I've had the chance to drive around my '73 Coupe and put on a few miles. I'm running a 454, 3.36 rear end ratio, and a TH400. My plan is to put a few performance mods in the 454 and I already decided to change out the 3.36 rear end gears for the 3.08's because I want to cruise on the highway at 70-75 mph and keep the rpm's low. I'm considering swapping the TH400 for a 700R4 to drop the highway rpm's down at cruising speed.
1). If you are running the 3.08 rear end gears, and the TH400, what rpm's are you turning at 70-75 mph?
2). If you are running the 3.08 rear end gears, and the 700R4, what rpm's are you turning at 70-75 mph?
I've read prior threads about the pros and cons, however I'm just looking for the rpm's at cruising speeds to help in my decision.
Thanks

 

This is a handy tool

http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

If it were me do the 700r first you probably wont want to switch
Dont want to lug your motor too bad esp if you add a cam keep the motor in its happy zone.

You will love the difference in 1st gear and having the option of 4th.

 

I had a 700R4 in my '79 for many years with 3.55 gears and it was fine. First gear was a little deep, but nothing I would consider changing gears for. Try the trans swap first and I doubt if you will still want to change out the gears.

 

A 700r4 has a .70 OD. I used s 4.11 for about 20 years with .70 OD. I could effectively use WOT in OD because My 700 was built with no kick down function.

Effective rear ratios. I always thought that a 3.90 rear end was about best for performance. A lower revving BBC might like 3.55

My C-5 had a ZF 6 speed with a .50 OD 6th gear. I installed a 4.10 in it


4.11 X .70 = 2.877
3.90 X .70 = 2.73
3.73 X .70 = 2.61
3.55 X .70 = 2.458

 

3.36 gears are perfect for an overdrive auto transmission in your C3. I swapped my THM-400 for a 700-R4 (with 3.36 gears) and I love it. At 70 mph the engine is running about 2100 rpm. If you put 3.08 gears in it and add the 700-R4, you will need to run in 3rd range anytime you are cruising at less than 45 mph. Otherwise, it will jack around between 3rd range and 4th range and jockey lockup on/off forever.

Just the advice of someone who made the swap...

 

There's the possibility that you could go with different diameter rear tires and save a lot of money/time/trial and error.

Google a tire/gear ratio/rpm chart/formula to see what tire diameters might work.

 

To make the same change as installing an overdrive trans (.67 to 1 gear ratio in overdrive), you would need 36" diameter rear tires. Don't think that's going to happen.....

 

7T1Vette, Thanks, that is the information I'm looking for. I want to keep my 70 mph cruising speed with the engine at about 2,400 rpm. I'm just trying to decide if I should do the tranny swap or maybe keep the TH400 and swap the rear gears from the 3.36's to the 3.08's.

 

It will just feel slower (gear change) wont do much in cruise rpm

Do the 700r win/win

 

It really depends on what you want with your car. If you have some desire to retain the original transmission, then changing the rear gear would be a decent way to go. With 3.08 rear gear and a THM-400 trans, your 70mph rpms would be around 2400. You will get somewhat better economy on the highway (maybe 18 mpg), but the THM-400 is NOT an 'efficient' transmission, so that's about as good as you can do.

If you keep the 3.36 rear end and install an overdrive trans (either 700-R4 or 200-4R), then you will only be turning 2000-2100 rpm at 70mph. You will also be in lockup mode and save another 10% on mileage, so you can expect around 20 mpg or a little better on the highway.

Simplest approach would be to change the rear gear; but standing start acceleration will be pretty weak. Cost will be much less than the O/D tranny swap.

'Best results' method would be the trans swap. And, when starting with a THM-400, it will be easier to install a THM-200-4R than the 700-R4. Basically the same swap, but the trans mount and driveshaft won't have to be changed to go to 200-4R. Life will be much easier (quieter, cooler running at highway speeds, less engine wear, better mileage) with the O/D trans. You will need a lockup (L/U) controller to manage the L/U clutch properly. Folks have installed a simple switch, but there are pitfalls with that approach. For about $100 you can get a controller that will do a good job of L/U clutch management and prevent damage to L/U clutch and/or transmission.

Choose your 'weapon' and go for it!

 

I would pull and rebuild the TH400, so the purchase of an overdrive trans won't be much more expense. Besides the ease of installation, what is the difference between the 700r4 and the 200r4? Will the 200r4 take the same torque and horsepower as the 700r4?

 

Neither of them handle much without upgrades. 88-up 700 is a little better dont know on the 200r

being as hes talking about engine upgrades was motivation to steer him away from the 3.08 more cam needs more gear etc is all.

On either one use a GOOD trans cooler skip the radiator altogether

 

I popped two 700r4 trannies in a year behind my 406.......Went to a th400 and never looked back.

When I ran the 700r4 my 1st gear was a complete waste. Waaaay too steep when combined with the 3.70 rear. Then if I got a chance to get into OD,...and ran about 2500 rpms, the engine felt like it lugged a bit. So for me,..the OD wasn't ideal.

There are guys here that will swear the 700r4 is the way to go. But I called dozens of tranny shops to rebuild mine, and when I told them I had 500- 550 horsepower, none would rebuild my 700r4. And if they did,...no warranty. And for that reason, I went th400. Mine is rated at 800 horsepower, and I have no hesitation hitting 7000 rpm shifts with it.
Once I destroyed the first 700r4 with a single 7000 rpm 1/2 shift,....I was gun shy with the second 700r4.

Keep in mind I only had a 406. With a 454,...and any sort of cam, you'll be pushing 500 horsepower easy.

 

Have a guy out here that builds a stout 700r has held up behind some healthy big blocks strokers etc. Doesnt go crazy on $$ parts just knows what to do and where. Reasonable too far as I know nobodys broken one over the yrs not saying it cant be done .Pm if interested
Done good by me over the yrs

 

I have 525 HP with a built up Th700r4 and 3.73 gears.... about 7-8 years now.. haven t broke a thing... build it right, it will handle 600 HP ... 4th gear cruises 80 mph @ 2550rpm