I have a heavy-half ton ford van with a strait 6 4.9 liter, strong slow motor 15-16mpg city 20 mpg highway FULLY loaded. I also have a 98 chevy astro 4.3 liter V6 at best 15mpg city 19mpg highway. Not worth the difference if you are hauling any amount of equipment unless you are vlm only and still you would have to go with a honda or toyota for better mpg.
I own two FWD mini-vans, one gets 20mpg hi-way, the other gets 22 hi-way
My Super Cargo extended gets 18 hi-way fully loaded.
I hear the Chevy the Astro is in the 18 to 20 mpg hi-way range.
Which, BTW is the only mini-van I'd consider if you're considering putting a small TM in a mini.
Engine size doesn't always equate to better mileage either.
The Super Cargo I had with 4.9ltr straight six (a great engine BTW) only got 15mpg hi-way with a tail wind.
My Super cargo w/5.4ltr has more power and acceleration and gets 18 hi-way
Any fuel savings would be eaten up by tires, brakes & shocks. Besides you will take longer to setup & breakdown because it will be harder to load/unload any tools like vac, 175, blowers etc. I am looking to go bigger for my next truck, not smaller.