Help with my MPG before i throw in the towel

wind cold air rain all these elements add drag or resistance kills fuel. for every 10degree in temp drop equals 10,000lbs of resistance at 65mph in cold air . You gotta drive like there are egg shells under the excelarator pedal . lower thr rpm the better . Progressively build up your speed and rpm as you pull from a stop. find a sweet spot for your engine when pulling heavy loads. I drive a 2010 Kw T660 3.55 rearends i avg 6.5 with winter blend fuel during cold months. During the spring summer months I Avg 7.5 8 mpg . I run 60 when im light 63 when im heavy . I keep my rpm under 14 as much as possible. Yea it suks driving slow but the pay checks are sweet and the miles are low 2100 2500 on elogs. As a Lease driver i made 148,000 on 104000 miles paid it can be done .

 

How is the engine pulling does it have good power it's possible you have boost leak. Also it could be something with the overhead if possible to readjust tell the mechanic to make the intake tighter and make the exhaust a little looser within specs of course you might also want to try replacing the boost sensor it's relatively cheap and they get plugged up sometimes and don't send right info to ECM. Let me know how it turns out

 

I believeyo ur truck is geared to high. Probably 3.90
To be running 1600 rpm at 65mph is bad. I had ia rig that ran
at those same speeds and got even worst fuel mileage.
It was an fld120 condo with a s60 Detroit and super ten tranny and 3.90


Just my experience with my rig. Long gone now

 

Yes there is a difference in fuels for mpg, any bio diesel seems to take .3 mpg off my cat, or loves, mobile, Texaco, most TA;s and Pilots fuel.

 

A 98 model truck will probably have a made in 97 engine. Even if not, the cam timing change on the Detroit engines was in 99. The 97/98 s60 don't run we'll below 1500. So your rpm is fine for that engine. Turbo wing is your biggest problem. It will cost you 1 to 2 mpg. Even more if pitch is angled high, and speed increased. They work fine to creat downforce on a race car, but creat a lot of drag. Back in the day, myself and several friends ran them. All with the same results. Loss of 1 to 2 mpg. Never again no matter how cool they look!