I recently purchased a 1994 E350 15pass clubwagon with a 7.3 powerstroke . lots of miles on it. 370,000ish km. around 230,000miles?

it runs great now that I replaced the cam/crank sensor.
1st tank I calculated got me 20.2 mpg . (our gallon is 4.54 lires/US is 3.78 litres). combinedhighway and city , and it has 10ply 245/75/16 snows on it.
I am putting a set of bridgestone rib tread 215/85/16s on this week and might go with a 6" rim on the front with them instead of the stock 7" rim.
better mileage is my goal!
filled up 2nd tank yesterday but can't find my calculator.
it will definitely hold more than 100litres and it looks like an 800+km range.

I'm looking for better, I figured it would get between 22 and 24.

from my research this van should not have a powerstroke in it, but it does?

I'm looking into a chip/programmer for milage improvements as it is our main family vehicle now and I hope it will be for a long time.
I've always wanted a diesel clubwagon, and the more I drive it the more I like it! wife loves it and so do the kids

update: 2nd tank was 22.0 mpg thats more like it