Can anyone tell me what I should look for? My truck is getting a consistant 12 to 12.6 miles per gallon in city and 15 to 16 hwy. Mods in sig. Lift pump,fuel filter,& vp replaced 10k ago. Mileage is done hand calculated, not by trip computer. Air psi in tires is checked weekly. Air filter is clean. Odometer/speedometer verified with GPS. Truck is not hauling anything except a tool box.

 

sounds just like mine.....

some say it is the 4.10 gears

 

try driving below 10psi of boost for a full tank and tell me what you get then. never go above 2k even on the highway. i have 4.10's and 285's and i know 2k is a little under 65mph it sucks but when im trying to conserve diesel it helps for sure. last time i calculated my mileage i got 12mpg mixed, but my tranny is slipping a good amount and that probably costing me atleast 2mpg if not 3 even. and i was blowing a lot smoke for that tank and racing around everywhere, like my gauges was up past 10psi all the time. really it depends on how you acclerate. and how good your tranny is. and if the stock torque converters in these truck suck alot.

 

Forgot to mention I have 3.55 gears

 

did the mpg's drop around the same time you added some of the goodies in your sig? Try checking for fuel leaks. What kind of mileage were you getting prior to the poor mileage?

 

Winter fuel blend causes a drop in mileage also.

 

Your tires are really hurting you for mileage in town and they are probably hurting you on the highway some as well. The biggest factor in fuel economy assuming that your truck is running well is wind resistance. It makes a huge difference to slow down some. I think that you will find most people claiming good mpg also don't exceed 65mph most of the time.

As far as physical things that could be wrong with your truck, I would first look at stuff like brakes and wheel bearings.

 

Took the 265's of at 300 miles after I bought the truck and switched to the 285's. Truck was getting 20 mpg on hwy pulling 16ft flatbed trailer cruising 75 to 85 mph before. Had 315's for a while and was getting 18 mpg hwy. Brakes are changed every 25k, rotors every 50k. New calipers at 100k. Wheel bearings have no play but to me do feel like they drag. How easy should they turn with calipers off? Had a shop look at it and they said that was normal. Truck was getting same mileage before mods and new vp. Doesn't change on stg 1,2,or 3 with programmer. Whether if I drive it like I stole it or baby it the mileage is the same. I do see fuel around the filter canister but have been unable to find any leak, never leaves a puddle. No excessive smoke unless I stomp it off the line. If I had a timing issue I would think it would run funny. The valves were also adjusted when the pump was replaced. Sorry for long post just getting tired of putting all the money in the fuel tank.

 

That sounds really low. I get around 20 on the highway and 16 around town in the winter. TC or your programmer may be causing trouble. Kurt

 

well mine is about like kurts i had a local mech. tell me to make sure my intercooler hoses are ok sometimes the fendr will get up agianst them and after time rub it and u could loose some boost and not see (on a gauge) i had trouble with truck not doing good on mpg then i started running (every other tank) that lucas injector cleaner and changed my fuel filter every 2000 miles and put 70 psi in my tires and i get around 18 to 19 in town and 20 to 22 on highway.....jope it gets better 4 ya...

 

Was it the same in the summer as it is now? I know i got 19 mixed before winter hit now im getting 14-15 if im being nice.

 

my mileage is around the same with out propane (16 highway) we have no city driving out here
with propane i get around 20,21

 

The drivers side fender cut a hole in the lower boot on my truck. I had to bend it out so the sharp edge wasn't against the boot. My mileage varies a lot according to my driving style. I've noticed the overhead comp to be pretty accurate when compared to hand calcs. Your mileage seems a bit low for these trucks from what I've read on this site. Kurt

 

My mileage was 12.5 when I bought the truck. I was rather disappointed, then put some Power Service in it. After two tank fulls the mileage was at 18 and stayed there. After the new VP44 it remained at 18.5 mixed city / highway driving but went up on pure highway driving.
I would suggest trying some PS, does not cost much.

 

have you reset the speedo for the 285s? that will lower your # another thing that can cause this (not trying to scare you) but if the VP$$ diaphram is going out your pump cant advance the timing and will also affect mpg my 01 HO gets a solid 24 mpg at 62mph at 75 it drops to 19 towing i see 11+-
my 01 SO has a utility box and the gvw is at 10k and gets 15 at 55-60