Best diff gears for application???

[drobertson]

Car is a 1973 Charger, 318 / 727. The 318 has quite a few mods, including a fresh rebuild. 325 cid now! But is making about 286 horse. I'm not all for acceleration, but I want a good top speed of maybe 120 to 130. Would 2.71 gears be good for that? I would imagine my gas mileage would improve as well. With 2.71 gears what range might be my top speed??? Car weighs about 300 pounds less than factory.

 

[GetX'd]

That would be a good ratio giving you a lot of top end if that's what you're looking for. Wouldn't do much for you out of the hole.

 

[terzmo]

Car is a 1973 Charger, 318 / 727. The 318 has quite a few mods, including a fresh rebuild. 325 cid now! But is making about 286 horse. I'm not all for acceleration, but I want a good top speed of maybe 120 to 130. Would 2.71 gears be good for that? I would imagine my gas mileage would improve as well. With 2.71 gears what range might be my top speed??? Car weighs about 300 pounds less than factory.

would need tire size to compute speed at rpm

 

[1wild&crazyguy]

Car is a 1973 Charger, 318 / 727. The 318 has quite a few mods, including a fresh rebuild. 325 cid now! But is making about 286 horse. I'm not all for acceleration, but I want a good top speed of maybe 120 to 130. Would 2.71 gears be good for that? I would imagine my gas mileage would improve as well. With 2.71 gears what range might be my top speed??? Car weighs about 300 pounds less than factory.

You can't cruise at a 120 or 130 without some OD trans or you will be turning 5500 rpm or so.

But if you just wanna buzz it up to that speed for fun i'd say go with 3.73's.
3.73's aren't a factory stock ratio but they make them for the mopar diff's or most of them.

Anything more than that and you will be turning 3400 to go roughly 75 mph cruise on a 25.5 tire, 3200'ish rpm on something bigger like a 27-28'' tire.

 

[bm02tj]

For a cruiser I would go 323 or 355
but not 271

 

[PurpleBeeper]

I don't recognize the 2.71 gear ratio (what rear end? 8-1/4"?), but you are on the right track. I ran 2.76's in a '77 Cordoba with a '71 440 in it and was able to hold a little over 150mph for 5+ minutes. You should be able to do 120-130 with no problem at all & you will not need an overdrive transmission with that rear end gear ratio (see above).

'71-'74 Chargers are some of the more aerodynamic B-bodies (besides the wing cars). I looked up drag coefficients many years ago. Make sure you have good speed-rated tires.

 

[greymouser7]

I don't recognize the 2.71 gear ratio (what rear end? 8-1/4"?), but you are on the right track. I ran 2.76's in a '77 Cordoba with a '71 440 in it and was able to hold a little over 150mph for 5+ minutes. You should be able to do 120-130 with no problem at all & you will not need an overdrive transmission with that rear end gear ratio (see above).

'71-'74 Chargers are some of the more aerodynamic B-bodies (besides the wing cars). I looked up drag coefficients many years ago. Make sure you have good speed-rated tires.

At about 140mph, the front end catches enough air in 71-74 chargers to test the suspension pulling the front end up.

With 2.94's, I have driven 150 in a built 360 powered Road Runner (no overdrive needed) I recommend against it after experiencing it.

The recommendation of 3.21 or 3.55 gears is an excellent recommendation for what you want while retaining comfortable street acceleration.

 

[bm02tj]

Every brand of rear end and gear manufacture has different ratios some 4.10 some 4.11 or 3.54 and 3.55 and 3.21 and 3.23
so numbers close is good enough as the 8 1/4 's has available 2.71 ,2.94 ,3.21, 3.55, 3.73, 3.90, 3.91, 4.10, 4.11, 4.56 & 4.88
some of those I think would be crazy but think of your purpose & power available