Which overdrive gear is best?

bishop1911 · Jan 30, 2007

I am going to be purchasing a tremec 600 for my 67 fastback which has 390 engine probably around 400HP. I am running 3.73 gears in the back and would like to install a 5 gear tranny so that when I go to shows that are farther away, the engine is not reving to high and screaming. My question is which overdrive gear ratio would be better? They have a .82 overdrive and .64 overdrive. I was thinking the .64 would be better for highway driving. Just slap it in 5th and go. But for some reason, that is cheaper than the .82. Anyone know what the reason is for this? Any suggestions for one or the other? Thanks guys.

Jon

PJx5x · Jan 30, 2007

.82 is for road racing. go with the .6x

96 DOHC Cobra · Jan 30, 2007

Where are you getting it, and how much? I'm thinking about the same thing. I have a 67 fastback also. Only my combo is a 428 engine and 3.50 rear gears.

bishop1911 · Jan 31, 2007

Tranny

I am not sure where to get it from yet. I was looking at Summit Racing and Mustangs Unlimited where to this point I have gotten about all of my parts. But, in an different post someone suggested lookin into dark horse performance and they do seem to have some good prices and have complete package setups. I did email them the other day though and still no response. I will call them tonight if still nothing by today, but already that doesn't look good on the Customer Service end. Let me know if you find any good deals or places to buy from. So far, lowest price I have seen for tranny is about $1820 for Tremec 600 with .64 overdrive.

Jon

Pbum5 · Jan 31, 2007

Be aware that you might have to do some tunnel surgery to install that Tremec with the proper driveline angle.

To be honest I don’t know that for a fact.

I hear you do have to raise the tunnel on 67, 68’s and that you don’t.

Still have not had a definitive answer on that.

I’d like to know myself since I plan on putting in a 5 speed someday in my 67.
That’s why that new Richmond 5/OD looks good right now.

juiced · Jan 31, 2007

I have a T-5 with .68 overdrive. I run 3.89 gears and 26 in tall tire. At 65 mph I get 2420 RPM. There are calculators online, sorry I do not have link handy. I think the .82 would be too high an RPM cruise for a long drive...

rbohm · Jan 31, 2007

what overdrive gear you choose will depend largly on what rear gear you have, what your torque curve is, and what you plan on doing with the car. if you plan on using a 3.50 or lower numerical gear, then the .82 ratio is what you want. for a higher numerical gear the .64 is the one to use. these are street number by the way. if you plan on road racing, then the .82 gear is best.

68RCodeConv · Jan 31, 2007

I have had both 4.11 and 3.89 rear gears in my car. I have the .62 overdrive and I like it.

Now if I had 3.25 or 3.00 rear gears the .62 would be too much IMO. Wouldn't be able to use it below 70 mph.

bishop1911 · Jan 31, 2007

Great info

Thanks for all the great info. Looks like I will be using the .64 overdrive. Can anyone else confirm or deny the whole tunnel issue with a 67 and a tremec 600? Will there have to be any modifications to the tunnel to use that or should it go right in?

Jon

LMan · Jan 31, 2007

Its tough to nail down, due to manufacturing variances...some have to mod, some dont. Its like the 'cut the driveshaft' issue, some do, some dont.

I didnt have to. But others do. Manufacturing tolerances in the 1960s werent as accurate as today.

Which overdrive gear is best?

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