Never owned, driven, or rode in one but I have talked to a guy with a 4.10 geared auto Mach before. He said his mileage went down some, but the difference in acceleration was well worth the a couple MPG's he lost. I saw the car in action and from a dig or low roll it moved very well and looked pretty quick.
I dont know if auto or man trans makes a diff with 4.10 gears but i heard the gears do drop gas mpg and then i heard it doesnt. But maybe this mite help.
Love my gears. Gears, converter, DR's made for a pretty quick mach for me. I recently switched to nitto Invo tires so the HOOK isnt there like b4, but I will be going back to dr's soon. MPG didnt really change much and the car accelerates easier with the gears. At 80mph, Im at 3k rpms in OD.