Sorry if this is brought up a lot. I did some searching but came up with nothing. I just bought a 1999 2500 24valve cummins, 266,500 miles, Ext cab, long bed, 4x4, Auto. It was all stock except the owner before me put a Airdog 150 pump on it. Since I bought it, I have put 4" straight pipe, BHAF, and have done all fluid and filter changes.(engine oil, engine oil filter, tranny fluid/filter, fuel filter, water separator,) I got through 3/4 tank of diesel and hand calculated my mileage, and it only came out to 13.1 mpg. This is probably 60% highway and 40% in town driving. I have tried my hardest to take is easy on the throttle, but every now and then the pedal dose get away from me . This seem kinda low to me?? I don't have any aftermarket gauges on it as of now, but I will have them by next week. Fuel Pressure, Boost, EGT Temp. The owner before me did put a low fuel pressure light in, but that don't tell me nothing. Where should I start from here? Low fuel pressure? vacuums leaks? Out of spec intake and exhaust valve lash? It has stock tires and im not sure on the gear ratio. Thanks guys!

Put a "raw egg" under the throttle pedal and try not to break it!!Your mileage will improve.

Make it an extra large raw egg. The best I get is around 18mpg highway and 16mpg around town, and my truck has 76k miles on it so it's a little bit tighter. Check which size dif you have. If it's a 4:10 like mine you're a little out of the ball park. If the number is smaller, 3:72 or 3:55 your mileage should be in the 20's for highway. Get your vin# and call the dealer,they should be able to tell you what size it is. Some others may be able to tell you how get better mileage but the egg works for me.

Make it an extra large raw egg. The best I get is around 18mpg highway and 16mpg around town, and my truck has 76k miles on it so it's a little bit tighter. Check which size dif you have. If it's a 4:10 like mine you're a little out of the ball park. If the number is smaller, 3:72 or 3:55 your mileage should be in the 20's for highway. Get your vin# and call the dealer,they should be able to tell you what size it is. Some others may be able to tell you how get better mileage but the egg works for me.

I believe I read somewhere on the door tag or core support tag it has 3:72 but I'm going to double check that tomorrow. IBMobile, I see you have the edge EZ. I was looking into getting one. How do you like it? Did it improve you mpg any? Thanks,

I believe I read somewhere on the door tag or core support tag it has 3:72 but I'm going to double check that tomorrow. IBMobile, I see you have the edge EZ. I was looking into getting one. How do you like it? Did it improve you mpg any? Thanks,

Any "box", or "chip", or "downloader" will give you the "POTENTIAL" for better mileage……………………….90% of folks never see it because they have a "HAPPY" right foot due to the extra power and torque!! Then comes the "bitching and moaning"!!!

I mainly use the truck to tow my 5er with the boat behind that, It's not my daily driver. As far a extra mileage goes I only took one trip in 04 with out it, I think my mileage was about 10mpg and any little hill the trans would kick down. After putting the EZ, boost elbow, and 4" exhaust on the trans doesn't kick down as much and the top end going up hills went from 55mph to over 65mph. I know now that going camping with a head or quartering wind 10mpg, with out 11.5-12mpg, and going home 13-13.5mpg. There are a lot more down hills going home.

most likely its a 3.55 rear diff. And when people get better mileage from a chip/programmer its because they pay attention to how they drive and do their best to get mileage.

most likely its a 3.55 rear diff. And when people get better mileage from a chip/programmer its because they pay attention to how they drive and do their best to get mileage.

That's what I'm after. A little more power for towing, but mainly mileage. 13 just still seems low for a stock truck. I'm in Eastern Kansas so its mostly flat, not many hills.

A rule of thumb I use (just like the raw egg) is to keep the boost below 5 PSI and EGT's under 600*F and I can cross 20 MPG without thinking...

^^ x2

Keeping an eye on the boost gauge is what really helps me. The more boost you are making, the more fuel you are using, the hotter the EGT's. When I am going for mileage, I never go over 10 psi, even if it means being the slow guy on the highway. I always average 5-10 mph under the speed limit on the freeway but I am making real good mileage because of it.

Just curious as to how you calculated that 3/4 tank. Did you fill it up and get the gallons or just figure you burnt near 27 gallons by the gauge? The gauges on these truck are not very accurate by just reading the gauge.

Just curious as to how you calculated that 3/4 tank. Did you fill it up and get the gallons or just figure you burnt near 27 gallons by the gauge? The gauges on these truck are not very accurate by just reading the gauge.

I filled the truck up full. Ran it till there was only 1/4 tank of diesel left according to the fuel gauge. Filled it back up, took my mileage that was on the odometer and divided it by how many gallons the Gas station pump read.

I filled the truck up full. Ran it till there was only 1/4 tank of diesel left according to the fuel gauge. Filled it back up, took my mileage that was on the odometer and divided it by how many gallons the Gas station pump read.

I know that sounded like a stupid question but I have seen folks think the gauge is accurate to the gallons. Do you have stock size tires? That can throw the odometer off. I see around 18 to 20 on the highway and 14 to 16 around town, depends on how many short trips I take. It has been mostly like that since it was new. I ran a Banks chip for quite some time and never lost any mileage, but gained some nice power. I did see some improvement with the addition of some rv 275 injectors, both power and some mileage. The mileage mostly occurred while towing the 5th wheel.

I know that sounded like a stupid question but I have seen folks think the gauge is accurate to the gallons. Do you have stock size tires? That can throw the odometer off. I see around 18 to 20 on the highway and 14 to 16 around town, depends on how many short trips I take. It has been mostly like that since it was new. I ran a Banks chip for quite some time and never lost any mileage, but gained some nice power. I did see some improvement with the addition of some rv 275 injectors, both power and some mileage. The mileage mostly occurred while towing the 5th wheel.

I have 265/75/16s. The Sticker on the door says stock is 245/75/16. Is there a equation that will tell me the difference? Thanks.

I have 265/75/16s. The Sticker on the door says stock is 245/75/16. Is there a equation that will tell me the difference? Thanks.

That is possibly stock. Mine came with the 265's as an option, but the door tag says 245. I dont what was available in 99. My 96 had 245's. I would think there is some equation to it but would not know what it is. If you have a gps you could make a run down the highway and see how the gps compares to the odo. Or just compare the odo to the mile markers in like a 50 mile run down the interstate. Not very scientific but would give you ad idea.

Tire size calculator:Tire sizes are metric so you have to convert to inches to determine size. 265/75 is a metric number so you need to determine what that is in inches unless you understand the metric system very well (I don't)1) divide the width of your tire (the first number of the tire size) by 25.4 (25.4 mm per inch)2) take your answer and multiply by your aspect ratio (2nd number on your tire size) This is the height of your tire from the rim to top of tire.3) multiply that number x two (top and bottom of tire) 4) Add the size of your rim to previous answer (16 in your case)EX: 265/75/16265÷25.4= 10.4 inches (overall width of tire)10.4 x .75 = 7.82 inches from your rim to top of tire7.82 x 2 = 15.6 inches (top and bottom of tire)15.6 inches + 16 (rim size) = 31.6 inches overall diameter of tire.JR