Run 60 get 28, run 65 get 30, run 70 get 29.
 

Mercedes 3.0 TD
Seems like the turbo diesel boost helps more than the aero hurts. My sample size is small but am going to continue driving 65 in my Mercedes for more data. I think I am comparing similar driving conditions. The engine is supposed to have peak torque at 2000 and 65 mph is 2800.

The car supposedly has a cd of .4 and is fairly small. yet pretty heavy for it's size. The engine is strictly old school IDI TD, no computer.

Comments?

MPG should be inversely proportional to MPH.

Don't buy it. Maybe if it had taller gearing. Keep us posted as you get more data.

Should probably include data points for 50 and 55.

Tell us how you did your testing.

Tony I won't argue as to the theory. But I drove it for 3000 miles at 55.

My VW gets better mileage at 55 (55 mpg). It gets only 2-3 psi at 60. The Merc gets 5 psi at 60 and 8-9 at 65. Trying to find a way to get more boost at less speed.

Drop a gear? :p

Nicky It is a 4 speed auto and 27 years old. Everything above 30 is in top gear. You might be right.

If everything above thirty is in top gear, then thirty five is probably where you can get best economy.

Of course, that's problematic on US highways where going thirty is just as good as standing still...

When I put a turbo on my diesel I picked up at least 2 to 3 mpg on the highway. And thats compairing driving 55-60 when N/A to +65MPH w/turbo.

Diesel engines are nice because the more air you can cram into the cylinders the better the fuel econony and power out put will be.
If you had a boost gauge you may see why there is a fuel economy jump at the higher speeds. At 60 the turbo maybe under such light load it may not be kicking out any boost, at 65 you could be developing 1 or 3 PSI and that small change can make a lot of difference.

Water injection