My 04 cummins 2wd has ATS Aurora 3000 AFE big Cannon 2 intake 4 inch straight pipe into custom stacks ( by custom i mean i did them myself in the parking lot in front of my Auto Diesel class) ATS intake mainfold ATS Sub-Zero Inter-cooler Pulse Flow exhaust manifold with dual CP3s and 50 horsepower injector with AirDog 150GPH pump and stage 5 tranny with Smarty TST stack never been under 24mpg intown or high way and thats actual tank to distance calculations too. propain and W/M comming soon for more MPG and power. also Dyno'd at 717 Hp 1618tq. so not too shabby!

 

i get about 50. (in my tdi)

 

that's irrelevant... shove another 2400 pounds of lead in the trunk and see where your at i do love me a TDI tho

 

I just got back from a 400 mile round trip towing our 8,000 lbs. pull behind camper driving 65 mph. Once again, hand calculated I got right at 12.1 mpg with very little or no head wind. Maybe I got my hopes up to high with this diesel thinking I would get much better fuel economy and also trusting the overhead computer which reads 3 mpg higher than hand calculated.

On the trip back, my buddy who drives a Ford F-150 with a similar size and weighted camper got 10.5 mpg. Outside of the awesome low end torque and the longevity of my truck, I sometimes wonder if I would have been better off buying a gaser with prices 20-30 cents less per gallon.

 

You're still money ahead even with only 2 or 3 more miles to the gallon!! And you could always go have him try to keep up with you pulling a hill!! I enjoy out running gas rigs on hills.....

That said: I think you all better hang onto your trucks! Mine has always gotten 14 - 15 (I always hand calc.) since new. The mileage has never changed as the miles have accumulated (40,k of them now....). I also say it is slow or weak power wise. We just got back from an over 400 mile road trip (Running empty!) and got 15.01 mpg. She drove 75 down and I drove 78 back. In all fair ness we had a strong head wind both ways...
While you can't trust the over head; my other 2 Cummins were closer than this one. They were usually less than .5 off and almost never more than 1 mpg off. This truck is always at least 3 mpg higher than actual and often sneaks up to 5 or more higher than actual. This just seems strange to me. Why would my other two be closer? (And more powerful and get higher mpg?)?
Don't take anything for granted. Even stock your speedo and odometer can be wrong. I don't know what the actual statistics are; but it occurs more often than you would think. I have had trouble using GPS to check my speedo with. It just seems like they don't respond fast enough to be very accurate. There is error built into the GPS system on purpose (I would suppose to keep it from being used to guide missiles or rockets by terrorists and such). All I'm trying to say is you can be as accurate as you want in testing your equipment. If you have a 10% error in your speedo you are approaching the 'autograph' zone... The one where you get to pay 'special' and much higher taxes than those who don't get to give their autograph to the nice police man.
The 5 mile speedometer test sections are for you to check your speedo, not your odometer. The most accurate way to check your speedo is with a stop watch. One mile at 60 mph will take exactly 60 seconds! The easiest way is to get out on a hwy / freeway (Nice and flat too to help you hold a steady speed!). It doesn't matter how fast you go (As long as you stay out the 'zone' mentioned above!) but you need to hold it at a steady speed for the whole mile! Record how many seconds that mile takes and divide 3600 with that number of seconds. For example: say it took you 69 seconds to travel that mile. 3600 / 69 = 52 MPH. Another - if it took you 42 seconds: 3600 / 42 = 85 MPH.
The reason you can't use this short 5 miles to check your odometer is that it is hard to tell if it is off a few %. Best way to check this is to go on a trip and write down the beginning mile post and your odometer reading. Travel a good long distance (A 100 miles is pretty good) and write down the numbers again before you get off the freeway. Subtract the two and this will tell you if your odometer agrees with the mile posts. I have a vehicle that the speedo and odometer are off different amounts from each other......
As far as the dealer telling you it didn't matter what size tires you install.... They are wrong. If you think about it; it doesn't matter where the speed sensor is on your vehicle. The tires are 'sized' in RPM (Revolutions Per Mile) which is the same as the distance traveled per revolution (This is based on the tires Circumference). The vehicle has no way of knowing how far it has traveled except by the revolutions of what ever it is measuring. Most do have a sensor on the output shaft of the trans. (which is the same as on the driveline). If you think about it, it doesn't matter if it is on a wheel or in the rear end on the ring gear. As configured from the factory, one revolution will always result in the same distance traveled. This will remain constant until something changes. If you change out the ring and pinion gears this value will change. If you change tire size then it will change also because the tire circumference or distance traveled has changed. With my luck, the officer wanting to write the ticket would not let me off because my dealer said my speedo still had to be correct.
Here is a neat Excel spread sheet based on Dodge Cummins equipped p.u.s It is kind of neat to compare the different packages available and see what differences it makes. Another neat aspect of this sheet is that you can change certain values to see what differences it will make. For this discussion, near the top are boxes that contain the values for the size tire the truck came with. You can enter new values for the tires you now have (or are considering getting). This will change the "Rev's/Mi" for the tires. This will also change all the speed calculations below. This will get you pretty close to what engine rpm and transmission gear will give you on your speedo. If your speedo was close before making the changes, this sheet will get you close to what the new values are.

klenger.net

Hope this helps a little.... (Sure was long enough though ) . . . . LOL

 

That is crazy. Of coarse tire size will effect the speedo.

 

Regardless of where the sensor is the tire size has to effect the speedo. It is electronic so you will need to hook up and reprogram either the rev's per mile setting or the diameter of the tire itself.

Think about it a smaller diameter tire spins faster (more rev's) to travel the same distance that a taller, slower moving tire does. It is all based on pi(r, squared).

If a dealer told you that either they didn't understand the question or they missed the boat.

 

seems to me, 4.10 gears get better mileage, on the whole. seems stupid by conventional gas engine thinking, but diesels are made for torque, not hp.

i run 3.73s in my auto 4x4 quad with 315/70r17 BFGs and 147k on the truck. mileage is around 16-17, adjusted for tires. power's okay (better than any gas i ever had) with an ez on the 1st setting,...maybe i ought to test mileage on other settings.

i too expected 18-22mpg. i think the culprits are:
1) 3.73 gears
2) factory variance in timing (i'm gonna 'd.i.y.' the several degree advance soon)

 

Sorry, doesn't work that way.

Although generally speaking you will can get better highway mileage on those gears with bigger tires, it isn't the gears its the tires. This however doesn't offer much when you consider stop and go driving where the greater rotational weight comes into play making the mileage suffer over a smaller tire which would have less rotational weight.

So, 3.73 is better than 4.10 but not as good as 3.54 or 3.42 or anything of that sort. This however comes at a price if you tow big loads because it will take longer to get that weight moving with a "highway gear" vs. a "towing gear" like a 4.10.

Your auto transmission plays greatly into this role as well. If your TC is goofed then don't expect decent mileage at all.

 

Tires make more of a difference here than you might think.
My bro has a Jetta TDI. He usually uses his 17" wheels during the summer and that is when he makes more road trips too. He just made a long road trip and had to put back on a set of tires and wheels closer to OEM. He came home and put his 17" wheels / tires up for sale. He got a consistent 5+ more MPG with the other tires! Just looking at them you wouldn't expect any difference between them . . .