I am getting the diff in my 69 350/300 T400 trans rebuilt, it has the 3.08 currently.
Would like to get the revs down when cruising, not looking for quick off the mark but just good for
long drives and cruising.
What would be a good ratio to go to ???
Who is the best parts supplier for rebuild kits ?

Andy

 

I think you can go to a 2.73 but honestly since your gearing is already at 3.08, not sure you will be happy with the additional RPM reduction since numerically it is not going to be much, about 300 RPM less at 65 MPH, 2200 versus 2,500 with a 27 inch tire.

If you have the time and money, I would go the other way. Change to a 3.55 rear gear and a Tremac etc 5 speed with a .64 OD. You would get the best of both worlds, MUCH better acceleration and MUCH better cruising RPM's, 78 MPH at 2,200 RPM in 5 th gear OD.....

 

Jeff is correct, if you want to drop your RPM you have to go to an OD trans. 273 was once offered by Tom's & GM years ago but no longer made. Trying to find a set is going to be difficult.

 

I have a 3:08 in my 1980 with T350 and just went with a 700R and 65 mph is now 1800 rpm now with Lockup or 2200 rpm at 80 mph and I love it

 

How fast do you drive with the gear you have 80 is 3100RPM. Your engine should be happy there. You know engines have a sweet spot for mileage. It's not always at a lower RPM. The newer fuel injected engines are designed to cruz at very low RPM. The older stuff not so much.

 

All excellent advice given above.
I went through the same questions and answers when I started putting the new drive train together for my 73, 454, TH-400 with 3.36 gears.
I wanted to go to gearing that would give me the opportunity to keep my rpm’s low, but also to keep them up high enough to be within the power band of my camshaft, plus not be bogging the engine while cruising down the highway at 70 mph.
Not always an easy thing to plan and going to a 700r4 was out of the question for me because the .72 4th gear was to high and my cruising rpm would be way to low to work with my camshaft and engine mods.
I wanted to stay with a auto trans, but in the end I went with a manual, Tremec TKO-600RR with a .82 5th gear and a set of 3.08 rear end gears.
My 70 mph rpm will go from 3100 down to about 2300.
2000 to 2100 rpm is about as low of cruising rpm’s as you should go with one of these older V8’s running a carburetor.
There are so many different combinations to choose from, so do some research and base your decisions on your driving style, what you will be using the car for, and engine mods.
Plan ahead.
If you are thinking about doing some serious engine mods in the future a 700r4 may not handle the additional power and the cost of custom torque converters are very expensive.
If you plan to keep your engine mods reasonable, the 700r4 would make a good swap with a set of 3.55 or 3.36 rear end gears.
Good luck!

 

I have a 409 stroked 474 and 700r behind it with a 373 rear. The 409 likes about 2600 for good mileage. But that is about 82MPH. Which I like to drive but CHP do not

 

Yes and no! There is a sweet spot for the engine RPM's where the motor will run most efficiently usually at the RPM for the highest TQ rating. Another words, for my 1978 OEM L-82, the highest Torque rating was 260 Ft-lbs at 3,600 RPM (4,000 RPM for some years) which is the engine RPM at which the engine is most efficient at making power but is not necessarily the optimum RPM for max MPG since many factors go into that result: Engine Piston Speed, fuel needed at that certain RPM, frictional losses at higher RPM, transmission losses at higher RPM etc.

Almost every engine will get better/best MPG at lower RPM's, pretty much universally......not just more modern EFI engines. Lower the RPM's and the MPG's go way up, even on SBC V8's with a carb.

How do I know? Personal experience from MANY C3 owners who have added 5/6 speed OD transmissions to their engines with zero other changes and MPG's with OEM and Built 355/383's in the low to mid 20's MPG range. There are folks with C3 425+ Gross HP SBC engines that can get 24/25 MPG HWY with a carb with OD tranny's...not great science/engineering here, just lower Rev's.

BTW- Do not be deceived into believing cars built in the last 20-25 years getting great highway MPG was from just technology..it was NOT. It was mostly about simply lower the rpm's, using steep OD transmissions, both auto and manual,starting about 20-25 years ago. It literally all about steep OD autos and manual transmissions.

How does my 20 year old Pontiac GP 3.1 V6 with 225,000 still get 29 MPG cruising at 80 mph? Simply because it turns 2,000 RPM at that speed. How does my 10 C6Z06 with a SBC 427 V8 7.0 liters get 28 MPG at 80 MPH with 505 Net HP? ....1,750 RPM at 80 MPH is how. I know that if I ever add a steep OD 5 speed to my L-82 355 with 425+ Gross HP and 3.70 gears, my car will easily get 23-24 MPG at 70 mph with the same carb and everything else..just lower the rev's...not rocket science.

Last point my 08 300 3.5 V6 has highest TQ at 4,000 RPM and theoretically should get best MPG at 4,000 RPM on the hwy. I guarantee at that rev it would get much worse mileage at 70 MPH then if it was turning 2,000 RPM at 80 MPH..its all in the revs for old and newer cars, mostly....EFI has allowed more efficient fuel deliver and lower revs with more radically cam timing. The bigger mileage gains (35-45 MPG with good power), mostly in the last 5 years, is more from technology improvement beyond lower RPM's on newer cars....

 

Pretty much the cam specs will determine the RPM range of an engine, not the design of the engine including a GEN 1 SBC. I know guys with OEM L-82's with non stock flat tappet cams that don't like revs less than 2,000 RPM...with an OEM motor. With my rebuilt upgraded L-82 355 with a howard roller cam (operating 1,500-5,600 RPM, duration 219/225, .525 Lift, LSA 110) with my AFR heads, my motor can run all day long at 1,500 RPM or even less (can idle down to 900 RPM in 4th gear and pull with no hesitation to 6,000 RPM with 1:1 gear). Now this same motor with a 230/235+ duration cam would not be able to do the same and cruise at 1,500 RPM...all in the cam....

There are lots of wife's tale about OD trannys (and what exactly an OD is for...it's not for racing, for most people) and what low revs mean to any engine for gas mileage and engine wear......

 

Flat ground VS mountain roads which is where I live. So it's not apples for apples. But yes I get it less throttle better MPG

 

For whatever it’s worth, my 1980 L82 with Promaxx 183 aluminum heads 64cc chamber and Comp XE262 can maintain or pull 55 mph full Lockup at 1400 rpm no problem at all , all day lonG

 

There are lots of wife's tale about OD trannys (and what exactly an OD is for...it's not for racing, for most people) and what low revs mean to any engine for gas mileage and engine wear......[/QUOTE]



The Crane cam in my 496 is a hydraulic roller, 234/242 @ 50 with a 610 lift and 112 LSA, with an operating range of 2000-6000 rpm.
I wanted a cam and gearing that would cruise all day and pull nice from any cruising speed.

I choose the .82 5th gear in my Tremac TKO 600 RR because I did not want the difference in gearing from a 1.0 to a .74 od which was close to 900 rpm difference.
My Sunday drives can start at 17’ elevation (home) and within 2 hours I can be at over 8000’ elevation going up to Lake Tahoe.
Down shifting out of overdrive, up or down a steep grade or through a twisting mountain road is unenjoyable if the gearing is wrong.

There is no right or wrong gearing combination.
Its how you make everything work together, your intended use of your car and your driving style.

Heck, I did my share of burnouts 50 years ago and I’ll probably never do one in my Corvette.
At least not intentionally!

 

What Can I say???^^^^

An almost 500 CI BB motor in my opinion should have no issue in any gear, pulling from idle in 4th, 5th or 6th....If I had a motor like that, personally, my opinion, I would want the steepest OD I could find. My L-82 SBC 355 now can pull from any gear from idle..no issues...It could easily pull with a .64 OD on the highway, if I ever decide to switch tranny's, flat or mountain roads.

My 10C6Z06 with .50 OD easily accelerates in 6th gear from any rev's as low as 1,000 RPM....I almost never have to downshift to accelerate on the highway....I guess I am totally perplexed why a BB 496 would have any driveability issues, regardless of gearing and overdrive.....from the tranny....at any speed.

 

This!!!

 

Sure!