Engine/Transmission (1994 - 1998) average mpg?

[Ahopper]

i was just wondering what people are getting for mpg on the highway? my tuck in the signature runs 295/75/16 bfg all terrains i belive its like a 33. 5-34inch tire. with cruse control between 65-75 mph i am only seeing 13mpg. is this right? i was getting the same with in town and highway combined but i figured it was low because of the intown. now i know its not. any suggestions? i was going to back off the plate but i thought that only affected wot. also i have a set of 215 injectors in the truck. they have 160k miles on them. dont know if that makes a difference or not.



also i have 3. 54 gears

 

[EKlem]

How long have you had the truck and has it always done this or is this new? Have you corrected for the increased tire size? If you have corrected for the speedo and odometer, your mileage sounds a little low although you are running much bigger tires which can hurt in town mileage a lot. Since you have gauges, what are your boost and egt numbers for driving on a flat section of highway at 65mph?

 

[arthr31]

eklem brought up a good point, have you corrected the mpg/miles travelled for the increased tire size? E. g. i'm running 265s and for every 100 miles i do, its more like 104.

the injectors don't matter, nor does the plate at a cruising speed. maybe something else is up. i would expect your set up to get 19-21mpg on the hi-way.

also, what about ulsd, are you running ulsd? i just got my first tank and i'm not happy with the results... drop in power, drop in mpg, no perceived diff in smoke ('course the truck smokes a ton anyway. . )

do you have other symptoms, like loss of power? Does it seem to take more throttle to do the same ?

 

[CumminsPower98]

70 is 2k rpms on mine and running 70 I get 20-21 mpg (265/75 tires 3. 54 gears+6 speed). I now have 358k on the original injectors. I generally get pretty good in town too. 13 is really low if you have corrected the speedometer. I used to run 33x12. 5x16. 5 tires and got the same. Those tires put me at about 72-73 at 2k rpms. Any idea what your timing is at? I know this can make a significant difference in mileage. I know mine went up some when I advanced to 16 deg.

 

[Ahopper]

when i got the truck it was stock engine and transmition with 285/75/16 on it. i never saw what mpg was at stock. i put 295/75/16s on last summer and on flat ground at 65-70 pulling a two place jetski trailer i got 20mpg. that was on mexico diesil also. right now i am running ulsd i belive ( i fill up at pilot). that was befor the new trannie, 100 plate and, 215 injectors. my boost sites around 10 on flat ground and 700-800egt. my rps at 70 are at 1950 and at 75mph 2100. my timing is at 16. 4*

 

[WDixon27]

my combined flog gets me about 16. 5 mpg, 20 miles to to work one way 16 of which are interstate, the only thing i do for mpg is try to generally keep it 70 or less on the hwy the rest of the time it's pretty much on the floor, flatbed cab and chassis with big wind catching towing mirrors, 4. 10s and a 5 speed, 2250 rpm for 70 by the math with my tires 4. 4% too tall. on flat ground at 2250 rpm unloaded around 700* egt and 4 psi boost, 330 marines, stock plate full forward, bhaf and dual straight pipes

 

[DESL PWR]

with my lift and 35's I get 16-17. 3. 54 gears.

 

[Ahopper]

WDixon27: how did you figure out if your tires were too tall?

 

[bdcraig]

My 3500 gets 11-13 mpg on the interstate between 65 and 70 mph and 15-16 mpg in town and on country roads. It has done this since new. My 2500 gets 19 to 22 mpg at speeds of 65-70 and 17-19 in town and on country roads. Gear ratio is the difference.



brian

 

[WDixon27]

the old trucks came with 215/85/16s i have the common upsize 235/85/16s which according to a pretty cool gear/tire calculator i found online puts the speedo about 4 low at hwy speeds, http://www.bncoffroad.com/tire_gear_cal.html kinda fun to play with

WDixon27: how did you figure out if your tires were too tall?

 

[DonTX]

My 97 in signature does 22. 90 to 98 empty, rolling with cruise at 65-70 indicated on the speedo. It was set up this way when I got it, and I wanted the 3. 54 gears for that purpose. I also have a Chrysler minivan, it has NEVER got as good of mileage as my one ton dually!

In total fairness, my speedometer speed reads several mph slow according to my GPS (about 66 actual when showing 70 on the speedo). The tires are lacking a lot of original tread and I attribute the speedo error to this, and of course this would make the odometer show actually more miles than actually travelled.

 

[EKlem]

It seems to me like your motor is working harder than it should to keep the truck at speed on the highway. With essentially the same truck as you except for having a stick, I only see egts of 650-700 and boost pressures around 3psi in the same conditions that you are reporting much higher numbers. If our two sets of numbers are accurate, it seems to me that the difference in our mileage would be caused by something other than the engine. Needing 10psi of boost to keep your truck at speed on the flats is a lot. Was it this way when you were getting better mileage.



Have you checked the obvious stuff including tire pressure and brake drag? It may be worth putting a hand on each hub and tire after driving for a bit to see whether there is a large amount of heat build up in one area.



Your fuel plate shouldn't affect your mileage unless you like to go wot all the time. If you have a really bad injector, you could lose some mileage. Also, if your timing slipped or your valves were poorly adjusted, you could loose a little but I doubt you would drop that much.



Good luck figuring it out.

 

[Ahopper]

to tell you the truth i dont remember what my psi/ etg tems were when i was getting the better mpg. i have checked for brake drag and i do not belive they r. my hubs do not seem to be that hot. could my fuel filter be to blame at all? i just bought one to replace the one thats in ther. (the old one has around 9k on it)

 

[arthr31]

I'd change it, specialy if you are picking up any ULSD in your refuels... I'm keeping an extra filter in the truck now that I'm getting ULSD- I suppose it's going to free up some crud sometime and i'll get stuck...

 

[Ahopper]

ok i changed the filter yesterday and i drove about 80 miles today on the highway. i leteraly filled up. got on the freeway did 70 miles an hour with cruise control got off filled up and i got 14 mpg. this way without stoping at all! i look for fuel leakes i couldnt find any. im starting to get mad!i thought the bad gas milage was because i drive so fast every ware in town but now i know its not that. its just crappy all together.



btw at 70 mph my rpms are at 1950 my boost is at 8-9psi and my egts are at 800 if a small hill aproches on cruise control my egt is at 900 and boost around 12-15psi

 

[DonTX]

Like has been said, your empty EGT and boost are about the same as my pulling readings, and your fuel mileage is about the same as my pulling too!

Can't recall, has the air filter issue been looked at? A dirty or wet air filter can play heck with fuel economy too.

 

[DieselDoghouse]

1998 2500 weighing 7640 lbs with 4. 11 on 285 AT KO's corrected for tires currently getting 17. 5 - 19 depending on whether I'm making sure the governor is stuck or not. I would like to get to heaven, I'm not seeking sainthood!! :-laf

 

[Skydiver]

Figuring Speedometer error using a stopwatch.
Before GPS I would have to hold the speed steady as possible for 1 mile marker. Lets say It took 47 seconds to travel 1 mile.

60 is a constant that has something to do with there being 60 minutes in a hour.

60 divided by 47=1. 28
1. 28 X 60 = 76. 8 MPH
Will need to figure % error to add to speedometer reading.

Lets say my speedometer was showing 71 MPH and now I know I am traveling at 76. 8 mph. 76. 8 divided by 71=1. 08.

Now I know I can multiply my speedometer by 1. 08 and get my true MPH and the same holds for odometer readings.
The error is 8%.

 

[bdcraig]

Don/TX



His boost and egt readings along with milage are the same as my 3500. It does weigh 9500lbs. with no trailer.



brian

 

[DonTX]

Interesting. I suppose the 4x4 adds some weight and drag, but still that don't seem realistic to have that much difference. I also just realized I do not know the exact empty weight of my truck. Gotta get that done. Mine just absolutely loafs at 65-70, 3-5lbs boost, egt around 500-600. When a hill comes up the ol boost and egt starts winding. He has more mods than I do of course too.