Overdrive Swap Economics

[cal30_sniper]

Originally Posted by Ford Assassin

I get 13-14 mpg in town, 18-19 mpg highway in my K5, I have a built 4L80e w/a billet 3600 stall converter and its been hassle free from the word 'Go'.

Keep in mind this with a supercharged engine driven by a guy with a heavy foot.

Details in my sig....

And you've got a significant mpg advantage in nearly every respect. He'll never be able to come even close to that kind of mileage with a 1 ton unless he does a diesel swap. You're using a LS based engine, with all the advantages that go along with that (including computerized engine controls, I assume). It's fine for a light K5, but would be a gas drinking hog in a 1 ton. In fact, that's where almost all of your increased town mpg comes from. That 2wd K5 is literally thousands of pounds lighter than a big block 1 ton truck or 3/4 ton sub.

You've got some real advantages on the highway too. Your 3"/5" drop means that you're pushing much less air. If you lifted it back up to factory height, I bet you'd see about a 2mpg drop right off the bat. You're also running low-profile street tires. If you were running anything close to the factory tire diameter and weight, and you'd lose more mpg. The low profile tires and lightweight truck allows you to run 3.42 gears. He'll never get away with that in a 1 ton with full size tires. More mpg lost. Your 10 bolt rear is also less of a power drain than the gargantuan 14 bolt that he likely has under his truck.

Overdrives make much more sense in small, light cars (which your K5 is bordering on). This is where you see the claims of a 25 or 30% mpg increase. They're not pushing much air, they've got small engines, and their light. The same recipe does not work in a heavy truck. You drop engine RPMs, but the engine has to work much harder to produce enough torque to keep that weight rolling at the lower RPMs. That's why you don't see much mileage on the highway. You'll never see good mileage with a big inch motor in a heavy truck around town. The overdrive has no impact on mileage at all below 45-50 mph.