Technical What rear gear for 700r4

Hello all, I'm restoring my 66 c10 and went with a 250. Will be going g with a 700r4 trans. The truck has 3.08 rear gears now. I feel like this will be to much gear for the smaller motor and final overdrive gear. Keep running rpm and gear calculators and keep coming up with 3.73 or 4.11. Think the 4.11 would definitely get the truck moving faster but might do better with the 3.73. Any one run either gearing. This will be just for crusing. Thanks

 

The 250's that ran with 3:08 rear gears and no overdrive, say a 350 trans, were a good factory combination. I ran a 250, 150hp six with a 700r4 with 3:25 rear gears and it was pretty doggy when going into overdrive and a lockup converter. Especially going up a hill, installing long reach plugs and an HEI distributor helped a lot but it was not fast. I took the six out and installed a 350 V8 and used the same trans and rear gears; no comparison for power on hills and passing power. Not telling you what to do, just giving you my results. Going to a 3:73 would probably do wonders but I can't say from experience.

 

My later work truck with a v6/700r4 and 373 rear was a good combination for 250000 miles. The next one had 325 gears and was a dog.

 

You could always add a dash switch to drop out the lock up in fourth gear.. you might try that first since it's going to cost you about $6 and an hour of your time...

 

My experience was the 700r4 had a lower first gear so I went with 3:55 but on my car the tire diameter was only 26". If I would have been running a 28 to 30 inch tire, I would have gone 3:73. runs at about 2000 rpm at 70 in od.

 

Thanks, I have gone back and forth with this truck about what engine sbc, ls or inline. I keep coming back to the inline. My dad changed the factory 3 speed to 700r4 with 3.08 and a 350 engine. Truck ran great and got around 18 to 20 mpg. But many things changed with the truck over the years and has been since restored and want to go with the inline. It will eventually be rebuilt and hot rodded some. May even get a turbo thrown in. But I know it is still not the 350. Thanks for the information.

 

If “restoring “ then may want to do a manual transmission and OD on it. If just making it all look as original, do the V8/700R4 and keep the hood closed.

 

700R4 has a 0.7 overdrive ratio. So a 3.73 would be like a 2.61 in OD. Or not really close to what a T350 with 3.08 gear. For discussion, 4.11 is 2.88 which is closer and probably what would be better choice.

For fun, the 3.08 is like 2.16 in OD. Way too tall for the 250 six and your aerodynamic as a brick truck.

 

Probably the best place to begin is with the question "What highway cruising rpm/speed would you like to achieve?"
Then work mathematically backwards from that figure to see where the rear end ratio will be.

 

Cruising eh. I'm not sure about the rest of the crowd, but cruising in a 6 cylinder and wishing it was like you be cruising in a V8, no matter what gear you choose, you will be disappointed.

That said, and if your willing to forgo fuel economy, the 4:11's will make it seems like your closer to the V8.
Damn the gas want to have fun, the 4:11 will move the truck along.

But the mature sensible guy would do the 3:73's, seeing it as a decent compromise. It will still have some jump, just not the tire spinning strip laying potential the 4:11's would give ya. So what's cruising for you?

 

I ran my Henry with a 700R4, 29" tire and 4.10 gear for a while. 1st gear seemed useless but it was fine at highway speeds in OD. I switched to a 3.55 gear and like it better but it does loaf along at lower highway speeds.

Gary

 

4.11 is the best gear for a 700r. If you run to high of a gear ratio, then you have no passing power while hiway driving because your rpm's are to low. I had an ot chevy p.u. new in '92 350,700r, 4.10 gears, it ran great.

 

My average highway speed would be 60 but will see some interstate speeds of 75 or so. My tires are 28"
I have also thought about using a 4.11 but adding a gearvendor overdrive. Read on their site it helps with split the extra low gear jump from 1st to 2nd on the 700r4 or 4l60's. But I could disengage the final over unless I would be on the interstate and needed a higher speed and to cut the rpms down. But just a thought

 

I am running a 3.23 rear gear with a 305 sbc and 700r4 on 225 70 r15 tires. Rpm at 70mph is around 1800 with the converter locked up. I have the lockup wired through a vacuum switch so it unlocks when passing. Makes for a great cruiser and gets decent gas mileage.

 

If the 3.06 or whatever low in the 700 is the problem, go find a 2004R. Most have a more “normal” low of 2.5 or so. Great little transmission, smaller and lighter than a 700. Most standard parts will live fine with your six.

 

you are over thinking it. keep it simple. 4.11

 

3.08 x .70 = 2.156
3.55 x .70 = 2.485
3.70 x .70 = 2.59
4.11 x .70 = 2.87

Yuppers that 250 would be lugging bad with the od and 3.08 gears.
The 70 C 10 that my son had that came with a tired 250 in it had 3.7 gears but had a 4 speed and that truck did great in the lower gears and not bad on the highway. It could have used that od out on the long roads.

 

I had 3.08 gears with a 700R4 in a 62 GMC and at highway speeds it kept shifting into passing gear. I would not recommend 3.08s, maybe 3.55 or 3.73

 

Keep it a Six, but put a 292 into it.
Up the compression and add a stump pulling cam to it.

The 292 could be made a lot stronger than a similar sized V8 [283-305]

A 3.23:1 is a good choice.

With stock 225/75 R15 tyres [28" diameter] and a 700R4 it would cruise at 70 MPH @ 2008 RPM.
Kick it down into 3rd for hauling a load and it'll do 70 MPH @ 2713 RPM

Just add a GM manifold vacuum switch to kick it out of O/D under load

A 3.23 would be a good ratio that would not beat your eardrums to death, plus the low 1st gear would help it off the line.

 

4.11:1

 

4.11:1 rear ratio with a 700r4 gives a very deep 1st gear overall ratio (12.5766), probably too deep to be of much use.
An overall 1st gear ratio of around 10:1 is very quick off the mark, and about as deep as you want to go.
The math shows that this will need a 3.30 rear end ratio (or as close as we can get).
1st gear at 3.06 x 3.30 = 10.098:1
2nd gear at 1.63 x 3.30 = 5.379:1
3rd gear at 1 x 3.30 = 3.30
and overdrive at 0.7 x 3.30 = 2.31

One caveat with the 700r4 transmission behind a mild engine is that, if you want to get up and going like a stabbed rat, you will need to hold 1st until the revs are really up there so the drop into 2nd is not such a momentum killer.
Of course, if you have bulk torque, dropping into 2nd earlier can be an advantage.

 

I have a 700R4 along with 235/75/15 rear tires. Originally had a 3.25 rear axle ratio and it was a dog around town. Swapped it out for a 3.70 and the combo was much better and got full use from the trans as opposed to driving in 3rd gear around town to avoid the “lugging” with the 3.25 gear.
With SBC for power in my instance a 3.50 or 3.70 works well, with a 6 cylinder YRMV.

 

I have 373 in my ‘41 K1 pickup, its a good combo with the 700R4 and V6. My vehicle only weighs 3200 lbs,,,bit lighter than your C10.

 

I haven't seen anyone running a 3.89:1 gear with the 700r4. I run the 3.89 in a 2500# car with 28" tires. Didn't want to go all the way to 4.11's. Runs 2000 rpm at 60. Works good, but 1st gear is just about useless. I ran 2.79's in the Maverick 8" for a long time before changing to the 3.89's. I actually liked the 2.79's. 1st gear is just about right with the 2.79 gear. The only time I ever stuck it in 4th gear was when I got it out on the highway. Had to be running at least 50 MPH to get it in 4th gear. To me with 4.11's, 1st gear would be entirely worthless, but you could run around town in 4th gear.

 

If you do this, make sure you have a good transmission cooler. Pulling a numerically low gear ratio up a long hill with no lockup will cause a lot of converter slipping and the associated heat that is generated.

 

If the 350 and 700r4 are still in it (or the truck is still set up for them), why would you go back to an inline? It won't be "stock" if you toss a turbo on the 6 so why go that route at all?

 

Depending on rear tire diameter you wont want a really low geared rear differential! And regardless of tire size you'll find the very low 1st gear on a 700R4 wont ever work well with 4.11 rear gears! And even 3.73 is a bit marginal if you don't use a 29" rear tire, and have an engine that winds out pretty well. You wont get through an intersection before it shifts to 2nd, so it will have a great launch, but pretty short legs.
If you had a SBC that wound up well, and had taller tires the 3.73 would be great. But otherwise I'd go with 3.55 gears. The current 3.08 gears will be the opposite problem in 4th OD and engine rpm's will be too low unless you're going 70 mph, and even then they'll be under 1800 rpm's with 3.08 gears.
I'm running 3.73 with a 700R4 and 29" tall tires, with a SBC 350, and it just gets through lights when it shifts to 2nd, unless I've got my foot in it a little and then it shifts later. I love the low first gear with my heavy '39 Chev, and an engine that likes to wind up.

 

This is a sound rule to follow.

When choosing between two rear end gears, choose the lower one (numerically higher).

It is always better to rev an engine a little higher, with better mechanical advantage, than it is to rev it a little lower, with poorer mechanical advantage.

The former, despite turning a bit faster, will not only give you better acceleration, it will also give you better fuel economy too.

What is double-plus-ungood about slow turning an inline engine, with an extra-long crank, is that it will increase torsional twist in that crank, which can add up to a whole lot of things that you, and anyone that uses the road that you smeared engine parts on, just don't want.

 

I am never concerned about how often my automatics shift, or how often I have to shift my manuals.

I run 5.14:1 gears in the rear of my Falcon. It has TWO overdrive gears, and has been to Bonneville and Los Angeles, from San Francisco, several times.

I forget exactly, but I think that it is at about 2400rpm at 75, in 6th.

It has been set up this way since 2014, and despite everyone telling me that this is impossible, it works just fine.