data logging got up to 78 for a while which was acully 83 and there was still some people passing me they must of been going 95-100 as fast as they went by.