fuel milage help.???

hello to all
my 97 AWD LS has a complete tune up nothing skip i just had the tranny rebuilt $2,500 about 3 weeks ago
the van drives like a dream it coast like free wheeling nothing binding what so ever
except the the fuel milage is bad the best i can do on the highway is 300 miles per tank on a 25 gal tank
where i read it should be closer to 400.
any one done or know of a mod to the airfilter box to get more cool air in.???
or an adapter plate underneath the throtle body for better fuel metering.???
thanks

I service a fleet of Astro and Safari cargo vans from '96 to '02 and they all get 20MPG. Doesn't matter if they're loaded or empty, city or highway, beat on 'em or drive 'em civilized, 20MPG. The largest tank is in a '98 and it has a 20 gallon capacity. Are you sure yours is a 25 gallon tank? Have you actually checked the fuel mileage? Is the Service Engine Soon light on? Save your time & money on the modifications, it's been proven that neither one of those work.

Replace or have the ECM rebuilt. I am told they last about 10 years.

good luck keep us posted