I see a lot of people talking about 3.84 ratios and that they get better gas mileage than other gears. What ratios are they talking about and how can I get that on my truck for better gas mileage.

 

Huh? Changing gears is a matter of physically changing the gear ratio. 3.84? There is no **magic** gear ratio.

Who are these people you see talking???

But mileage is a combination of different factors...rear gear ratio, transmission ratios, tire height, rolling resistance, aerodynamics, engine efficiency, driving style, etc.

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No factory 3.84 ratio in a Ford 9-inch. There were various 9-inch ratios available from the factory during the 9-inch rear end's production run (1957-1980 in the Ford cars and 1957-1986 in the Ford trucks).

There were more ratios than this but, some of the main factory 9-inch ratios were: 2.47, 2.50, 2.75, 3.00, 3.25, 3.50, 3.70, 3.89, 3.91, 4.11, 4.30 and 4.57:1.

A high ratio gear is low, numerically. A low ratio gear is high, numerically. The lower (numerically higher) the rear end gear ratio, the faster the engine will spin at a given speed and the more fuel it will consume at the same speed.

If you have an FE engine with a carburetor, it isn't going to win any mileage awards no matter what gears you have. A ratio change might help a little but don't expect big MPG gains for a change to higher gears (lower, numerically). The biggest thing that it would change is the noise level inside the cab at highway speeds.

The biggest things that will help with fuel mileage is optimal rear end gearing, fuel injection and an overdrive transmission. It just depends on how much you want to spend for these.

 

AERODYNAMICS!!!! are the biggest factor...