I know in some GM models, if you had a rear axle ratio that differed significantly, they had different transmission drive gears. I think the T-5's have different drive gears, something like 6 tooth vs 7 tooth. The plastic driven gears will adjust the ratio about 4.5% to 5.5% per tooth change. So your 4 tooth change for a 20% correction sounds about right. But if you made a large change in the rear axle ratio and and or tire sizes, you may flat out be out of the range of adjustment of the plastic driven gears. You may need to find out if a different drive gear is available and crack open the transmission to put on the different gear on the shaft.
Another alternative is that some companies make speedometer correction gear units. I have never used one, but it changes the ratio that it spins the speedo cable.
Doug
