Technical What rear gear for 700r4

Not sure if this adds anything to the above content, but anyway….
700R4 gear ratios are are are 3.06, 1.63, 1, 0.7.

I have a Model A with a 348 Chevy, 2500 stall converter with a lockup clutch, 700R4, 4.11 gear, and 31.5 inch tires.

Great fun starting in first gear, shifts into 4th and converter locks up at 40-45 mph under light load, easily cruises down the highway. My tires are probably taller than yours and I have more HP and torque.

I suggest you go with what’s been said already: go with the higher gear (4.x), get a lockup converter and a kit to make it activate and/or a switch to turn it off, and enjoy. You’ll be happy.

EDIT: Here’s another piece of info you might consider: factory ratings for the 250 show peak torque at 1600 rpm. You might do the math and determine what rpm different rear gear ratios will give you at your 60 mph highway target. Hmm…..

John

 

Sounds like it was 'hunting' in and out of lockup in 4th gear. When these transmissions are computer managed, they will not lock up the converter under a set road speed. When retro-fitting without the computer, you can use a tach with a shift light and run the control wiring through the shift light circuit to only activate lockup over the pre-set shift RPM (added advantage of being adjustable). This stops the unwanted converter clutch cycling when in 4th around town. Or, you can run a simple speed switch. I have done both, and both will work well. A speed switch with adjustable hysteresis is the best method (example: locks at 50mph - unlocks at 45mph).

These transmissions work exceptionally well if all the parameters the computer managed are still implemented. Yes, it can be done without running a TCM, but it takes more work than most are prepared to put in. The basic lockup wiring techniques do work, but they will only give a basic result. Always a compromise.
The 700r4 is capable of much more if you put a bit of effort into it......

Your trans should not be trying to pull that hard up a hill in the 4th gear. It should be kicking back to 3rd way before this becomes a problem. If not, you should re-assess the way you are controlling the shifts. Sounds like the TV (throttle valve) cable is not set up correctly. This is vital with a 700r4.

You are right about having a good transmission fluid cooler installed. Heat is the enemy, even if everything is set up to run perfectly.

 

I had an 86 k10, 350, 700r4, 3.08 gears. It was a dog. 32 inch tires made it worse. I put 3.73s in it performance was better, and my mileage went up. No tach, but that 86 350 had the horsepower of a well tuned lawnmower, so should be similar to your set up.

 

For lock up of the TC, look into wiring it through the OD/4th gear solenoid. Seems easiest way. Ya gonna drop the pan to change the fluid anyways

Edit: that should read pressure switch, not solenoid.

 

My old 6 cylinder F100 with engine size close to what you have ran 3.89:1 diff gears from the factory.
It was fine but lacked highway speed.
I'd say that ratio with overdrive in your T700 should be a very good match.
If you are feelinbg adventuous maybe 3.73:1, but if you want to do hills and towing probably 3.73 may disappoint you.
My Pickup was running 235/75/15 tires.

 

Look on spicerparts.com an you can calculate based on your tire diameter, desired rpm anit gives you the mathematical speed. Not downplaying the significance of any actual experiences provided earlier. Just another input... this math obviously doesn't consider torque range of the engine or going uphill

 

90% of the customers cars ive converted to overdrive over the years ive allways used 3,73 gears or thereabouts , i think the math worked out to roughly same as 3.08 as a final when in overdrive ,
lockup kits for the the converter from companies like painless make it simple too,
fabricator john
miss you dad

 

All true for sure. Maybe I should have said "long, slight slope" instead of hill, where it will likely stay in 4th. Consider that, in overdrive, the engine is running at reduced RPM's which means if you manually switch off the lockup, the converter isn't nearly as efficient at those reduced RPM's and will have quite a bit of slippage.

I've never been a proponent of manually switching off the lockup. If the trans is in good condition and the TV cable is adjusted properly the system will do what it needs to under all conditions. Pre-computer transmission assumed, since we are talking about a 700R4.

 

My truck picture previously had the 223 cubic inch, 6 cylinder engine with factory 3.89:1 diff ratio.
That would have to be a pretty fair comparison to what you are working with.
My truck now runs a 350 SBC engine with T700 tranny.
I went from 3.89 to 3.54:1 diff gears.
My feedback on that is, if you are running an empty truck mostly and not towing etc, they are fine.
However, in my case, my truck is required from time to time to haul weight including a trailer and I would have much prefered 3.71:1 diff gears in hindsight.

 

My 250 runs best at 2200 - 2400 rpm on the highway, 3.08 gear, T350 trans, and 28" tire. Mileage went down with 3.42 gear, with 3.08, 20 - 21 mpg at 65 - 70 miles per hour, '37 Chevrolet truck, 3300 lbs. I doubt it has enough power to pull at any lower rpm.

 

Most of the 700r4 transmissions had the TCC lockup controlled by the vehicle's ECM (engine control module).
Yes, it is just an electric solenoid switch that allows the clutch to be hydraulically activated, but the management parameters for lockup were still assessed and sorted by computer.

Here's the really cool bit. ALL of the factory required inputs can be utilized in a retro swap, and they can all be done using analogue tech............

 

Changing from a 3.08:1 to a 3.42:1 rear gear ratio will mean you are running at a higher rpm at the same speed.
Higher rpm would naturally use more fuel at cruise speed.

 

I should have said, mileage went up with 3.08 gears.

 

I use a simple hydraulic pressure switch setup from Superior Trans Products to lockup my 700R4 trans. If I'm driving easy it drops into 4th gear at around 45-50 mph, but slowing down it stays in 4th OD lower than 45 until I step on the gas pedal and then it downshifts to 3rd. The trans doesn't have any issues with nuisance up and down shifting between 3rd and 4th, and I'm really happy with this simple pressure switch controller.

 

I used similar lockup as @1971BB427 , haven't driven it very much, but seems good. No wiring.

 

3.08 gear is the most common gear for the 6 cylinder pre '67 C10.

6 cylinder OD you are going to want the 4.11:1 gear. Unless you live in the flat or are running really short tires.