Engine tuning, tire selection, and proper hitch alignment can help improve fuel efficiency.

Dear Andy:

I really enjoy reading your articles and have been educated by them.

I have a 2005 Acura MDX and I pull an Aero Cub C-160 HTT. I have a weight distributing hitch and brake control as well, and everything seems to pull quite nicely. However, I've been reading that some of your combinations (minivan and travel trailer) get quite decent gas mileage. I currently get 27 l/100km when towing the trailer at 100 KPH. I tend to be towing in 3rd gear most of the time. Are there any secrets to getting better gas mileage?

My trailer is level (maybe a bit nose up) the tires are all inflated to maximum pressure. The MDX is a 5 speed. However, the transmission automatically shifts to 3rd most of the time. I usually leave the transmission selector in 4th when towing, but it usually shifts to 3rd automatically and stays there at 100 KPH. It has the same drive train as the Honda Odyssey.

I could put on smaller tires to increase the final drive ratio a bit but my tires are P235/65R17 Michelin Cross Terrain SUV. I currently get between 10-11 l/100km when driving at 120 KPH with no trailer. I can't believe that I get 27 l/100 km with the trailer driving at 100 KPH (pretty consistent these last couple years). Is this normal? I can't believe that some of your testers get 16-17 l/100 km with much larger trailers than mine (with slightly larger frontal areas as well - my trailer is 7.5' wide). Would lower elevation than where I live in Edmonton have any effect?

Any recommendations would be great!

Many Thanks

Brian.

Hi Brian:

Though the trailers we tow are longer than yours, aerodynamic drag is about the same so fuel mileage on a highway run tends to be about the same as well. Weight has very little effect on mileage except in stop and go driving.

However, your mileage does seem rather low, and having the transmission shift to third gear all the time also seems unusual. We have several customers with MDX’s and have found them able to tow in 4th gear most of time. I wonder if your engine control computer is causing your transmission to shift down easier than it should. I would have the dealer run a diagnostic on it and see if everything is normal. Adjusting the hitch so the trailer sits slightly nose down instead of up will help slightly but not likely enough to keep you in 4th gear.

At higher elevations you loose some power due to less barometric pressure, but you usually pick up fuel economy due to less aerodynamic drag. This was not the case in the age of carburetors but these days fuel injection calibrates the mixture for the atmospheric pressure so the engine is always optimized.

We do notice that the Odyssey delivers better performance and mileage when towing than the Honda SUV’s do. There is more drag in the 4 wheel drive train than the front wheel drive, but a big part of it is the larger tires on the SUV’s. This puts you in an RPM range where you have too many rpm in 3rd gear and not enough in 4th to be in the power band.

If you were to change your tires to 235/55HR x 17" they will be the same diameter as the Odyssey tires which is a 7% improvement. This should put you into a much better RPM Range in fourth gear and you should be able to tow in 4th 90% of the time. You will also notice a considerable improvement in handling both towing and solo. Solo fuel mileage should stay the same or improve slightly, even though the engine RPM will be 7% more the reduced rolling resistance should make up the difference.

If the MDX has the driving computer, use it to adjust your speed for conditions. In head winds especially it can be handy - sometimes slowing down just 5 KPH can improve your economy 20%.

There are three downsides to the lower profile tire: one is that the speedometer will read 107 when you are traveling at 100 KPH. Software for recalibrating the speedometer should be available soon however. The 55 series tires will not be quite as good in snow, and the tread life will be slightly shorter. The third downside is that the lower profile tire has less load capacity than the taller tire; however it should be more than plenty for the MDX.

For readers considering engine calibration and tire size change it is a good idea to weigh your combination’s individual axles and make sure the tires you install will have enough load capacity before changing them. Feel free to send an email if you want an opinion on a size change before you do it.

I hope this helps, feel free to send any further questions. (Page Top)