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Trans snd rear combinations

Got a 99 freightliner with a 500 detroit and 10 speed direct with 390 rears and 24.5 low pros .i wanna lower my rpms for better fuel mileage and wondering what is the best lower rear ratio i can run.i pull light and heavy but mostly on flat roads.

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I agree with Kenneth, you don't have a direct drive transmission, you have an over drive 10 spd. 9th gear is direct and 10th gear is over drive. It really depends on your top speed that you want to run. The only way to really lower your rpms is to change your rear end ratio. With an over drive transmission like yours this is what I recommend:

55mph 3:90

60mph: 3:73

65mph: 3:55

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You wouldn't be going very fast with a 10 direct and 3;90 gears . Are you turning 1950 rpm at 60 mph? A 10 over drive with .73 or.74 overdrive is what you probably have. A 10 direct transmission with 2;79 rear ratio is a perfect 58-62 mh truck. A 10 overdrive with 3;90 gears is almost the same overall ratio. That detroit will be happiest at about 1450-1500 rpm.

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