Re: getting better MPG on E350???
I'd stick with the 3.73 gears as it's an 'inbetween gear'. Going with 3.55's will give you worse mileage around town as the van will need more power to get the wheels turning but will get better mileage if you drive 80% highway miles or more with it. Going from 3.73-3.55 won't net much of a difference to make the cost worth while as it'll take years before you actually see a profit from that particular modification.
A few 'cheaper' mods that MIGHT increase mileage (and slightly increase performance) are Taylor 8mm wires, single platinum spark plugs every 50K or less without towing, A free flowing air filter helps. Most major brands don't make kits for vans however but a trip and $50 at the home depot in plumbing parts plus a $20 cone filter can increase air flow. Home builts are also cheaper than the $300-400 kits that aren't proven to net mileage anyway, so why spend the money if you don't have to!
If you have any excess weight in the van that doesn't need to be in there, leave it at the house. Every 50lbs decreases mileage by 1-2% and adds up after a prolonged period.
Make sure tires are at proper inflation pressures and when it comes time to replace them, find the most aerodynamic ones available that suits the climate conditions for your area. Another point is to take anything off the outside of the van that would cause additional wind drag and keeping the speed at or below 65mph will net the best mileage! Good luck
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!