Meaning best mileage. Anybody try to keep an RPM, or just MPH? On the open road, I do the speed limit plus 5 MPH and haven't really tried for better mileage. Just curious because I'll be driving to Colorado with a light load and will have plenty of hours to experiment.

 

When my truck had all the parts on it the best mileage I ever got was at exactly 65 mph on the highway. Any more or any less and mileage went down. Now that I did the deletes I really don't know where it's at because I have not been on the highway in the last couple weeks. I know that cruising around town at around 45 mph gives me really good mileage as long as I don't need to keep stopping....

 

Wind resistance is the biggest factor affecting mileage on my truck. I always get the best mpg's (over 20) traveling through the mountains where speed is limited to under 60 mph. Up and down over several mountain ranges seem to balance each other.
Very seldom get over 20mpg on the flatland where speed limits are above 65 mph and there is always a head wind coming and going.
The lowest rpm always produces the best mpg's. If I can keep rpm's under 1500 mileage will be over 20mpg in most situations. I do not tow or carry a heavy load.

 

My best MPG is in 6 th gear and RPMs are between 1600 and 1800

 

Agree - between 1600-1800 is the sweet spot.

 

I run based on RPMs and not speed due to having 3.42 gears and 35" tall tires. I run it right at 1600RPMs and have gotten my best mileage there. I saw a dyno sheet of a stock 6.7L once and it showed that the 6.7L reaches max torque at 1600rpm so this makes sense. 1600 RPM for me is 63.5 MPH, GPS verified. If i drop to 1500 RPM the mileage drops due to lugging the engine. If i raise to 1700 RPM the mileage drops as well. Trucks with different gearing might find this to be not the case though.

 

Thanks for the replies so far. RPM is what it is regardless of gearing, tires, etc.

 

Yes but with numerically higher gears you would be going considerably slower at 1600 RPM. I don't think driving 50-55mph on the freeway is feasible, 64mph is fine around here. I get passed a bunch but am not driving dangerously slow. Also the larger tires create considerable more load at low RPM lugging. A stock tire'd truck might be able to still get good fuel economy driving below the max torque peak but my truck doesn't like it. That is the only reason I mentioned those variables.

Although, as far as the gearing is concerned, i read somewhere that the final drive ratio of a G56 manual transmission equipped truck with 3.42 gears was just about equivilent to a 68RFE automatic transmission equipped truck with 3.73 gears. So, my point might be moot on a 6.7L truck.

 

It's been a while since I was on the highway but I think I'm somewhere around 1600 rpms at 65 mph too...