Why Such A big Difference In Fuel Mileage
 

There seems to be large difference in reported fuel mileage between people on this forum with the same rear ends in there CTD. With 3:54 I have seen anywhere from 19 to 28 MPG and with 4:10 anywhere from 16 to 24 MPG,I don't see how this can be,I can see a mile or two either way but not the difference I am seeing here. I get 17MPG in the city and 18Mpg 0n the freeway with a 4:10 rear no mods. 285X75r 16 tires and an empty truck.So do some of us have something wrong or a problem with our trucks?

mpg
 

This is a good topic. Since I got my '97 2500 4WD, manual trans, about 6 weeks ago, it appears I'm getting about 19 mpg. Our son who has the identical truck except 1996, gets 22/23 mpg all the time. I'm still checking mine frequently, but I'd like to hear more about all this. BTW, mine has 128K miles and I plan to have the valves adjusted because I don't know when, or if, they have ever been done. I was told locally there is no timing adjustment on mine. Other than valves, which was recommended, I don't know what else to do. And I don't drive much over 65 mph for economy reasons.

??

Dennis

Some may have problems such as poorly matched mods or poor maintenance, but there is a better answer, IMO.
First off 28 mph is, by far, the exception to the rule.... or it's that one trip (with a tail wind or coming out of the mountains) that we all brag about but have never seen duplicated. :)
Primary reasons for mileage differences...
-12v vs 24v vs Common Rail (3rd Gen)
-4x4 vs 2x4
-Large Range of Tire Sizes
-SRW vs Dually
-Single Cabs vs Ext Cab.Long Beds
-Automatic vs Manual
-5 sp vs 6 sp
-Empty vs Working Truck (empty weights vary as much as 1500 lbs)
-Geography, Climate and Altitude where we all live
-Finally... Driving Styles

I haven't even gotten into the guys who trust their overhead readouts.... or base mileage on 1/2 tank, filled at different pumps... or have mismatched tires and speedo program... etc.

There are probably a few other reasons but you get the idea! ;)

RJ

I quite sure this statement is not true... but my lack of knowledge on 1st Gens is mind boggling. :D

RJ

i was told injection pump timing has alot to do with it.

might not be all true, im at stock timing and i get better mileage than most.

I have to agree with rj - bought my truck used, drove it like a 3500 Cummins heavy truck, 1st tank, mostly hiway, 26mpg, give or take, 20-21 back and forth to work, daily.

Jacked it up, #10 plate full forward, diddled the AFC, 6-speed, 13" clutch and flywheel, started keeping up with or ahead of traffic, occasional full-throttle runs to get the plaque off my teeth, heavy black smoke at full-throttle, not to my liking, but the power was breath-taking at 30psi - mileage dropped to 14-15, rat real quick-like - sometimes low as 10mpg, but my teeth stayed real clean, then.

Recently went back in, tweaked the plate and AFC, no more smoke but with reduced power, lightened up on my style, and mileage is back up to 19-21, depending on if I just hang with traffic, or break ahead on occasion.

Oh, yeah - and now I start in 3rd, rather than 2nd, a little iffy with the 3.54's, but the increased fuel mileage is good - real good.

That's my story, and I'm stickin' to it..............

My 95 get's 14.5 running around, 15-16 freeway, the best I've seen was 17.3 @ 75mph up and down hills in NV. (cant understand it. I dont drive that fast normaly as I have 4.10s, every one says keep it under 60, but my best mileage was 75[duhhh] ) My tires are 265/75 16s.

You get a lot of mechanical loss in the stock 47R, with it's 2200rpm stall TCC - shift kit and torque convertor with 1200rpm stall would likely turn your economy around, with tighter hookup anytime the TCC is unlocked.

Hills are not conducive to fuel economy - I'm a flatlander, so you should not compare to my results.

I agree about the TC and shift kit. they are on the short list of next Mods.

more confused....
 

Well, I'm even more confused. RJ brought up a lot of good points and reminded me I should have given more information. He also brought up some questions....at least in my mind.

12valve vs 24valve vs "common rail," 3rd gen: I thought, generally, the old 12 valve (2nd gen in my case) was at least as fuel efficient as any of the rest and the newest Rams were the worst on mileage, but had more power from the factory, (which was the trade off). Am I wrong? Or do I have it backwards?

Certainly, 4WD, dual rear tires make a difference. In my case I have 4WD but single rear tires and a club cab, (little more weight in the case of the latter I suppose).

I run 2.65 x 75 x 16 tires. Modest driving habits, light, if any loads, most of the time. A lot of flat country, but fair amount of two-lane roads too.

Local dealer's shop said there was no timing adjustment but I am beginning to seriously doubt some of their ideas. I plan valve adj. but have no idea about timing. Is there really any adjustment? I plainly need to learn more.

I ran 150 miles today hauling two culverts home for the farm so will check mileage again in the next couple days.

Dennis

Check yours and son's diff ratios - 3.54 would give better economy than 4.10's - it's on the options tag, underside hood just behind the grille

gearing
 

Ooops. Sorry, I forgot dif. gearing: My son and I both have 3.5's.

Dennis

THere absolutely is timing adjustment... You have a p-pump...Take it to any diesel shop, or Cummins dealer in your area, $150 later an your timing is bumped to whatever you want.

All the above provides several good reasons why there is difference in mileage mechanically, but I tend to think the single biggest reason is drivers with the right shoe about 3 or 4 sizes larger than the left!! [laugh] Case in point: When I ran new RV trailers out of Bristol IN, I sometimes would hook up with another driver going the same way, at least when leaving the lot. We would start off thru town a couple stoplights and VRROOOMMM!!! The other driver would be hell-and-gone then slamming on the brakes at the next light. [nonono] By the time we cleared town(just on the 2 lane still) they would be clean outta sight, calling on the CB "Where'd you go?". I'd say "Right here behind you, why are racing foot to the floor like that?", and they'd say something like that's the way you're supposed to drive a diesel, or no reason to waste time, or any number of things. Of course you can guess what happened on the interstate; they'd set the cruise @ 75mph or more and I'd set at 64mph tops. Bottom line is they wasted fuel and still complained about it. To each their own, no? :)