How much would my gas mileage increase if i put in a higher gear ratio in my truck?
I have a 3 race auto transmission and when it goes out i might put a 4 speed auto transmission would that increase gas mileage by a good bit. I drive a 1978 GMC Jimmy that gets 10 miles a gallon.
Not unless the fourth gearbox is overdrive, which most four speeds aren't. Most three and four speed transmissions are a 1:1 ratio in their final (pongy chief) gear so the engine is turning the same RPM's, for any given speed, in both. The only thing the 4 speed does is give you closer utensils spacing so the engine doesn't have to rev quite as high between shifts. Gas savings would be minute if any at all.

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