So I finally got my Doug Nash overdrive installed along with the new speedo gear etc. The vehicle is a 1988 E250 with a c6 and 3.54 gears.

My first impression is that I really like it and that it makes it much easier (almost too easy) to cruise at higher speeds on the hwy. I put a total of 318 miles on it with about 80% of it being hwy. Mileage was 14.9mpg. This is on the upper end of the mileage I would get for the same route prior to the install (I would range between 14-15mpg). However, instead of having the cruise set at 60-63mph I set the cruise at 73mph. Wind resistance has a big effect on mileage of my van since it has the aero of a barn. With that in mind I think slowing down to about 65mph should show some pretty good gains.....maybe a couple of mpg.

Flatter terrain would help too. I took a road trip into Ohio a few weeks ago (to pick up the DNE2) and was able to get a little over 16mpg with the cruise set at 65mph. Thinking the overdrive I would have been able to hit about 18mpg.....if I can get in that range I will be happy.

All the temp gauges seemed to run a little lower.....EGT, Trans and Coolant.

Acceleration, while noticeably slower, isn't too bad and the O/D can always be turned off with the push of a button. I am also thinking it will help if I am towing anything a little heavier since I will have a 2.5 gear available.

The real test with be in 2 weeks when we head out on a 2000 mile trip.

 

I always seem to get the best mileage keeping the EGT's under 600 in OD at cruise, and anything close to 70mph that's just impossible.


My egt's actually go up with double-OD, and the lower RPM's just make it all a wash.

 

I am assuming you are turbo'ed.....doesn't that drop your EGT's quite a bit? I wonder what a comparable NA EGT would be.

You are correct though, the EGT's do not stay below 600* at 73mph. For the most part they stayed in the 600-675* range. I am thinking that dropping to 65mph would probably help a lot.....60mph even more, but I don't think I could cruise at that speed on a long trip.

 

Either way, turbo or not, its how the fuel is being combusted, creating the EGT... It takes a certain amount of energy to get you down the highway... BTU's...


I would say that more energy is wasted out the exhaust on an N/A setup, especially if its not tuned up, but I just don't have that much experience with one, so I couldn't tell you for sure. Id still bank on EGT's being fairly consistent no matter what.

 

Did some highway driving yeasterday and kept an eye on my egt gauge. Cruising at 65 mph seems to keep theegt below 600* ecxept on bigger uphills. I am going to try and stay at 65mph to see if it produces any improvement.