Fuel Milage Question

[1971ChevyTruck]

I have two rear ends...I have a 3.07 and a 3.73. I am running a 350 with a 350 turbo basically stock everything with a 600 carb. Anyone have close to this etup with either rear end? What gas milage did you get. Oh its a 1971 Chevy 1/2 ton 2wd.

[WorkinLonghorn]

I've had both rears with 350 ci and it seems like it's hard to average more than about 12 with the 3.73 and with the 3.08 I got the same (different truck).

[1971ChevyTruck]

Wow only 12?? I have a 1979 Chevy 4x4 3/4 ton and I get roughly 15-17 with it no matter what.

[WorkinLonghorn]

What's your set-up? And what about your '71?

[1971ChevyTruck]

Basically stock everything...3.07 rearend right now...

[Cannon007]

ive got a 3.73 rearend and I got about 18mpg...but there is a catch I only went 40mph tops and RPM's stayed under 2200 cause my engine has some problems geting passed 2600rpms, some vacuum leaks i believe but anyway its also a tuned port injected topend from an 86 vette on a 406ci sbc. If you do highway driving you will see a huge advantage in gas mileage with the 3.07 gears

[mnunn454]

If mileage is your concern, I think I'd approach this in 4 areas:

I'd opt for the 3.07 ring and pinion,

I'd spend a lot of time with an air/fuel meter adjusting the carb so I was getting 14.5 to 14.7:1 air fuel ratio at highway speeds and conditions.

I'd make sure my exhaust system is very low restriction and that the headers (perferred) have the optimal length and diameter primary tubes to deliver the torque at low RPMs that you need.

I'd look for a TH350C tranny that has the lockup converter and upgrade to it when budget allows. OR, if you went with a TH700 tranny, then the 3.73 R&P would be fine, plus it comes with a lower 1st gear (eliminating the stall converter need) and it has a lockup converter like the TH350C.

If I wanted performance plus mileage, then I'd lean more toward the 3.73/TH700 combo but I'd still do the exhaust and the carb fine tuning.

JMO.

[Ackattack]

I got a best of 13mpg with 3.08s, TH350, and a cammed 327. Usually more like 11.5.

Now with the TPI 350, 700R4, 3.73 I've gotten 18.5

[piecesparts]

There are numerous things that will affect your mileage and I did not see any reference to the tire size in your discussions. The change of a tires diameter by just a inch will affect your engine's revolutions by as much as 200 RPMs. I spent some time with my 84 and my son's 86 Chevy trucks trying to get things straigtened out with OD trannys. During this time, I found that the SBC engine is greatly satisfied at around 2200 RPMs on the road. That puts it in it's general power range and allows it to overcome hills and traffic without working the engine more to maintain speed. The combo of a 350 engine, 350 tranny, and a 3.08 geared rear end is the most accepted, due to the RPMs achieved from that setup. A change from 3.08 gearing to 3.73 gearing will bring the engine speed up 500 RPMs----this is using the same size tires and same tranny output. If you are running at 2200 RPMs now, then you would be running at 2700 RPMs

I run my 84 truck with a 700R4 tranny and a 3.73 geared rear end at 2500 RPMs (at 70 MPH) on the open highway. I run a lower profile tire (around 27") and I get approximately 15 MPG. When we put the 700R4 in my son's truck the highway RPMs went to 1800 and the truck had to work harder to overcome hills and maintain speed on his cruise control. His gas mileage went to 11 MPG. We installed a set of 3.73 gears and his RPMs went up and so did his gas mileage, he n ow gets around 14 MPG. There is defintiely a breaking point in this game.

[lilpup69chevy]

yeah i have to agree with that just because you put in a higher gear does not mean you are gonna get better gas mileage especially with an od transmission, i've played around with the same mess on 4x4's as well, in my 84 gmc swb 4x4 6inch lift and 35's, goodwrench 350 and th350c lockup trans with 2.73 gears, (yep that was stock lol) it was a dog and it only got like 8 to 10, left everything else alone switched to 3.73's and the truck was a whole different animal it would actually do a burnout and even spin the 35's with ease, plus my mileage went from 8-10 to 12-14 mpg and this wasn't all highway driving either

[Longhorn Man]

I'm always trying to tell ppl that on here, but too many think it'd BS, so I stopped for the best part.

[mnunn454]

Originally Posted by lilpup69chevy

yeah i have to agree with that just because you put in a higher gear does not mean you are gonna get better gas mileage especially with an od transmission.......

I certainly agree.

There's a "sweet spot" for engine efficiency that is different with (unique to) every setup. Above that spot (revving higher than necessary), you're pulling excessive CFM (air+fuel) through the engine and running mostly on the carb's less well metered main jets. Below that sweet spot (lugging), the low vacuum is mixing in more fuel than is needed.

It's usually easy to find the best spot while cruising with a vacuum gauge and a wide band air fuel meter. What you're looking for is the highest possible manifold vacuum at cruising conditions while maintaining an A/F ratio in the 14.7 range. EFI vehicles do this for you of course.

There's a neat article if you haven't read it in CHP Mag a year or so ago called: "Old School Meets New School :How to Get EFI-Quality Mileage From Your Carburetor (By Stanford Curry)" . It's located at
http://www.chevyhiperformance.com/te...age/index.html

A few more How-to's are located at Innovative's website at: http://www.innovatemotorsports.com/r...FI_mileage.php

While all the tinkering takes time, and the WB02 setup isn't cheap, I learned a lot personally, and if you do this to a carb'd daily driver, the payback would likely be less than a year through reduced fuel consumption.

So to answer the question originally posted, either rear end could work if it's results in the engine running at its optimal efficiency at highway cruising speeds, but my bet is it would most likely be with the 3.07 gears using the TH350 tranny or the 3.73 or 3.42 gears using the TH700 tranny.

[1971ChevyTruck]

Thanks for all the help...I have done some checking and I do have 3.73 gears...I found the stamp on the gear...My tires I have now are 255 60's which at 65 mph I would be turning 2900 RPM. I want to drop this so I need to go from a 28" tire to a 30" or 31". If this is right I should get down to 2600 RPM. Now my question is if I switch to a 700R4 tranny how do you wire that tranny. I think that will be my best bet since I can't get a 30 or 31 inch tire in the style I have on the truck right now and I can get my hands on a 700R4 tranny pretty cheap.

[mnunn454]

The 700R4 will drop your RPMs by 30% when in overdrive. So 2600 X .70 = 1820 RPMs. That seems a little low to me (but as I mentioned above, it might not be, depending on where your engine runs most efficiently). 2900 RPMs X .70 is just over 2000 RPMs and that should be a little closer to ideal, I'd think. So don't run out and buy those taller tires just yet. You may want to keep the ones you have.

Regarding "wiring" the TH700, there's only 1 wire to worry about and that's the one that locks up the torque converter. It's a 12 volt supply wire that you can run from the key-on side of the fuse panel. Most people run it (in series) through a cruise control type brake switch so that TCC lockup disengages when you step on the brake. Next stop is there should be a 4th gear switch inside the tranny that only allows lockup in 4th gear. You'll want to verify it's there and working "before" you fill the tranny w/fluid. Additionally in my Elky, I ran my 12 volt supply wire through a vacuum pressure switch so it would disengage when I was passing but not aggressively enough to warrant a downshift to 3rd.

If no one's mentioned it so far, there are a couple of other considerations when switching from TH350 to TH700.

Unlike the TH350's kickdown, the 700's Throttle Valve (TV) cable is a bear to get adjusted correctly. Read as much as you can on this. Adjusting it incorrectly can shorten the life of your tranny.

Also, I'd shy away from the early TH700's because they're not as strong. Get one that's at least an 89 or up (someone please verify the year).

Finally, the 700 is a longer tranny so the driveshaft will need to be shortened and the tranny cross member will need to be moved aft a little.

Read more about these mods by doing a search of our forum threads or searching the web, or by visiting http://www.chevelles.com/techref/ftecref11.html or even researching several vendors' websites like www.bowtieoverdrives.com and similar.

Good questions and good luck.

[1971ChevyTruck]

I found a 700R4 out of a 1989 Chevy truck. How do I check to see if it has the 4th gear switch anyone have any pictures or anything?? I am ordering the wiring kit and stuff from B&M.

[Dova]

I have a 327 with 3.08's and at 65 i get 14mpg with the TH350. 12mpg at 70-85 and 10mph anything higher than that

[mnunn454]

Originally Posted by 1971ChevyTruck

I found a 700R4 out of a 1989 Chevy truck. How do I check to see if it has the 4th gear switch anyone have any pictures or anything?? I am ordering the wiring kit and stuff from B&M.

http://700r4.com/tech/tcc/internal_install/700r4.shtml

When you get there, you'll find the links on the left answer most of your questions.

Good luck.

[piecesparts]

Originally Posted by mnunn454

The 700R4 will drop your RPMs by 30% when in overdrive. So 2600 X .70 = 1820 RPMs. That seems a little low to me (but as I mentioned above, it might not be, depending on where your engine runs most efficiently). 2900 RPMs X .70 is just over 2000 RPMs and that should be a little closer to ideal, I'd think. So don't run out and buy those taller tires just yet. You may want to keep the ones you have.

Regarding "wiring" the TH700, there's only 1 wire to worry about and that's the one that locks up the torque converter. It's a 12 volt supply wire that you can run from the key-on side of the fuse panel. Most people run it (in series) through a cruise control type brake switch so that TCC lockup disengages when you step on the brake. Next stop is there should be a 4th gear switch inside the tranny that only allows lockup in 4th gear. You'll want to verify it's there and working "before" you fill the tranny w/fluid. Additionally in my Elky, I ran my 12 volt supply wire through a vacuum pressure switch so it would disengage when I was passing but not aggressively enough to warrant a downshift to 3rd.

If no one's mentioned it so far, there are a couple of other considerations when switching from TH350 to TH700.

Unlike the TH350's kickdown, the 700's Throttle Valve (TV) cable is a bear to get adjusted correctly. Read as much as you can on this. Adjusting it incorrectly can shorten the life of your tranny.

Also, I'd shy away from the early TH700's because they're not as strong. Get one that's at least an 89 or up (someone please verify the year).

Finally, the 700 is a longer tranny so the driveshaft will need to be shortened and the tranny cross member will need to be moved aft a little.

Read more about these mods by doing a search of our forum threads or searching the web, or by visiting http://www.chevelles.com/techref/ftecref11.html or even researching several vendors' websites like www.bowtieoverdrives.com and similar.

Good questions and good luck.

The 700R4 TV cable is a little more of a consideration on adjustment, but it is not a major change from the "Kick down cable" on the 350. If it was so bad, it would not have lasted this many years. You just have to make sure that it is not engaged at normal driving conditions. There are numerous companies that sell connection sets for wiring or go to Street and Perfromance for a "TECH" read on the steps to do so.

The change out of the tranny may not be as complicated as you think, the tailshafts are different on some models, but not all. On my 84 and my son's 86 the changeout was like for like in length. The older model trucks may have to consider a driveline redo. That is not a bad thing, It might be a good time to check the u-joints.

Definitely look for the newer models of 700 R4 trannys. The early ones were JUNK by any right. Also most 700 R4 trannys will have some wear in the valve body parts and a change to upgrade the shifting of those parts is a good idea. I would recommend a shift kit, at a minimum.