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Old 11-14-2012, 06:31 AM
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Originally Posted by Paul E Cain View Post
Recently purchased a 1941 Ford with a chevy 350 and automatic 3 speed transmission. @ 65 miles per hour Tack reads 3,100. Should I change to a four speed transmission, or can a different differential be installed. Looking to better fuel millage. Engine is a high performance Edelbrook.
You could do either. In most cases it will be cheaper to change the rear gear ratio, but there's also something to be said for an OD transmission because of the low first gear ratio plus a .7 OD ratio (or thereabouts depending on the tranny you use) gives the best of both worlds- a low first gear along w/a high cruising gear. The cheaper rear ratio change will give you a good cruise rpm- at the expense of good acceleration in low gear.

Some numbers:

The way you sit now, with a 30" diameter tire (middle of the road size) your rear ratio works out to be about 4.25:1.

With a .7 OD the effective rear ratio becomes about 3:1 and at 65 mph the engine drops to just over 2000 rpm. This is a hard to beat combo IMHO.

You can play with these calculators to see what you can come up with: Wallace Racing - Automotive Calculators.

Good luck.
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