MickeyP, I think Jerry (Hotrod Lincoln)
gives a better explanation of what needs to happen and why when checking the axle ratio this way.
If you're only jacking up one wheel, the action of the spider gears doubles wheel speed. You'll have to make two turns of the wheel to get an accurate estimate of the ratio. For instance, if the driveshaft makes just over 4 turns to get the wheel to turn twice, You've got a 4.10:1 ratio gearset.
A more accurate way is to make a chalk mark on the driveshaft, and roll the truck straight forward one full turn of the rear wheels, counting driveshaft turns as you go. Since both rear wheels are on the ground, the speed-doubling function of the spider gears doesn't become a factor. That's why we never exceeded 30 MPH when doing on-car wheel balancing. The actual wheel speed was 60.
Jerry
On the corrective action, others will most likely chime in here soon. But for my $.02 worth, you will probably need to get an adapter mounted to the transmission speedo drive output to get your speedo reading more accurately.
Edit: hcb3200 and Joe H beat me to the post button!
