I have a 1989 Ford E250 2WD extended chassis van which has been modified into a motorhome (Falcon 190 by Invertec) It currently is driven by an unmodified 351W fuel-injected motor and c6 transmission. I really like the motorhome layout but am disappointed by the fuel mileage. On several trips that I have made I am getting 8-9 mpg on a good day driving at 60-65 (no trailer)! I therefore have a 2 part question: 1) What modifications can I make to the 5.8 which will significantly increase the mpg, if there are any? and 2) if I decide to make an engine swap (diesel or gas), what would be my best mpg options? I would like to stay with an auto trans and am considering 7.3 (IDI and Powerstroke) and/or a Cummins 5.9. Are there other diesels I should consider? If I stayed with gas I gave no idea what engine would give me the best mpg without having to put computers everywhere. Yes I know any swap would be a major undertaking but I do love the layout for my cross-country use. Some late model Class B motorhomes get much better mpg (16-18 ) but cost upwards of $50K!! My question boils down to if I am committed to keeping this motorhome, what would be the cheapest/most practicable way for me to get 16+ mpg regularly? Thanks for your thoughts.

 

8 to 9 mpg... even NEW motorhomes or RV's / busses, etc... don't get much better. Some max out at 13, most get 7.
there is NOTHING you can do that is WORTH any of the $ you are going to put into it.
Not unless you put in 100K on the clock in a few years.

if you go gas, an LS swap is king, no exeptions

if you go diesel, its a 5.9L cummins 24v pre 2000's

if you go ***** deep, a VAW turbo diesel with massive boost and injectors can get you 400HP and shave 500lbs.
the problem is the air, not so much the weight.

just go slower!

if you have the $ to do something like this then you have the $ to not care about fuel! past this, no amount of parts you put at it will ever become a real $ saver.

 

With a carbed 360 Dodge powered '86 Falcon I got about 11mpg highway but cruised right at 60.

Let 'em pass. So slowing down a little should help.

Gearvendors. Expensive but much less than an engine swap.

 

The engine isn't your biggest problem. The C6 is. Changing it for an E4OD would probably add 2-3 MPG. That will save you 25-33% of your fuel cost.

All you would need to do this is the transmission, a wiring harness, a shortened driveshaft, and the correct PCM for a 5.8L/E4OD.

Changing to any engine and keeping the C6 would not make a huge MPG improvement. Actually getting to 16 MPG is probably impossible.

 

and a lift, and good weather, and a bunch of tools, and a trans rebuild
IMO never put a used trans into a thing that gets real miles with out actually having it serviced properly.