One other thing to remember, is that with a diesel, the torque is available is much higher than with a gasoline engine. Having said that, by giving up some off the line performance, you can gain gas mileage by running larger tires.

As an example, by putting the largest tires on an 83 300D that would fit in the wheel wells, the "indicated" gas mileage (miles travelled on the odometer/gallon used) didn't change. What changed was the actual number of miles travelled however, which increased. On a test on the freeway, when the odometer said 65 the car was doing 80. That is a 23% increase, so if you were getting 28mpg before, you'd be getting 34mpg after.

Obviously there are other factors (like the fact that acceleration was slower) but on the 300D I could drive as fast as I wanted to, which was probably faster than I should have been going in most cases. Having an automatic really lessened the effects of the larger tires (simply because off of the line the torque converter can slip allowing you to run at a higher rpm to accelerate as fast as you'd like), but I had slightly larger than stock tires on my 77 240D and I couldn't tell the difference.

You can also change this a good amount by changing tire pressure or tire brand. In my experience Michelins will give you more mpg. There may be another brand that has less rolling resistance, so if this concerns you, look around. I lost 2mpg (out of 21 that I started with) on an SUV changing from Michelins to Generals of the same size.

Anywho, I was getting an easy 30+ mpg on my 77 240D. I never paid attention to the 81 240D, and on the 83 300D (turbo) I was around 30 on the highway.

-Tad