Hello all, I've got a 96 cummins and have been having poor fuel economy, IMHO. I bought it with bushwacker fender flares, and installed a chrome roll bar with Hella lights. The first tank, with a heavy foot, I averaged 14.8. The second tank, with a pretty light foot, I averaged 15.4. Both tanks saw mostly highway driving with some town driving. I guess my question is, does the roll bar and bushwacker flares hurt the mpg's that bad? I've been thinking about getting the valves checked and reset, and the timing advanced to about 16-16.5, that should help right? I'm not exactly new to the cummins world, but I'm kind of a newb to the ol 12 valve, I've mostly been around 24 valves so far. I was hoping to see at least 17-20 for mileage, and will be driving this to college, so all the mpg help I can get would be awesome. Thanks

 

Before timing, I would make sure fuel filter and pre-filter are good and clean, then get a fuel pressure reading to make sure it is in spec. I always lost MPG when my fuel pressure was down.

Are you sure it is going into OD and TCC lockup? With 3.55's and stock tires at 55-60 I always got 21 plus straight Hwy, and never less than 17 unless I was on winter fuel or I was gouging on it. Above about 65 MPH though, fuel economy dropped like a rock.

 

I just put a new fuel filter on before the last tank, when I changed oil. Got 15 mpg. Where is the pre filter exactly? I guess I've never heard of it. My TC locks up fine and the overdrive works, I generally try and drive around 60-65, my rpm's are around 1750. Sometimes though, around 40-50 mph, it kicks in and out of overdrive sporratically and if I either let off completely or put the hammer down it will quit. It doesn't do this often though, so I'm not sure what the deal is? I'll try and check the fuel pressure here soon and see what it reads. I'm running 285/75/17 BFG all terrains for tires, I don't think they'd hurt the mpg's much over stock, but I could be wrong?

 

The tires are giving you wrong mileage readings unless you changed the speedo drive gear. Those tires are almost 4" bigger OD than the stock 235/75/16's. that's 80 RPMs difference which ain't a lot on seat of the pants feel, but it is a lot in Odometer reading. You are actually going a lot further per tank than your speedo shows. Even one size up in tires needs to be corrected for accurate readings. here's a link you can compare tire sizes.http://www.discounttiredirect.com/di...foTireMath.jsp

 

Oh yeah, at 65 you were really doing around 74 !

 

Well, I'm not sure about that. I went through a speed zone with one of those Dept. Of roads radar speed sensor deals, and it said I was going 64, and my speedo was at 65....

 

check your fuel pressure and timing if all the filters are clean might be time for some new injecters too.
I just changed a set of injecters for a guy who was getting 14-14.5 now hes getting 19+ depending on his driving habbits

 

Did you just put stock size injectors back in it? Or slightly bigger? I've heard a bigger injector can actually give you better fuel mileage and performance, up to a point. I kind of hope my injectors aren't bad though, the pickup only has 149K on it and I'm a lil short on cash!!

 

just go and do what Obama wants you to do to get better MPG, inflate your tires. But really like was said before, you might need to change your fuel filters.

 

Before you get worried, get a pre-filter kit, about 8 bucks from Genos, the Pre-filter is just ahead of your lift pump, the fuel heater is right above the pre-filter bowl. Just unscrew it, change the screen, and reassemble.

Then get a pressure reading , if you don't have 25ish PSI at idle and 32-35 at 2500, then we need to find out why.


I think you should be OK with the tires, even though they are 17" rims, the sidewalls are shorter.

Oh yea, it may not help, but dosing it really hard with Power Service can sometimes help clean the Injectors if they are starting to spray funny. I usually hit mine a couple of times a year as preventative maint. I would put 16 Oz or so into the tank and run it thru.

 

Alright, my local parts store didn't have the prefilter kit, but I was curious so pulled it off anyway. It was dirty, but not plugged by any means, so I just cleaned the bowl with carb cleaner and used an air hose to blow out the screen, and put it back in. We'll see if that helps, I changed the fuel filter less than 200 miles ago, so I know it's clean.

 

My truck gets way better mpg at 55-60 mph compared to 65-70 mph. I suspect it takes extra fuel to push the air out of the way at 70.

 

we did 40 hp on it, and even though it only has 149k on it the springs inside the injecters have had pressure on them now for how many years?

 

Yeah true that. Ok, after cleaning the prefilter, and driving like a grandma, I still only achieved 14.8 mpg. I drive mostly highway, it's about 30 miles to work one way. I'm going to try and get an appointment to get her in the shop next week and get the f.p. checked, and I suppose do an overhead and timing check at the same time. slipped timing could be a factor of bad mpg, right?

 

http://dodgeram.org/tech/dsl/filter/pre-filt_clean.htm

http://www.cumminsforum.com/forum/94...-12-valve.html