I have a 89 D350 that I have put an intake on and straitpiped. I now average 21-23 mpg most of the time and 25-27 going 60 on the highway. I have the 5 speed getrag. I have adjusted the fuel screw for less fuel. I was wanting to boost power and maybe mileage a little. I was wondering if puting a hx35 on would hurt my mileage if I drove it the same. I am assuming I would have to readjust my fuel screw if I put a hx35 on. I was also wondering how much work it is other than putting on an intercooler. I was also wondering if putting in a 4 inch downpipe and running the exhaust out right behind the cab might help. I have also seen injectors that claim to help fuel economy and want to know if anyone has any experience with these. Most of the time I drive it like an old gramma, but occasionaly I like to toast some japcrap at a stoplight

 

man 20-23 is absolutly fantastic. if you are getting 25-27 on the highway in a truck that weighs 7000lbs+ someone is looking out... i get 15 driving like a grandpa in my 95 a little better in the 90 with the 5spd. that is as good as my geo tracker!!!!

but a hx35 is a great upgrade over the H1C and you can rig them up where you can run intercoolerless.

 

That's some great mileage! You must drive slow
Turning up your fuel shouldn't have much of an effect on mileage as long as you keep your foot out of it still.