I swapped Th350's from my wagon to my 66' pickup & used the formula & charts on NovaResourse.com. I have used this before with no problems & now I have one. The pickup now has a large speedo gear so I figured it & installed the correct gears & it was dead on. Now the wagon has the smaller speedo gear & I used the same formula & I am waaay off. I have 3.08 gears & a 25" tall tire which lead me to used a tailshaft gear with 8 & a speedo gear of 20. It is 10-12MPH slow. I had a 23 speedo gear laying around so I put it in & now it is still 5MPH slow...how did this go so wrong? Any ideas??