what is the best rearend gear ratio for my truck that is going to have more than likely a hopped up 390 in it, the guy that had it before me put a low gear rearend in it to pull wagons to town on the farm, the truck is a 1975 f150 has a shot 360 in it right now, i want to race with it is what im getting at, i hate people that think there chebby will smoke me., is a low geared rearend better or a higher one for racing with a hopped up 390 in it.
i have been a fan of 3.50 gear ratio or so. Its not to high or to low. If you get 4.10s you will have no top end and 2.75s you have little acceleration. If your 390 makes good power and have small tires you could get away with 3.25s.
very good advice so far, I'd like to add that the best gear ratio depends on how much "race" and how little "street" you are willing to put up with. The size of you tire will effect the rearend ratio as was mentioned, if you have large tires on your truck then you need an even lower (numerically higher) gear. If you have 31" or larger tires, it will be a dog with anything less than a 4.10. Might not be a dog, but it won't accelerate like you'll wish it did. For racing, the lower the gear you can use, the better. For street, the ideal gear for fuel economy with a 3 speed automatic trans is around 3.0, but this number works for 29" tires (255/70/R15). Anything taller would require at least a 3.25 to acheive optimum efficiency.
Keep in mind also that if you want a ratio any larger than a 4.56, you will have to use a "Daytona" type pinion carrier if you have a 9" rear. The super-low (numerically high) gears have so many teeth on the pinion gear that it will not fit in the carrier.
John F. Daly III
'77 F-100 LWB, 100% Aftermarket 302-R.I.P. (Stolen)
'95 F-150 SWB, 300 I-6 sold @ 331,000 miles!
'96 F-150 SWB, 300 I-6, Eddie Bauer and 5 gears to row (current rig)
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