5,000 pound diesel, 26mpg at 65mph
 

Been a while since I have been on here, but I thought of you guys today. Just got my 89 Dodge diesel back today, was busy getting a 4spd trans (47RH) with a lockup converter installed. Also running a whopping 27 degrees of timing from idle on up. Truck sounds like it's going to blow up pretty much all the time :D.

Anyhow, was crusing at 65mph or so with the window down, no aeromods, and a rocking chair and table in the bed (not so good for aero). It got 26mpg, backed up by 24mpg at 70-75mph next fillup. I think I could break 30mpg if I drove 55, the EGT gauge only reads 400 deg at that speed. The timing made an immense difference in low rpm power, and I could pull slight hills at only about 600 deg on the EGT gauge. The '89 Dodge diesels had 3.07 rear axle gears, so it has one heck of an overdrive, other than that, the timing is the only other real mod that would help fuel economy. Just figured I would share!

Very nice! That's the way a vehicle should be: serves a useful purpose, has enough power to get the job done well, and get good mpg in the process. I envy your gearing, by the way. You get better mpg at 70+ than I do putting along at 60 with a tailwind.

Amazing MPG for such a heavy Truck. If it were over 6000 GVWR you could write off the cost.

Tax Write-Off for Trucks, Vans and SUV's

I would really like the tax code to be rewritten to allow us to write off cars under 2000 lbs!

That's very good, higher than I would expect. But I have a Mercedes 240D and a Jeep Liberty both nearly 5000 lbs, and both are capable of nearly 30mpg at 60-65 mph as-is, no aeromods or driving techniques.

Weight isn't the issue at steady highway speeds. Aero drag is.

Heavier vehicles have higher rolling resistance, which is proportional to speed. Heavier vehicles also tend to have higher frontal area.

True, unless you drive in hill country... I thought Oklahoma was flat until I started trying to find a good place to test the truck.

JQmile, that is pretty good. My 4.7L Gen3 will get about 24mpg at 65 mph, but I do have a couple aero mods installed.

Aerodynamic drag is significant, however rolling resistance is still 25% of the energy loss, and that is proportional to weight and speed.

Maybe, maybe not. It depends on engine efficiency versus gearing. If your truck's engine drops out of a favorable part of it's BSFC map going from 65mph to 55mph then you won't see much of an increase.

Have you looked at any intake or exhaust modifications? I would think a turbo back migh have some benefits, especially in the EGT temperature area.