Better fuel economy out of overdrive

[Big GMC truck]

My 1984 GMC K1500, 305, TH700R4, RV cam, rebuilt carb, and Edelbrock intake, does 1300 rpm at 65 in overdrive. I'm assuming that this is due to numerically low differential gearing. I'm getting 11 city and 17 highway. Is this what I should be getting? Due to the large load on the engine, do you think that it would get better fuel economy if I kept it in third?
By the way, how do I identify what gears I have?

 

[Chevy305]

1300rpm at 65mph that is really low. I am willing to bet you have a 3.08 ratio, but it might hepl if we knew the tire size. Also 17mpg is amazing for a Blazer! I get like 9mpg with my K10 with a 305 and the SM465.

Also I really hope you have a tranny cooler because you must have large tires and stock ratios. Both of which equal burnt tranny.

 

[Big GMC truck]

Sorry, the tires are 31 inches and it's a half ton longbed truck (5000 lbs).

 

[CyberSniper]

Sounds like you have 2.73s.

While it is gutless, you're kind of inbetween points for gas mileage. 3.08s and 31s is about ideal for highway fuel economy. Overall fuel economy 3.73s is best with 31s.

When driving around town, keep it in third gear. On the highway, it depends on the load on the motor. I know when I had 2.73s I got the same fuel economy in third and fourth. With 3.42s I get a tad better fuel economy in fourth but still have no power.

The best fuel economy I ever got was with 3.42s and 33s... 19mpg at 63mph.

 

[loafer]

The best fuel economy I ever got was with 3.42s and 33s... 19mpg at 63mph.

Did you correct your mileage for the odometer discrepancy when you figured your miles/gallon?

I assume your stock setup was 31's w/3.42 gears? In that case your correction with 33's and 3.42's would be:

19mpg x (31/33) = 17.8mpg

This is still much high than I ever got; I used to get 12-13mpg highway with my K5 and 33's, 3.42's, 350 TBI, 700R4. This is also only going 65MPH. If I drove 75 it would drop to 10-11mpg.

 

[CyberSniper]

Did you correct your mileage for the odometer discrepancy when you figured your miles/gallon?

I assume your stock setup was 31's w/3.42 gears? In that case your correction with 33's and 3.42's would be:

19mpg x (31/33) = 17.8mpg

This is still much high than I ever got; I used to get 12-13mpg highway with my K5 and 33's, 3.42's, 350 TBI, 700R4. This is also only going 65MPH. If I drove 75 it would drop to 10-11mpg.

Yes, that is corrected since I did all this using a GPS. The truck originally came with 235x75r15s and 2.73 gears with a 305/700R4. It got decent mileage with that setup and sharply went down with 31s. With 33s and the original motor with 2.73s it wouldnt' even hold overdrive.

When I had a warmed up 350 and 2.73s/31s it got really good gas mileage. Even around town it got 12mpg.

With my heavily modified 305 (190hp@rubber) the best I ever got was skinny 33x12.50 Mud Kings and no lift with 3.42s. That's when I got 19mpg. It was pretty gutless in overdrive. Any kind of acceleration made it unlock the torque converter. It's like driving a 6.2L diesel.

Once I lifted it and put wider tires on it the gas mileage went down 2mpg to 17mpg. This is all at 63mph (measured with GPS). At 80mph the truck gets 10mpg irregardless. Towing it gets 10mpg whether going 55mph or 70mph. Around town, if I baby it, I get 12mpg. If I drive like I normally do (Mario Andretti) I get 9-10mpg.

 

[Blue85]

I'm getting 11 city and 17 highway. Is this what I should be getting?

No, it generally should be lower

 

[Chevy305]

Was the 2.73 even available for an 84? I thought the standard was 3.08?

 

[CyberSniper]

You could get 2.46s in 84.