I don't consider this a real problem but I thought I'd post it to see what response comes back. I bought an 83 300D turbo with 167k miles back in August and have been fixing minor issues and tuning it since. After a couple of tanks of fuel, I checked milage and was seeing 32 mpg. Valve and timing adjustment got this to 34. I bought a set of monark nozzles and a pop tester and rebuilt the nozzles along with setting pressures. I tested each injector before and after and saw an improvement with the spray pattern on a few. The old nozzles were opening around 1700 to 1800 psi and I set the new ones at approximately 2100 to 2150. After getting everything reassembled, the engine ran smoother and quieter, but I lost 2mpg, which is the opposite of what I was expecting. Any ideas on why that would happen? I was careful not to mess with anything else so that I could see the effect of the new nozzles.

From what I've read, 32mpg still sounds pretty good for this car but I don't push it at all and I keep it at or below 2500 rpm on the highway (thus the username).

 

Thanks for the reply! Just checked my tires and they are 195/70-14 and the differential case has 3.07 stamped on it. Is there something I could look for on the instrument cluster that would identify it as from an 85? The car came with a large collection of maintenance records that seem to indicate the accumulated mileage is real and it certainly looks and drives like it could have just the 170K it's now showing.

 

Looked up what google maps says the distance is to work (22.7) then checked it against my odometer both ways (23.7). With some fudge factor, one could believe my odometer is expecting a 2.88 differential. Please let me know if there is a flaw in the following logic:

The ratio 2.88/3.07 = 0.938 (rounded). So for every mile my odo says I've traveled, I've really gone 0.938 miles. If I multiply .938 times the milage my odo reads then divide that by the gallons used, I'm seeing 2mpg less than what I thought. It also turns out that I can just multiply that ratio times the milage I was seeing and I end up at the same place. So the 32 to 34 mpg I thought I was getting appears to actually be 30 to 32. If I just take the ratio of the milage my odo saw to what google maps says, it's .958, which would result in slightly higher fuel milage but the .938 seems more likely to be correct, since it agrees with what one would see with an instrument cluster swap.