While towing my 8000 lbs trailer to San Diego from AZ, I was averaging 10-11 mpg at 65-70 mph. no real head wind to affect it, and the only modification done to the truck was slightly larger tires (285/65r20 instead of stock 285/60r20). this seems low as I use cruise control to hold a steady speed, nothing in the instruments seems to indicate a problem. I had just replace the air filter back to a mopar air filter from a bosch one. I had heard that mopar filters are just the way to go. do i need to have it looked at? i know that im probably looking at maybe 2-3 mpg increase but hey thats something.
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2021 2500 diesel getting low mpg while towing... ???
- Thread starter ace22398
- Start date
Blackpowder
Active Member
I tow a 9K travel trailer with my 3500 and my mpg is 11-13, depending on terrain. 13 mpg is on a very good day and flat terrain. I typically set the cruise to 62. I'm retired so no hurry 
H3LZSN1P3R
You're doing it wrong
You can spend big money to save you very little chasing that MPG. There is no point it wont get much better. All the brands get about the same milage. You can check if the trailer brakes are dragging thats about it. Your millage is on par with everyone else that can accurately calculate fuel milage.
Have you adjusted the speedo calibration for the larger tire?
Also, use nothing but the Mopar air filter part number: 53034051AB or Fleetguard AF27684
Also, use nothing but the Mopar air filter part number: 53034051AB or Fleetguard AF27684
I did not need to calibrate, as i check different gps speed they ready accurately to the speedometer.
H3LZSN1P3R
You're doing it wrong
5% taller should change the speedo unless the tires are smaller than advertised?
How are you checking?
If you're using your phone and it's connected to the truck radio, the GPS speed shown is not actual.
I went from 285/60R20 to 285/65R20 and had to change my tire size.
Your MPG calculations are off because your speedo is off.
MPG calculations are not being done using my speedo, just simple mileage to gallons used. it will not matter if my speedo is calibrated.
here is an extensive fuel history from my fuelly app.
Dude, if the tires are 3% larger and the speedo is 3% off, the mileage is 3% off and you've traveled 3% farther.
H3LZSN1P3R
You're doing it wrong
If your speedo is off how can the millage be correct….
What is 3% of 10mpg? 14mpg? Not significant enough to show the kind of difference I've noticed. But thanks for your input, it has been noted.
If you use actual miles (not indicated by your odometer, which is tied to your speedometer) and actual gallons in your calculations....
Never said it would make a significant difference. All I'm saying is that your calculations are off, no matter how much, if you did not correct the speedometer error and are using the trucks odometer, which will also be incorrect, to record the mileage. Using anything else, GPS, maps, etc. is inaccurate. 3% is based on tire listed size difference, not actual. If the Transforce measured slightly under and the replacements over, it could be 4-5%. Additional rolling resistance of the new tire could also contribute to worse fuel economy.
As to the air filter. It's not that the Mopar is likely to flow different enough to effective MPG, but it could affect emissions and regen. Now, more frequent regen would make overall MPG worse.
That said, looks like you generally get 10MPG towing, which is pretty much what almost all of us get towing. Do you track your DEF MPG? Do you track active regen?
If I got your empty MPG, I'd be thrilled. I'm hoping my mileage improves once the engine hits 30K. I don't run empty a lot except in the winter, and then it's usually inclement weather, hunting, and other use that's not generally highway cruising. I don't expect great mileage on winter fuel.
Those ugly July dips are pulling big passes in Colorado with 16K in tow.
H3LZSN1P3R
You're doing it wrong
Based on what? Anything other than actual miles traveled by the ODO would be inaccurate
I would imagine that your elevation has some kind of effect on it as well. My MPG is pretty stable, but I did get the new tires in May this year. If I did wanted to get the speedo calibrated, is that something I need to take to the dealer? at this point im not too worried about it, but possibly interested.
If your speedometer is off, your odometer is off. Hence the need to know the REAL/ACTUAL miles traveled ...