I’ve got a 1990 4x4 Suburban fitted with a 1989 Cummins engine with 35k since being rebuilt. I’m looking for any other low hanging fruits that would increase the fuel efficiency on this engine.

Here’s the setup:
NV5600 Manual Transmission
3.73 gears
Gear Vendors Overdrive
305/75/16 tires
2” lift
NP205 transfer case
3” exhaust
HX35 turbo with 12cm housing
Timing advanced 1/8 inch
Injection pump moderately turned up

On a recent trip covering 5000 miles, I travelled at mainly 65 mph (1400 rpms) and the truck returned 25.5 mpg for the whole trip. I know I could get better fuel economy with narrow tires, slower speeds and if I lowered the vehicle but at this point, I would like to focus on engine efficiency.

Here’s a partial list of things I thought up that might increase engine efficiency. Is there anything else to add to the list? And any comments / insight to any specific items on the list with respect to fuel economy? Which items on the list would provide the largest fuel economy gains?
Electric fan
Intercooler
Water Injection
Aftermarket Injectors
Muffler delete
Larger exhaust
Port & Polish heads

 

larger exhaust, electric fan, intercooler and muffler delete defiitely wouldnt be a bad start. i dont know if water meth will help your mpg out, especially in comparison with the cost of it. and certain injectors might help your mpg, but mine definitely didnt

 

Thats perty good mileage I'de say How bout some gears?

 

I'm trying to accomplish the same goal, increase efficiency. Soon, hopefully, I'm going to install a water injection system (not methanol) with a water pre-heater (most likely at exhaust). I'll have the system inject the hot water post-inter-cooler. My goal is for the water vapor to enter the super-heated state and expand rapidly as it absorbs heat. I'll keep you updated.

 

4inch straight through exaust, propane, cam, p@p head, get rid of the lift.

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switch all fluids to synthetic would gain you one xtra mpg

 

Yep, propane injection for sure. And, turn down your pump! Still, if you don't want to do that, and I wouldn't blame you at all, then your first list seems pretty complete.

 

Water Injection, Methanol Injection, Alcohol Injection Systems | Coolingmist

Might look into this.. We know W/M will cool EGT's to a manageable level and will also boost HP. Looks like these guys may have added 'Increase fuel mileage' to the equation.

I don't have a 'cruise' setting on my setup but the small nozzle does kick in at about 7 psi which I hit going up hill....Haven't taken a long enough trip yet to verify any mileage increase yet.

 

do you have p pump or rotary, ive been told ppumps are better and make it the engine more efficient
not 100%sure tho, would be somthing to check into

 

I have the rotary pump. I was also thinking the same. Which is more efficient?

 

ve is more efficient