I just got back from a 300-mile (that was one way) trip hauling a 500-pound snowmobile, on the interstate across South Dakota (fairly flat,) with my '98. I filled up before I came back and it made 14.4 mpg, adjusted for the speedometer being off 3 mph (checked with GPS.) The tach was around 2100rpm the whole trip. This is a 12-valve, 5-speed, brand new 4" exhaust--I expected better. (It is a 4x4 though.) Are my expectations unrealistic, to get at least 18 mpg out of this thing? Is there something I should look for to fix? I'm ready to get rid of it--with the price of diesel fuel 30-35 cents more than gas, I'd be better off driving my husband's 1/2 ton gasser with a 4.7 that makes about 14 mpg, although the thought of that depresses me.

 

14 sounds a little low.
Theres a lot of variables that factor in to actual mpg's.
On average, my 12valve gets just under 17mpg. (~16.8).

Besides the obvious, like:gearing, tire pressure, and overall mechanical condition, I understand that one of the largest key factors in our 12 valve performance between fueling stations can be engine timing.

Rich.

 

What about speed? If you have 3.54 gears and were spinning 2100RPM you were moving pretty fast and should be happy with 14mpg.

Cummins used to post somewhere on their website that for an ISB's best economy you want to run 1800-2100 and for best performance under load you want to be between 1900-2400. Seeing as how you were on the upper end of the economy ratings I don't think 14mpg is that bad.

 

aren't there differences between 1st gen and 2nd gen 12v's? I just clocked 21mpg's on my 2nd gen. And yea 2100 rpm's on my 4.10's i'm going like 68mph if I remember correct. U were towing somthing to, even if it's not heavy it's the drag that will also reduce milage, 14mpg going 75 or so ain't bad, if you went 65 I bet you woulda got closer to 18-20. Diesel is finally cheaper than unleaded here, thankfully.

 

Oh, and clean ur fuel filter, might help alot.

 

14 is on the low end of my mileage spread. I get 16-18 usually. I did have a tank get just under 14 recently, but that was a lot of short trips to the store (3 miles), lots of extended warm up idling trying to dry out the wet carpet with the heater cranked up, towing the sawmill and trailer around on narrow county roads and dragging logs out of the woods at my neighbor's place. So that got me about 13.7 mpg.

I did get 20mpg once, but only once. So I would say yeah, 14 is a little on the low side. I would change fuel and air filters and adjust valves if you haven't already.

 

i get 16.1 unloaded if i keep my foot light. got 12.9 with a load of horses last week. on the highway i can get about 22 unloaded if i am going 65. any faster and i get less than 20.

 

I get 17~18 empty on the highway and ~13 pulling my trailer. This is driving at 70-75 mph. BTW my RPM is fairly high at those speeds as I only have a 3 speed auto.

 

I am with Blitz_81 but I just added bigger injectors and I think I will see a better mileage especially when towing.

 

What gears do you have? I've noticed the 2nd gens are more speed sensative...maybe from pushing the front axle with no locking hubs or somthing, I'm not sure. I know if I run 60-62, a 24mpg tankful is not out of the question. Over 65 and it starts to drop like a rock. Beyond that, I agree that having your timing checked/set is a very good idea. I've heard they can slip quite a bit over the years/miles.


Yes. We 1st genners have dynamic timing

 

I should have specified--they're 3.54s.
The sled was in the back of the pickup so we weren't towing anything, but it was still weight and probably produced some drag. I'll probably take it in and have the timing checked, give it a chance before I sell it.
Also this truck seems to be really a dog at low rpm, like under 1500. At 1500 it really comes on and then seems to bog again at a little over 2000. Maybe anything under 1500 is lugging it too much, but my First Gen seems to be okay with anything about 1200 on up to 2500 when it goes against the governor.
Thanks for the comments, everyone.

 

Don't discount what a new air filter and a valve adjustment could do for you. I haven't had a chance yet to check normal driving mileage since I did the valves, but the truck just purrs now. I'm assuming I will see a bit of a boost in mileage.

And my air filter....

well, it was pretty nasty.

and that can't help mileage much either...

 

Oh wait, I thought u got that milage in the 93' w250, I didn't notice the 98' in ur sig, lol. You should definently be getting better milage than that with the 98'. Believe me, it might be somthing simple like ur fuel filter, and air filter, or somthing alittle more involved like ur timing.

 

when my filters need to be changed i go from 16 to 12 empty. i would check the filters. try a high flow air filter.

 

I can hit 18 in the city, 22 on the highway (approx, rather light footed, but not particularly stupidly light footed).

Probably 16 & 20 if I stop caring as much.

I've also had around 10 - pulling a 13,000 lb tri-axle 5th wheel with the frontal area approximating a semi through mountains.

Truck is pretty much stock plus a K&N. Those filters can fill up, I'd go around doing maintenance on those, valve timing, etc. If you see any black smoke, that means you aren't get your full value out of each squirt of fuel.