You got good advice there. The 700R4 has a 30% overdrive so with 3.90 gears you have a 2.73 final gearing when in OD. Can't stand much shallower gearing than 2.73 if your ever going to have any load or pull a trailer. A vacuum gauge is great for a tell tale on gearing. If it drops to under 15 or even 16 then your geared too shallow and costing yourself fuel. If your in the 18 to 20 range your getting the most out of your set up.
