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Discussion Starter #1
trying to decide on gear ratio on my 69 mach1 has 3.50 posi like to go to 3.73 or 3.91 with automantic used to have the original wheels on but changed to 15'' would 3.91 be to much for street driving not much highway and some 1\4 mile runs cars has 408 stroker which has a lot of torque. any ideas.
 

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How tall of a tire will you be running? 3.50 gears aren't bad, especially if you're making a lot of torque with a stroker motor. More torque doesn't require as much gear. Personally I'd say with a 28" tall rear tire, the 3.50 or 3.70 gears are adequate for mileage, as well as acceleration. Plus I assume you're running a 9", so you can swap out the pumpkin for your 4.11 or 4.56 gears on race day
 

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I have 3.89s in my Torino, it turns 3k @60 on 26.5 tires, I am going to 28s real soon, I can only do about 110-115.
 

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Your 265/60/15 tires are 28" tall. With 3.50 gearing, you're turning 2100 rpm @ 50 mph, 2520 rpm @ 60 mph, or 2940 rpm at 70.

With 3.73 gearing you'd turn 2238, 2686, or 3133 rpm at 50, 60, & 70 mph respectively.

With 3.91 gearing you'd turn 2346, 2815, or 3284 rpm at 50, 60, & 70 mph respectively.

With any shorter (lower) gearing than you have now, you'll get a lot of noise, extra engine wear, and poor mileage on the street. You just don't need those lower gears on the street with that long-arm, big-torque stroker, and you'd also have a tough time hooking up on street tires with the lower gearing unless you use drag radials. So 3.73/3.91 is not too desireable for street.

On the other hand, the 3.73/3.91 would be okay for drag racing with slicks, but still not optimum. Depending on how tight your motor is set up to wind, you'd probably need 4.10 - 4.56 to touch red line in the lights.

If it were mine, I'd leave the 3.50 in it for street (or even go to 3.27), and follow the advice of Sandman351 to have a spare pumpkin for the strip...if you're really that serious about the last possible tenth or two. Otherwise, run what ya' brung and have fun; with the proper stall converter, that 408 stroker will twist the 3.50's with a vengeance!

Steve Amos
 
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