3.08s and overdrive?

[speesh]

My 71 burban has the famous 3.08 rear gears. Is there any point to going with a 700r4 tranny? I would like to save on gas if possible, but not if its only .002 mpg! So would it still be worth the tranny swap or not?

[SWEET7T]

it would help if you swapped to a 700R4 AND swapped your rearend to 3.73's! 3.08 is just too high a gear with overdrive, unless your cruising speed is 90+ so the converter will lock.

Hope my file attachment works... Has some interesting stuff about the different GM trannies and rearends so you can figure out what you want. I found the page on the net quite awhile back and I just saved it, I don't have a clue what the URL is for this place anymore

[Bowtie67]

Newer cars are running OD with a 2.98GR Your setup will work but going into OD your RPMS are gonna drop to about 1500RPM you wouldn't be able to tow anything, haul any real heavy loads. The nice part about the 700R4 is the nice low 1st gear which you dont have with the 350 or 400 tranmisions. 373 gears do work well with the OD, your cruise speed should be around 2000 depending on tire size. JMO

[MrGoodpliers]

I think most trucks now with OD's are equipped with 3:42's.

[sam's 72]

I had a 83 chevy truck at one time, 700r4 with 2.73 gears in it, would not pull good at all, but it got really good milage,

[krue]

Your setup will work but going into OD your RPMS are gonna drop to about 1500RPM you wouldn't be able to tow anything, haul any real heavy loads.

You really shouldn't tow anything heavy in OD with a 700r4 unless you are looking to replace it. Most everybody I know that has had one die on them was towing in OD. Anything heavier than an empty trailer and I just tow in drive.

[stelth2002]

I wouldnt and i dont know any cars really that have overdrive and a sub 3:1 gear ratio. Most of the cars today have smaller displacment engines that are suppose to be wound up a little higher than a bigger cube like a 350. Trucks mostly have 3.73 gears unless you get to diesel ones. As for putting in an OD tranny with those gears i think you might actaully loose milage depending on your driving. If you drive in town much it will be worse. However, if all you do is flatter freeway, then you might benifit from it. With a stock motor, you will be downshifting from od on a hill though. And person opinion, i wouldnt.

[Longhorn Man]

I agree, over drive and 3.07 gears will hurt your MPG.

[Blazer1970]

Originally posted by sam's 72
I had a 83 chevy truck at one time, 700r4 with 2.73 gears in it, would not pull good at all, but it got really good milage,

Yup, for a few years there they built a bunch of half ton trucks with the 700R4 and 2.73 gears. A lot of them probably had 305s also.

[lukecp]

One of my friends has an '86 K10, 305/700R4/3.08 and 32" tires. It is a DOG.....runs 18.4's in the 1/4 mile with a Gtech, lol. However it does idle down the highway.....still only gets around 10mpg, but the engine is tired.

[Huck]

Hate to disagree with all of the above but----. The stock rear for a 350, auto truck is 308. And they run pretty decent. The difference between an overdrive and a stock turbo 350 at 65mph is about 900 rpm. If you did things to improve the engine performance like ---HEI, Headers, Dual exhaust, better Carb and intake----your engine will pull up any interstate highway hill in OD with no problem. Now if you pull trailers, you may have to downshift or run in 3rd gear verses 4th.
You can expect to see a 2-3 mpg savings with OD on the highway. Around town a 700R4 may help mpg a bit but the real bonus is on the open road saving those 8-900 rpm.
A 342 rear will allow for better pulling as you will be higher on your rpm curve.
My .02 based on personal experience.

[FirstGen]

My R/T has 3.92's with a 42RE tranny and a .69 overdrive. It put down 500 hp and gets about 8-12 in the city and NO LIE 20 mpg on the highway if I set it at 70mph!!

[stelth2002]

I believe that it gets that milage, its all about matching the stuff right. Have you ever dynoed your dodge?

[FirstGen]

512 and 486 ft-pds to the rears = 11.86 @ 126mph

[86k10]

Speaking from my experience, 3.08 and overdrive don't mix except if you are on level ground with a good wind behind you. The truck in my sig used to be a 305 with 3.08's and the difference in mileage wasn't much from about 17 in 4th to 16 in 3rd. not enough to matter, but at least in 3rd it would run pretty decent. I wouldn't go with anything less than 3.73 with a 700, especially with a small block chevy since they don't have any power unless they are wound up a little.

[rn2fine]

I have a Burb C10 with the following:

- 350 with a mild roller hydraulic cam
- Edelbrock E-Tec 170 Heads (Vortec style)
- Headers
- HEI
- Edelbrock 600 cfm carb #1406
- Performer Vortec Manifold
- 700R4
- And 3.07 Gears

It gets 15 mpg. The 700R4 has a steep 1st gear so the 3.08 will work well with it off the line. I have no complaints about not having enough power. RPM's are low on the freeway but my engine setup has a lot of torque. I am pleased with the 700R4 and do not regret it. I hope to put a TPI on the motor some day now that GM has a TPI base for Vortecs. I am hoping to get 17 to 18 mpg with the TPI, and gobs more power too. What kind of mileage are you getting now?

[speesh]

last time I checked it was about 15 mpg... which seems pretty good for these old trucks! That is with a 3 speed manual, which I am wanting to change to an automatic so my wife can drive it. ( o.k., because I am too lazy to shift.... ) I also have an edelbrock carb and manifold, dual exhaust, k&n air filter, and H.E.I.... I wonder how much mileage I will lose if I go with a turbo 400? I know a 350 is more efficient and will bolt in easier, but I already have a rebuilt 400.

[bpmcgee]

Originally posted by 86k10
<snip>especially with a small block chevy since they don't have any power unless they are wound up a little.

Brian

[Dean]

My El Camino SS has a 2.41 rearend...I'm ready for the salt flats....lol

[sixty8c10]

i'm going through this dilema right now and so far everyone i have spoken to about it has told me the same thing 3:42 or lower anything higher will lug the engine thus you would really not being saving any gas

[lukecp]

Originally posted by bpmcgee




Brian

I agree here....a stock SBC is an excellent low/midrange engine. They usually peter out somewhere after 4,000 rpm in stock form.

[nate68]

i'm surprised no one has mentioned cam selection. stock cam 350 will prolly be fine with OD and 3.08 gears. if you're going to go with a bigger cam then definately change the gears as well. i'm running 3.50 gears with a 700r4 (406ci) and i wouldnt run any taller gears. the sb 400s have the extra torque to handle the taller gears.
i went from 10mpg to a best of 13 when i changed the tranny. my mpg is pretty sensitive to timing. town driving is what kills me though.