How to getter MPG? Switch gear ratios?

[CHEMPOWR]

Hey everyone,

I am getting about 12 MPG on the highway at 70 mph (RPMs at 3000). Is that what most are getting. I have a 69 GMC short bed with 400 small block turbo 400 trans and 373 gears. Running edelbrock 600 carb.

Do you think running 308 gears may help or should I look somewhere else to get better mileage.

Thanks

[EA69TRK]

not sure about changing gears but what i have seen most do is switch over to a tranny that has OD like a 700r4 of the 200r4.
Personally i hav exp. only from the 700 its a great tranny if built correctly and has the tv cable adjusted correctly I hope this helps.

[cdowns]

alignment tire pressure synthetic grease in wheel bearings synthetic lubrication in tranny rear and engine replacing worn suspension front and rear are but a few things that will help// the 308s wwould help too//best is keeping the 373 and going for the 4speed overdive tranny just for the lower rpms and reduction in power loss from the 400 tranny

[Longhorn Man]

12 is the average on these trucks, but I'm sure you could get some with a 3.07 gear swap, and that would be a lot cheaper than a trans swap

[nicolas72gmc]

I get 11.46 mpg with my 3.07's, I don't think you'll get a whole lot better by switching to them. BTW, I have the 330 hp gm crate, 600 Elde, and stock 350 trans. My mpg is calculated with "a little bit of everything" driving. (in town, hwy, heavy foot, light foot, etc.....)

[CHEMPOWR]

Thanks everyone. This was the info I was looking for. I think I will leave it as is for now since everything is working good and I probably won't get much better gas mileage.

[csa daddy]

i had a 68 longbed i put a 700r4 in an it had a 350 with came holley strip dominator intake with 373 gears an i got around 17 or 18 miles to the gallon

[toddtheodd]

Put a 2 inch block under the accelerator pedal...

[rscam]

I have an NV3500 with 3.07 gears behind a 355 with about 300 HP. I run 1900 rpm @70MPH. I have not taken my mileage on just hwy driving. I mainly do some small town driving and country road driving and I have run 3 tanks through it 12.49, 12.95 and 12.04 mpg. The last one was running in 4th gear because I was told I might not be in the optimum RPM for gas mileage. I imagine if I did just hwy driving I could get 14-15. None of my calculations are with CREATIVE math either.

[Davetopay]

On long trips my k series will knock down 15-16 in warm weather. In town, in the mountains, and loaded....it will vary between 5/6mpg to about 11mpg.

[capev86]

replace that trans with an overdrive. i know that a 3.08 gear sucks in truck. a 3.73 x .7 overdrive gear will give you about a 2.60 ratio which is better on the highway than a straight 3.8. the 200-4r is a direct bolt in but the 700gives you a more aggressive first gear than the 200,350 or 400 which is good for takeoff. 3.73 gears are good for compromise between towing and cruising and if towing is what you do most of the time, you might as well stick with the turbo 400. remember that regardless of gearing that most cars get best fuel mileage around 50mph. at 70mph you are sucking a lot of gas to push through the air because trucks are just not very aerodynamic. if you kept it at a nice steady 55-60 you might see closer to 14-15mpg. my dad can get 18 out of his suburban with 3.73 gears (2wd) and overdrive. my 72 burb gets 10-11 with a 4.57 gear and keeping it well below 60. i will be swapping the 4.10 third member into the axle and adding the overdrive unit to the saginaw 3 on the tree and that should allow me to cruise at 60-65 while getting 16-17mpg.

[rcrahn]

Look into a Gear Vendors Underdrive/Overdrive....
This should improve gas mileage 22-28% plus reduce wear and tear on your engine/drivetrain. Check out link to GM trucks with 3 speed autos.

http://www.gearvendors.com/cg2wd3s.html

[LMN8UR]

I usually get 15 around town, and I've broke 19 on the highway with my setup. I really don't know how much I'm giving up off the line, when I bought the truck it had the 2.76 rear and a 2.8 first gear Saginaw three speed. Now that I have the NP833 with a 3.09 first she comes off the line better, not ready for the drag strip but on the highway she's only turning around 1400 at 65. I have no idea what the top end would be, I'm not that CRAZY!

[capev86]

you can get a good Od trans for a few hundred to a grand whereas the gear vendors will automatically require mods to the driveline and costs many years worth of money in gas savings! plus, they only have a .78 ratio vs. the factory .70.....not quite as nice for solo driving. they are rugged units, but your for financial and simplicity reasons, a trans swap is the cleaner way to go!

[Boog]

The most cost efficient method to increase fuel mileage is to slow down 5 or 10 mph. Like that's going to happen.
If you are considering a gear vendors type od unit there is a thread just a few pages back where one of our members recently put one in his c20. NOT just a simple bolt in deal. Check him out.