Veggie Van Gogh isn't going to win any MPG contests.
But I wanted to get it down as much as I could!
These step vans came with a GM 450 HD V8 gasoline engine. Frito-Lay burned them up at around 100,000 miles, and since the body and running gear was still good, they sent them off to Cummins for re-powering with a 4-cylinder in-line diesel (4BT).
This left several power-train issues. First, they were optimized for sprints in traffic between grocery store deliveries, and I wanted to optimize it for long-distance highway travel. So I put the tallest gears in the rear-end I could get; I think it's a .410.
Cummins left the original Muncie 4-speed tranny in, which was barely useful. Winding out 1st until the 4BT governed at about 2100 RPM put the 4BT out of its power band at about 1400 RPM after shifting to 2nd. This often resulted in forced down-shifting, to the annoyance of drivers behind in traffic. Other gears had similar nasty gaps.
So I found a Ranger "gear splitter" 2-speed that sits in front of the Muncie, and lets me split each gear (including reverse!) as needed.
I had to shorten the driveshaft by 7" and make some changes to the rear motor mount, and fab a new tranny cover, which gave me the opportunity to add a mount for a second seat, and some hatches to access my vegetable oil filter and the fuel economy gauge senders.
Now, I can split gears and more-or-less flow with traffic, while in 4-2, the 4BT is turning at its peak fuel economy (1800 RPM) at 63 MPH.
Wind load is now the biggest fuel economy factor, and I can do 17 MPG at 65 MPH between tanks, if the wind is right...
I know this is a far cry from those who are getting triple-digits of fuel economy, but you can't carry 10,000 pounds and sleep in yours, can you?
If this is of interest, let me know, and I'll post more, including details of the vegetable oil fuel system and instrumentation.