Actual overdrive gain

[70c10stepside]

Hey guys, any of you do an od swap? What was the actual before and after mpg gains? Put this one here to get a more accurate number,being we all have 67-72. Really asking because my turbo400 is starting to slip bad going into second. And would like t56 or 2004r,but would REALLY like to put a m20 Muncie back in it like it once had when dad had it. I understand the m20 is the wide ratio and blah blah. What was your true number gains? How long will it take for that extra cash to pay off? Lol I do have a 4.11 right now with a very happy 355. With the turbo 400 I CAN get 13-15 mpg driving back roads. With my motor, I think its impressvive. Anyone able to answer this?

[Dm1pfoy1]

I dont know how much it helps but with th350 getting about same mpg on 383. Have you looked at borg warner manual trans like t10 or super t10 they are very similar design to muncie with lower $$ gives you the manual trans you want with a wide selection of ratios. So in ideal world should work to give you all you want and keep cost low to make payoff sooner.

[RManson]

307ci with Saginaw 3-speed: 12 MPG.
307ci with T5 trans and 500 cfm Holley: 18 MPG.

Completely changed the dynamic of the truck as I was able to drive it on the highway without fear of being run over and it got better mileage to boot.

[hugger6933]

I don't have any numbers to give you as my trucks are still in the processes of being built one will be a small block 327[or 350]/T5 one will be a 454T400 and one will be a 454/4L60E. yep, I have lost my mind I have three builds mostly going on at the same time but I am pretty much use to it. It is gonna take a LONG time for you to recover the cost of the upgrade to the OD trans, unless 1, you just want one 2, it will help your motor live happier on the road due to deep gears or 3 you drive a bunch. you didn't say your current mileage now but I would hazard a guess in the 12 range. In a perfect world the best you could hope for from a OD trans would be like 4 or 5 mpg but that is a lot

[rsavage]

A m-20 is just a 4 speed with a final drive ratio of 1:1. It isn't an overdrive so it would not offer any improvement in mileage. It would probably be cheapest to install a 200 4r or a 700 r4 auto in your truck since you are already set up for an auto.

[Zeke's Garage]

With your 4.11 gears the Muncie is going to be the most fun, and even though the final drive is 1:1 just like the T400, you're going to get better mileage over the lossy automatic, especially when cruising without a lockup torque converter. While the 200 or 700 R4 route is a good upgrade solution, with your 4.11 gears the first gear of those autos will be useless, they were made with 3.73 or numerically lower gears in mind. The T56, while I'm unsure of the first gear ratio I think would be the best for a manual conversion, may drive the price way up for the cost of the manual conversion.

[leftybass209]

If I had to do some ballpark averaging from all the realistic numbers I've seen, including mine: 5-7 mpg's over 50 mph with carburation. Tbi could probably push that in the 7 - 10 mpg range with great tuning. Of course rear end gears and engine size and performance parts could bury those numbers on the low end.

[Longhorn 70]

I went from 12-13 to 15-16 when I changed from TH400 to 700R4. That is whith having to do math and add in a 10% speedo offset when going to the 700R4. The engine works a LOT less at freeway speeds. Went from 3100 RPM on the freeway to 2000 RPM or so when using the 700R4 and converter locked. With the 700R4 I get out of the hole quicker too. I have either 4.11 or 4.56 tail gears.

Just a note: TH400s used to last me 2-3 years. The built 700R4 has been in since 1997...

[slow-1]

I got 11 mpg with the original Saginaw 3 on the tree , I get 17-18 mpg with the 700r4.

How long it will take to pay for it will depend on how much you drive.

15 gal (about what you use out of a fill up) x 18 mpg = 270 miles

15 gal x 11 mpg = 165 miles

270 - 165 = 105 more miles per tank

105 / 18 mpg = 5.8 gal x $3.50 = $20.30 per tank that you save every time you have to fill up.

Fill up once a week...$20.30 x 52 = $1055.60 per year savings.

[67ChevyRedneck]

350/700R/3.73.

Was previously 250/3spd man/3.73

Original set up was 11mpg no matter what. City, highway, didn't seem to care.

The new set up is mostly 12-13 mpg (I have fun when I drive it.) That's mostly around town cruising, with a good bit of low speed 3rd gear driving.

I get 16 on the highway.

I did mine more for drivability. I've put over 35K on my set up since 2003. It's a great combo.

[Keith Seymore]

Originally Posted by Longhorn 70

I went from 12-13 to 15-16 when I changed from TH400 to 700R4.

Originally Posted by slow-1

I got 11 mpg with the original Saginaw 3 on the tree , I get 17-18 mpg with the 700r4.

Good data.

70c10stepside - one other trans we might suggest would be the New Process A833.

http://www.customclassictrucks.com/t..._transmission/

It was actually a MOPAR trans but was made available in the "squarebody" C/K/R/V versions with third gear as 1:1 and 4th gear set up as a .73 overdrive. Comes complete with a factory Hurst floor shifter.

K