Fuel Mileage Drop

[JohnnyR]

Towed my motorcycle (Goldwing 1800) from Houston to Ruidoso, NM on a 2-wheeled utility trailer. Bike and trailer were about 1500 lbs. behind my '05 Duramax 2500 HD. Had not carried the motorcycle on this trailer before but had towed this same trailer with a 4-wheeler on it many times and routinely get 17.5 to 18 mpg, knocking my empty truck mileage down about 10%. I only got 13.5 to 14.5 mpg on the Ruidoso trip and really can't understand why it was so low. Truck ran fine, no smoke from the tailpipe when idling or at steady speed. The weekend before we left, I replaced the stock Bridgestones with Michelin LTX M/S (stock size- LT 245/75/16) and I changed the fuel filter. The Michelins do ride a bit rougher, sidewall must be stiffer than the stock tires, and the tread is a little more open than the orig tires, but it's not a mudgrip by any means; and I had them on my "01 Duramax and didn't notice any change in fuel mileage. I even felt all the brake hubs to see if one was hotter than the others to no avail. Only thing I noticed was what looks like a light oil and dust stain where the intercooler pipe hooks into the blue hose at the rear of the engine. Would appreciate input on the mileage drop.

[Dakster]

You've changed too many variables at one to really knock a specific reason down. I don't now what type of quad you carried ebfore by a motorcycle will have a higher wind resistance to it, speaking of which you could've had a head wind. In addition, you changed tires, which on a another truck didn't change mileage...

When you got back home and drove around town do you still get reduced mileage? If you don't then I would bet either the extra wind resistance of the motorcycle or other environmental factors contributed to your loss of mileage. I drove four times thru Smokie Mountain National Park in ONE WEEK and got different MPG each time. Each time about the same weight was in the truck, although time of day was different and so was the temperature, wind, and one day it rained. (It only varied by 2 mpg, but that is still around 13%-15%)

If you do get lower mileage then the next easiest thing to do is switch tires back (if you can). Do you have any DTC's? Light oil and film is gonna happen underneath the hood, however, clean it off and see how quickly in that area it comes back. A leaking intake pipe can lead to bigger problems than lousy fuel mileage. (Dirt and Debris) I would check connections and bolts and how the intake pipes are sealed to each other why I was there anyways, just to make sure nothing was out of whack.

I'm sorry I can't help any more. I know it is frustrating when all of a sudden MPG's drop for no good reason... Just be thankful you don't have the regen process to deal with as well.

[rob from bc canada]

What speeds did you drive at??

My 05 empty will get 19-20 US mpg at 60-65, 18-19 at 65-70, and drop right down to 17mpg doing 75.

The more wind resistance your load has, the more difference speed will make.

Total load weight really only matters a lot if you're running hills.

[Mark Rinker]

My guess, if your speeds were similar, is that the drop can be attributed to a headwind/tailwind difference and the outside air temperature. Was it hotter than the last time you towed? My LLY mileage dropped badly in hot weather, especially while towing.

Also - check your air cleaner, just in case...

[Kennedy]

Like many have stated here, wind, speed, and even fuel quality can be major factors. You also did not say if you'd made this same trip before with the trailer. I fing that geographical changes can have a HUGE impact on my MPG.

As for the tires, I'm surprised you feel the Michelins ride rougher especially used versus new. To me new tires always ride better, and Michelins have been some of the best riding tires I've ever owned.

[JohnnyR]

Thanks for the comments. As for the Michelins, all I can say is it appears sidewalls have perhaps been reinforced some to better handle loads over the last set I had 4 years ago. I've e-mailed Michelin and asked that question. Not necessarily rough but definitely stiffer with 50 psi in front and 46 psi in rear for the light tongue weight I had. As for DTC codes, haven't checked yet and will have to go to dealer for that. I suppose I could run by Autozone and have them plug in a code reader, although not sure how accurate their's would be. First time I've been this route w/this trailer and motorcycle, but have pulled my 11,000 lb 5th wheel with a 12 foot face this route and gotten 10 mpg, so the 13.5-14 still struck me as low. What doe GMC charge to run diagnostics/warranty work after the 36K miles warranty is up, $100 or $150 per visit?

Haven't driven much empty after getting back but initial feedback from computer suggests about a 5% mileage decrease, but too early to tell. The Michelins, according to the charts, are 1.3% larger in diameter than the orig tires to begin with even though size designation is the same, so maybe I'm just being paranoid.

[DmaxMaverick]

The variables effecting mileage are too great to excect the same mileage as another driver and/or truck. As long as you can get the numbers in the ball park, which I think you have, you can tweak your driving habits, towing conditions and equipment to improve your mileage. Comparing my old 5'er to new (1995 Savanna to 2005 Mountaneer), my average improved by more than 1 MPG. Same truck, same driver, same habits, with only the trailer being different. New trailer is longer, 2K+ heavier, and about 6" taller. I attribute it to improved aerodynamics. The new trailer even seems less effected by strong crosswinds. At highway speeds, air is your #1 MPG killer, and even a small utility trailer can screw up your aerodynamics enough to make you scratch your head. Last January, I made a trip up to northern Idaho from central CA with a 12' utility trailer in tow hauling a pickup cab. Total load was ~3K, but my mileage was only about 3-4 MPG better than hauling the 14K 5'er.

Try upping the tire pressure, front and rear, by 5 PSI at a time. Your rear pressure is too low, IMO, to get the most MPG. I used to adjust tire pressure to get the most miles out of them, but after some very detailed research and calculations, I found mileage suffered more than tire wear, dollar wise. The $$ return is better, even at +1 MPG to pay for the tire wear loss.

Dealerships have a shop rate, and you can guess pretty close as to what it will cost for some jobs. Diagnostics are usually a 1 hour charge, or they will have a flat rate. Any code reader will do as well as the next, as you only need the number(s). You can then look them up online or post them here for more details on the DTC.

Tire rollout will vary for the same tire size according to the mfg's. I have found these figures to be very inaccurate and inconsistent. You have to measure them to know for sure what you really have. A 1.3% change can be significant or not. That difference is about what you will have from new to old, accounting for tread wear, and a MPG increase of 1.3% can be attributed to error or other variables, so the point is moot, anyway.

Another thing to consider is your method of calulating your MPG. The DIC may be inaccurate enough to put you out of your comfort zone. Any program updates you've had (by the dealer) may have changed this as well. If you are using any type of a programmer or box, the DIC will likely be inaccurate. Best to calculate your mileage by hand in any case. If you are a DIY'er, do yourself a favor and get a good code reader or programmer capable of pulling/clearing DTC's (I recommend Predator, but others work). With some of these, you can also adjust your odo/speedo to be more accurate, which will give you a more accurate MPG figure. In OEM trim, the odo/speedo's are generally not that accurate.