I think one way to improve the milage is to change the rear and and front diff axel ratios. This is an semi expensive thing.Of course you might lose some of your get up and go with a Higher ratio. But it will show in RPM.

Also Gear Star make Overdrive units for may cars. Not sure if they make one for the HUMMER . But it's a small unit that bolts on after the tranny and has normal and OD speed control. They are small units. Also you might wat to contact them and BS with them about your quest for better MPG.

Here's a Link... Scroll down and you will see listings for different trannies.

Go here.

https://gearstar.com/

https://gearstar.com/gear-up-to-a-ge...SAAEgJAn_D_BwE

 

I've been going through a 2003 H2 the currently has 109,000 Mi on it. Mechanically it seemed pretty well neglected. 10 miles per gallon highway. Changed all the fluids to synthetic, fix the rear main and oil pan gasket, oil pick up o-ring, oil cooler lines and the list goes on and on. Even new O2 sensors and injectors. Remove the $500 cold air intake and returned it to factory air box. 12.6 miles per gallon on the freeway cruisinet 63 mph average. That's what I expected in completely stuck trim. The last thing to deal with was the ECM. Previous owner obviously had some tune installed, I could tell as it was shifting a little bit late. Also had a few bizarre anomalies that would come and go. Like 10% of the time it would not signal the fuel pump for two or three seconds with the key on. New ACDelco fuel pump, filter, Etc. Picked up a refurbished ECM from flashmastersecm.com for $99 plus uploaded my VIN number. They also program it with the latest updates from GM available on all data . 3 days later I install it, go through the key relearn procedure and the camshaft sensor relearn with my China clone Tech 2. Going on two weeks now and my average is 14 miles per gallon. I'm in a rural area so I have 20 miles to town on State highways with Rolling Hills. Averaging 60 to 63 miles per hour, and about 30 minutes in town driving stop light to stop light.
Verified by manually calculating it at the fuel pump. 15 miles per gallon would be quite easy on a freeway Drive keeping it to 65. It was never my goal to get past 12 and a half miles per gallon.. it was quite surprised to see that in Factory trim it does as well as it is now. Everything on the vehicles completely stock, except for the Dorman HVAC unit which is working perfect, the aftermarket four core aluminum radiator, and the updated valve covers with the baffle to address the PVC issues with oil in the intake. The replacement ECM also took care of the BCM randomly failing a system scan. Best $99 I spent on it yet.

 

"it's lineage was the hmmvve that was intended for military use. the military does not care about fuel economy as we tax payers fill there fule tanks."

Easy, Us 🇺🇸 Veterans (101st Airborne Infantry) go through Hell to let all peeps sleep 💤 easy at night & we pay taxes too. Of course, personally I only got a ride in a HMMVVE twice. 😁 God Bless!