The Stovebolt Site Forums The Shop Area Engine Shop What rear end for 261/700R4?

Going from 3 spd/OD to 700R4 in my '56 1/2 tn--wife has bad wheels and has trouble running the clutch. After reading previous posts, I'm thinking a 3.73 ring & pinion would be best. What's the word on the 'Bolt?

3.73 is a good place to start. What engine are you running?

If you are running a high HP motor, you could go with higher gears, but a 3.73 w/ OD is about the limit for a 235. Others may disagree.

if you haul stuff often, the 3.73's are a good choice, however if you are empty often, a set of 3.42's is nice. 3.08's would be a little bit for the 261

What rear is under it now? If you still have the original 3.90, I'd leave it in there.

What drive ratio is the stock overdrive? I don't remember. I think it's comparable to the 700R4. It's a place to start anyway. The 700 has a nice low first, almost as low as the first in the old 3 speed. Are freeway rpms reasonable now?

Devin

Don't forget to factor in tire size, the actual outside diameter, a few inches makes a significant difference in effective gear ratio.
All of the above suggestions and comments mean nothing without knowing what tire size they have in mind.

For example, a truck with 26" tires and a 3.73 ratio is equivalent to having 4.16 (an imaginary ratio) and 29" tires. You can see in this example a 3" taller tire requires about one step up in gear ratio, 4.11 would be the closest available ratio.

This will help you figure speeds and rpm with tire size and gear ratios.
http://www.onlineconversion.com/bigger_tires.htm

If you like how it drives now, then compare the final drive ratios and gear it so you get the same thing, the same engine rpm for a given road speed.
Or if you feel the engine needs to run faster or slower at a given road speed then correct for that by changing gears and or tire size.

Grigg

Original rear long gone. Currently 3.31 gears in 12 bolt posi with 225 70 R 15 tires on 5-bolt Corvette Ralleys. The 261 was bored .30 over, head shaved .20, Clifford 6=8 intake, Holley 390, Fentons. Tires 27 1/2" tall and 86 1/4" circumference. Has always seemed gears were a little tall especially on hilly terrain north of K.C.

Great, that's a start.
What is the OD ratio for your current transmission?
From what I can find a 700R4 has an OD ratio of 0.70 to 1, is this right?

If you were to run the 700R4 with your current rear and tires you would turn 1,840 rpm at 65 mph.
do you have an idea of what rpm you want to turn at what road speed?

Grigg

I have a 700r4 behind a 350 in a K2500. It has 3.42 gears if I remember correctly, and I notice it shifts down into direct a lot on almost any little incline.

Its my most frequent alternate commuter over another 350 with 3.08 gears that makes the same trip in overdrive (manual so I control what its doing) and never seems to strain unless ol' lead foot decides to pass. The one with 3.08 gears weighs about 1,000 pounds less than the K2500.

My 270 powered GMC Suburban with 3.08 gears and no overdrive makes the same trip at similiar speeds with no strain (but no passing), and I suspect would behave similiar to the K2500 if it had a 700r4 instead of a SM420, dropping in and out of OD as it needed.

My point is that the 700r4 would give you the benefits of overdrive while dropping into direct or lower when conditions made it appropriate. Unless you are carrying heavy loads or pulling a trailer I'd assume the existing gears will be fine until experience tells you otherwise. With a 12-bolt already in place you are probably going to change the gears rather than the axle, so changing later won't mean changing anything you did during the trans swap.

here is a trippy engine- axle- speed- gear box
CALCULATOR. http://vexer.com/68rt/speed.html

You really have to try a couple different gearsets yourself to tell for sure. Some guys like to wind out a motor, others like it to chug along. I run a MY6 trans with 0.73 OD top gear & a 3.08 axle behind my 292. Depends on terrain, what speed you run mostly at, whole bunch of variables. I'd start with around a 3.2 to a 3.5 axle set & go from there.
Doug

My son, the engineer, came up with the following (all with
225 70R15 tires):

Current 3 spd/OD 3.31:

1st 2.8
2nd 1.68
3rd 1.00
OD 0.70

60mph OD 1701rpm
65mph OD 1843rpm
70mph OD 1945rpm
75mph OD 2127rpm

700r4
1st 3.06
2nd 1.62
3rd 1.00
4th 0.70

with 3.73
60mph 4th 1918rpm
65mph 4th 2077rpm
70mph 4th 2237rpm
75mph 4th 2397rpm

with 3.90
60mph 4th 2005rpm
65mph 4th 2172rpm
70mph 4th 2339rpm
75mph 4th 2506rpm

On trips to shows we typically travel at 65 to 70 mph and am currently almost to the floor at times.

On the floor at about 1,900 rpm will it accelerate?
If not you probably need lower gears, that's a pretty low cruising RPM.

2,500 rpm at 75, your last combination, sounds about right.

You'll also get better fuel mileage with it geared right. You don't want to lug the engine. When you put your foot down it should accelerate, if it does not you either have it geared to fast, or you just plain ran out of power for the speed you're traveling.

What's your best guess on gear ratios considering how it drives now, and how you want it to drive?

Grigg

If you don't like the way it performs now with the .70 OD the 700r4 will only make it better if it downshifts to accelerate or climb a hill. Otherwise you're looking at similar and apparently not inspiring performance.

Based on the way my factory stock truck with a 700r4 behaves I think that's a reasonable approach. If its not going to be acceptable for an automatic to downshift you better change those rear gears.

Provided the motor is running efficiently at your current cruising speed, I'm inclined to stick by my earlier suggestion of installing the trans and only changing the gears after seeing how it feels. You may still end up changing the gears, but if you like it without changing them you've saved time and trouble, not to mention a few dollars.

Grigg

Thanks for the comebacks. At 1900 rpm won't accelerate or have enough guts or vacuum pressure to hold the cruise control at speed.

Sounds like currently you have a bit to high of a gear.

Steve has an excellent point that the transmission will shift for you. That changes things slightly
Theoretically could gear it faster than if you had a manual, and let it shift when needed
If a manual with same gears you probably wouldn't shift as much as needed to keep the engine happy and running efficiently.
But even so you want that high gear to be usable most of the time, so if on the straight and level you currently can't accelerate in high gear, then even with an automatic you probably need lower gears.

Assuming you have the power for the speed you want given the shape and weight of vehicle, which I think you should, then do as Steve suggest and let it run in OD most of the time, but downshift on the big hills.

Once you get it setup if you find it doesn't need to downshift for much then you need faster gears and will probably find some more fuel mileage.
If it always downshifts and can't hold high gear then you need slower gears and again will find some fuel mileage.

Can you play with your different ratio choices before you take the jump by temporally swapping rear wheels and tires (smaller ones) to change the effective ratio? calculate here: http://www.onlineconversion.com/bigger_tires.htm

Grigg

I like the idea of swapping tires to experiement with different ratios. That might be a good cheap way to sort it out if you can find a variety of tires (mounted on wheels) that you can borrow.

Consider the transmission specs on a recent 6-speed Corvette. 5th and 6th are both overdrive gears with 5th being about .7 and 6th about .5. In that evnironment (I beleive) you rarely use 6th unless you're running highway speed and expect to stay there.

But when it is appropriate you can cruise with the motor turning real slow. While it is just a 700r4, my K2500 implements a similar concept, without requiring me to think about what the motor is doing. I'd be happy to try a 6-speed Corvette but haven't had a chance...