700R4 owners... what ring/pinion are you running?

[CentralCoaster]

Right now I have an open diff 4.11 (or 4.10) behind my 700R4 and mild 350.

I finished my converter lockup wiring, but it still tachs too much in 4th.

I'm thinking a 3.55 posi would suit me better. I want this truck to be a cruiser that I can drive anywhere, not a stump puller. I like driving fast (80-85 mph is fast for these trucks), and that's no fun taching at 2800 rpm for an hour or more. I also like getting decent gas mileage. I can squeeze out 15 mpg now at 70 mph, and 3.55s would probably help a bit more. 3.73 would be a better compromise if I ever do any towing.

I'm used to my vette purring along at 1800 rpm in double overdrive, so perhaps my idea of "taching it out" is way off.


What are you guys using?

(I have to check and see if 3.55 is even available for the 12-bolt)

[CentralCoaster]

Side question, are these 12-bolt carriers worth anything if I go to sell it, depending on its condition when I crack it open?

Can I install the posi in this open carrier or do I need a different one?

I think I'll look into setting up the gears myself this time instead of paying the shop. I've installed lockers before on Toyotas, but never done a posi.

[C10-Driver]

I'm running 3:73 and like the combination with the 700R4. First gear is very snappy and the overdrive really helps with RPM's and good mileage on the highway.

[earl87gta]

here is a wb site I use for gear size with out knowing your tire size its hard to say what gear would work best for you. I had 4:56 in my 700r4 blazer but it had 38 in tires. check this site out it will help you figgure out what is best for you.

http://www.et-studios.com/motorsports/gears/gears.html

http://www.grimmjeeper.com/gears.html

[special-K]

With your present carrier you can go to 3.73s.Any higher and the higher series will be required...probably want to swap a complete rear.Wouldn`t dbl o/d be nice?

[Smitty]

You'll like the 3:73's better. I went to 3:55 and kick myself the gas mileage was better before!

[SactoJim]

I have 3.73's with my 700 and so far seems fine. First gear is definately snappy!.......can't imagine what it's like with 4.11's.

I have a little blower on a 383 and I can't help but think that 3.55's would be better cuz of all the torque this motor makes.

I sold my "one legged" 12 bolt with 3.73's for $100.

[steveshow]

what size tire are you running? A cheap fix would be to go to a taller tire, which raises your effective gear ratio.

[screamin_c10]

3.73s are sweet with the 700

[piecesparts]

I have a 91 Chevy with a 3.42, 10 bolt posi rear end and it works real well with the 700 R4. The RPMs on the highway are around 2300 RPM at 70 MPH--running a 27 inch diameter tire. The acceleration isn't bad, but it is better with the lower rear ends listed below.

My 84 GMC has the same size tires, but has a 3.73, 12 bolt posi unit behind it and it runs around 2500 RPM at 70 MPH.

My son's truck now has a 3.73 rear end on his truck, but previously had a 3.08 rear end and it ran around 1900 RPMs at 70 MPH. This was apoor combination with the 700 R4 setup, he picked up 2 MPG by going to the 3.73 and getting the power curve up to where it belongs.

[Painter D]

ive got 4:10's with my 700r4 with 28"tall tires and it doesnt seem to rpm too high. but honestly i dont know what rpm its at because i dont have a tach.

[Ackattack]

I have 3.73s with my 700r4 in the GMC. Cruise right around 2000 rpms at 62ish. (18.5mpg )

In my 68 chevy I'll be running a 4.11 rearend with a 4L60E but with taller tires to keep the rpms close to 2000 at 60mph

[Patg]

I just put in a 700r4 in my truck it has a 308 gear in the rear end. It works fine for all around driving. On the freeway I get 1600 rpms at about 65mph.
But starts to run out of power going up hills. I want to get a set of 343's to move up the power band. If I go all the way to 373's youI will start to loose all the benfit of the overdrive.

[mnunn454]

I have a 3.55/TH700 in my 3900lb 454 70 El Camino with 275/50/15s in the rear. It's the perfect ratio as far as I'm concerned. Highway RPMs at 60 are about 2200. At 70, they're about 2400.

3.42's or 3.73s are as close as you can get with the truck carrier to a car 3.55 12-bolt. 3.73's will work on your current (4.11) carrier. 3.42's require a different carrier (which is no biggie if you're buying a posi anyway).

Powertrax makes a unit that replaces the spider and axle gears in your current carrier. I've heard they're fairly easy to install and don't require you set up the R&P all over again. Pros and Cons on both sides of this approach.

I have a 3.73 posi in my 69 C-10 with TH400/454. It spins too fast for me on the highway. I have a spare TH700 that one day I may rebuild and install in it.

I'd think 3.73/TH700 and your 350 engine would be a really sweet combo.

A cheaper alternative to your current delimma may be to just run rear tires that are about (3"-4") taller. That'll effectively reduce the highway revs by 10-15% (acting more like a 3.73 gear set). With the 3.06:1 low gear in the 700, you'll never notice a difference at the traffic light.

Food for thought anyway.

[CentralCoaster]

I have a 28" tire (275/60R15). Don't want to go any bigger than that since it fills out the fenders nicely and I don't the truck any higher than it is now. Any bigger would look stupid on Corvette rallyes. Tires lose about 3% of their diameter from weight, so here's the numbers. Damnit a 3.55-3.65 range would be perfect for me too. I'm leaning towards the 3.42s. It's hard to drive 70 mph here when everyone else is doing 85. The truck is noisy as hell at 80-100, but I can address that later with insulation, headliner, window adjustments...

If 3.42 is a little too tall I can always go down a bit on the tire size. I'm sort of picky about rear gears. It's a mod easy to screw up that will aggravate you on a daily basis if you're one of the many people who go way to far and lose drivability. If these 4.11s aren't original then I bet they have less than 10K miles on the install, and I could get something for them. Everything else on the truck is about 10K old at least.


3.42

60 mph @ 1800 rpm
70 mph @ 2100 rpm
80 mph @ 2400 rpm


3.73

60 mph @ 1900 rpm
70 mph @ 2250 rpm
80 mph @ 2600 rpm


4.11

60 mph @ 2100 rpm
70 mph @ 2500 rpm
80 mph @ 2800 rpm

[CentralCoaster]

Looks like I'm limited to Eaton or Auburn. Eaton's made in USA, Auburn is assembled in Mexico. The fact that Auburn has a "Pro" line doesn't imply great things about the non-pro version does it.

Looks like Tom's Differentials is the way to go on this.

Also, is Superior Gear any good? 2 gear shops I've dealth with weren't fond of Richmond, but there aren't many alternatives. Noise may not be an issue on the truck though? My vette rear end is practically inside the car and it sucks hearing it all the time that I'm not on the loud pedal.

[mnunn454]

I think now my mileage estimates above were a little high, given your speed requirements. Our trucks are about as areodynamic as a breadbox, so pushing them through the air at 70+ MPH carries a huge mileage penalty.

Still, with a decent a/f ratio at cruise, I'd think you can do a lot better than you are. I'd still start with the carb for sure, but the rear gears seem now to play an equally important (albeit more expensive) part of the equation.

EDIT 20 SEPT

Here's the reasoning behind my belief that greatly improved mileage can be had with a good carb tune (not just a simpe rebuild).

It's a Chevy High Performance article from a year or so ago called "Old school meets new school". It's on-line at
http://www.chevyhiperformance.com/te...age/index.html

In short, using some tech tools, this guy has a 500+ HP SBC and adjusted his Mighty Demon carb's idle air speed and high speed air bleeds to increase the operating range (RPM and load) of the idle circuit and delay the onset of the primary's main metering circuit. Result was smoother operation and 21 MPG on the trip home following a drag racing outing (granted he has an AOD tranny).

It's a technical article, so grab a cup of coffee before you sit down to read it. I think once you read it you'll agree that a lot can be done to improve mileage as long as you can see what your air/fuel ratio's doing.