I would say that that is scientifically impossible. It certainly flys in the face of all experience that I have had, and I have changed quite a few rear diffs to improve mileage, etc.

A higher speed will always require more power. More power requires more fuel.

Now a higher ratio rear end will produce better fuel economy at a given speed, though.

Tom W

Apparently my sarcasm goes unappreciated. My earlier statement reflects my real observations on the subject, and I agree that to a point, taller gearing will improve fuel mleage.

Just changed both front wheel bearings.

I have no idea how much my mpg has increased,
but I do know it will due to this change as the car
is MUCH slicker rolling now. Before it felt like my tires were going flat.
I travel much furthur after letting off the gas now.

One bearing was making only a tiny grind noise.

Low milage can result from a number of things. First, make sure you don't have any fuel leaks. Bad overflow lines can leak off enough fuel to drop you a couple mpg without leaving much fuel dripping off the engine.

Leaking fuel lines can cause air to enter the injection system, resulting in lousy combustion, hence more fuel used per hp output. Dirty injectors, carboned up prechambers, late injection timing, and so forth will also reduce the hp produced per unit of fuel. An Italian Tuneup (drive it like you stole it, for example) will work wonders. So will a hard several hours on the interstate.

You also need to verify that you do not have dragging rear brakes -- they can cost you up to 4 mpg before you notice much preformance loss, maybe more. Probably time to rebuild calipers by now anyway, they get "sticky" with age due to bad rubber seals. Bad front end alignment will eat fuel (and tires, look for uneven wear, especially inside or outside of both fronts). You usually notice dragging front brakes as they are never even and it causes the car to pull sideways. Other worn front suspension parts will cause the same trouble without excessive tire wear as it evens out more.

Old sticky diff oil will cost you milage, so will crappy tires, or tires that are out of round or out of balance. Inflate to the max numbers listed in the filler flap, it helps quite a bit.

A slipping tranny can cost you milage, of course, but so can driving style. I get 32 in my 87 300D is if stay off the turbo, 29 if I don't.

My brother gets between 27 and 29 in his 86 SDL, no AC. 30 would be unusual, its a rather heavy car.

Peter

Three words: fuel line magnets!!!

LOL

I can get a consistent 28-29 on the '86 SDL.

This is 90% highway and 10% city driving. Speeds on the highway don't exceed 70 mph. a/c is off or the above figures will fall a bit.

The key to the higher mileage is as follows:

1) Accelerate at 2000 rpm in gears 1 and 2. Hold 3 to 2700 rpm to keep with traffic.

2) All tires at 32 psi.

3) Avoid use of the brakes. Anticipate the road and lift the pedal without using the brakes unless some DB forces you to do so.


It's possible to crack 30 mpg with an SDL, but, usually, you'll need to keep the speed at 65 mph to do so. I have not done it in the past year.