Since You are talking about 700r4's and gears

[348tripower]

I got me this 300:1 gear with 29 inch tires and a 700r4. Seems to me, by reading the comments here that I need a different gear.
This truck gets terrible gas mileage. And it seems to be working hard with or with out the trailer.
Towing the teardrop trailer in drive about 11.5mpg. Around 2300/2400 rpm.
In OD only about 13 to 14.
The motor is a 350 with 202 heads and a 600 Edelbrock that I have leaned up.
 What do you guys think?
Thanks,
Don

[papastoyss]

I had a 'glas 37  3 window W/ 307 rear & 700r4 behind a 93 LT1 engine. Itwas a dog at medium speeds on highway, 60 -65 mph. I changed to a 370 rear gear & was amazed at the difference in performance w/ no loss in gas mileage. It seemed the 307 rear was lugging the engine.

[Learpilot]

I had a 'glas 37  3 window W/ 307 rear & 700r4 behind a 93 LT1 engine. Itwas a dog at medium speeds on highway, 60 -65 mph. I changed to a 370 rear gear & was amazed at the difference in performance w/ no loss in gas mileage. It seemed the 307 rear was lugging the engine.

I have the same problem with my 3.00 gear. Bad gas milage and lugging. I get about 14 MPG at 65 MPH and 22 at 75 MPH.  I am going to change to a 3.55 gear. I have the third member built , but I am working tooo much to have time to change it. One thing I have started doing is to not run the lock-up converter until I am on the highway. I seem to get up hills better.
Rick Harris

[Crosley.In.AZ]

what can happen with a carb and the over drive trans  with gears that MAY be too high,,,

you get into OD ..... the engine RPM may drop a little too low.  You step on the gas pedal a bit more to maintain the speed....

 Vacuum drops a bit , the enrichment circuit opens slightly and gas mileage goes down.

With electronic EFI , the vacuum may drop off , but the computer controls the fuel mixture with feed back from the oxy sensor.

[tomslik]

hey, at 60 mph, what are you turning in 3rd?
shoot for 3000 rpm or maybe just a touch more.

[348tripower]

hey, at 60 mph, what are you turning in 3rd?
shoot for 3000 rpm or maybe just a touch more.

Tom,
 I am turning about 2600. I will check it again. I really appreciate everybodys replies to this post. It will send me in the right direction.
Thanks,
Don

[tomslik]

hey, at 60 mph, what are you turning in 3rd?
shoot for 3000 rpm or maybe just a touch more.

Tom,
 I am turning about 2600. I will check it again. I really appreciate everybodys replies to this post. It will send me in the right direction.
Thanks,
Don

yeah, you need another500 rpm....
one other thing, you didn'tr go too lean did ya?
too lean= lousy milage, holes in the pistons, overheating....

[Topsterguy]

I've got a 700 in my 32 coupe with 3:55's and 29" 255/75x15 tires and it seems  perfect to me. I think it's about 2000 or maybe a little less at hiway speed in OD.

[Leon]

With the OD I think I may be going too slow since when I tow I have to keep the speed down till I get out of California.  I get 20 MPG solo and 15 to 17 towing, but I'm also going faster when I'm not towing.  If I'm out of state and can tow at 70, I get 17-18 MPG but 15 MPG at 55-60 MPH.

[Crosley.In.AZ]

Low RPM on the engine does not necessarily equate to high MPG.

[348tripower]

I drove the truck this morning. I am turning 2600 RPM at 60MPH in Drive ( third gear). Using the online calculators, to gain 500 rpm, I would need a 4.46 gear. The calculator says I have a 3.73 now? Is this really what I am looking for? I will do some more checking tonight to verify the findings.
Don

[enjenjo]

No. You should be pretty close now with that gear. I would start looking at the tune.

[1FATGMC]

............ Is this really what I am looking for? I............Don

NO, NO, NO.

If you are running a 3.73 or 3.50 gear you are right where you need to be.  I agree with Frank look elsewhere.  If you had the 3.00 gear you first posted then I would say you are too high.

What is the engine combo, heads, cam, etc.??

Sum

[tomslik]

............ Is this really what I am looking for? I............Don

NO, NO, NO.

If you are running a 3.73 or 3.50 gear you are right where you need to be.  I agree with Frank look elsewhere.  If you had the 3.00 gear you first posted then I would say you are too high.

What is the engine combo, heads, cam, etc.??

Sum

i dissagree, 4.10's would be better,4.56's might be better yet BUT it DOES depend on engine setup.


my cpa has 31" tall tires on his truck and we went to 4.56's, truck was happier with it unbtill he blew up the posi, went to 4'88's this time.
truck got better milage with the 4.56's over the 3.73's....BUT it has a LOT of camshaft.....

[Charlie Chops 1940]

We're not getting anywhere fast with this analysis, are we?

Don, give us the full details on this truck. Is it a half ton hotrod or a 2 ton mama? Engine, cam, carb, etc. Everyone's just taking shots in the dark at this rate.

Charlie