Quick update:

I swapped in a SOHC transmission with double overdrives (4th (.923) and 5th (.744)) in the I Mark last weekend. This new transaxle has a 3:57 final drive (rear end) instead of the 4:11's the DOHC came with. (And 5th was .829.)
I mentioned I was going to do this. (I did. <img src="/forums/images/graemlins/smile.gif" alt="" />)
It used to turn 2800 rpm @ 70 mph and now it is at 2100 rpm to do 70 mph.
It sure slowed down the wind up or take off, but the boost builds fairly quickly and is real smooth and quite cruising home.
A lot of people who hear it running have a hard time believing they're hearing a diesel engine.

No hard figures on fuel mileage yet.

Curtis