Affect of Car Top Thule Box vs. Small Utility Trailer on MPG



I have a '97 Explorer Sport with a large Thule box on top. My son says I would get better mileage by replacing it with a small utility trailer to lug my stuff between Canada and Mexico. Any opinions on this?


He MIGHT be right…If the trailer is small and below the roofline of the explorer, there MIGHT be less drag. But all things considered, since you already own the Thule Box, stay with it. A trailer means another registration to pay for (plus you must buy a trailer) and paying extra tolls on most Mexican highways… You will save nothing…


A small trailor will be hassle AND likely to be stolen.


There is no answer to you question “long distance.”

Our neighbor, and automotive engineer and heavy duty statistician, has a 14 year old son who has done school science fair projects on your topic.

The dad and he built a 4" diameter wind tunnel, powered by a leaf blower, and resting on an adjustable angle inclined plane for their experiments.

They used toy cars and trucks with various configurations and calculated drag for each variation.
Some were surprising; for example a semi truck with the typical long box trailer has less drag than a semi with a flatbed trailer. The reason for this is the box trailer has a sharp, abrupt top trailing edge which serves to “break” the bond of parting air and vehicle. The flatbed does not, and thus has more drag.

This is why one sees those small lips at the upper trailing edge of some SUV roof lines, and on the trunks of some cars. They work!

To conclude before even I fall asleep, the Thule box may present less drag than even a tiny trailer. One cannot know except by experiment and monitoring MPG over a long trip.


The registration counts a lot. If there are lights on the trailer, you’ll have wiring and an extra load on the battery, but not much. You won’t be able to back up at all. The wheel bearings will fail regularly on a small trailer and good luck if the tire goes flat. You want to keep your cargo undamaged. I had my own trailer question this Spring. I have two lawns to mow, my own and one at a relative’s house. The trailers all seemed to cost over $400.oo. I can’t use a push mower on hills. I went and bought a second lawn tractor and parked it in the other garage and bought an extra garage door opener remote. Stargate would call it a GDO or GDOR. Now I park the truck, push the button and ride off into glory. No loading or tying down, no registration and almost no worries. Plus, I don’t have a trailer parked outside all Winter.


Figuring out these things is complex. That is why the auto manufacturers build wind tunnels to answer such questions.

BTW while you may (or may not) get less wind resistance with the trailer, you WILL get more rolling resistance with two additional tyres bearings and additional weight.


Before you buy a trailer, you might want to rent or borrow one and try backing up with it on the car. Backing a trailer is presumably a learnable skill, but a lot of foks don’t consider it to be a skill worth learning. Given the difficulty of backing modern vehicles (due to poor visibility of where the corners are and seats that block rear visibility) you may choose not to acquire a possession that will make backing up even more difficult.


Ya know, I’m just a caveman! I don’t know anything about “wind tunnels” and “drag coefficients”.

But there IS one thing I DO know:

You paid WAY TOO MUCH for that Thule box, so if I were you I’d use it as much as possible to get my money’s worth out of it. A round trip drive between Canada and Mexico is a good start…


Thank you. After reading all the comments I think I’ll just stick with the Thule box. I have learned how to cram it full. It stays on top of my car all the time, so I have no parking or backing up hassles.


Over the last 35 years, I have gone both routes.

A small trailer rides in the wake of your vehicle, providing less drag. However a trailer is a PITA (That’s not a piece of bread, that is an acronym for Pain in the A…)


Besides wind drag, what actually goes into producing better miles per gallon? does anyone know where to surf to get the whole story? I know I probably should have posted this somewhere else but Im computer challenged so I just jumped in. I bought a Kia Rio 5speed and the window sticker mpg is 35 and 39. Im getting 20, the same as a Hummer. I want to research what could be wrong.


How about one of those trays that attach to the trailer hitch? It is about a 4x5 feet tray and can be attached into the receiver.


I would stay with the Thule Box. Last year my wife and I traveled up to Anchorage Alaska from the San Francisco area with a Thule (667TT - Excursion 13 cubic feet). I calculated the gas mileage over the 3,472 miles we drove on the way up (we also took a ferry for a few days). The average mpg was 24.7 mpg. Our 1999 Subaru SUS typically gets 26 mpg on the highway. The cost of using the Thule was then about 1.3 mpg (26-24.7). This is a pretty affordable cost for the extra storage.

Trailers have all type of problems. You will have an extra tire that can go flat, it can be difficult to back up in a hurry, finding parking can be a pain, and it could cause problems driving in wet weather on windy roads. Plus if you buy a trailer you will have to clean up the garage to store it.

Good Luck on your trip.



most likely, you are shifting at way higher RPMs than necessary.

do you make “jack rabbit starts?”


Check several things:

  1. Tire Pressure! 5 psi low can zap up to 5mpg.
  2. Are you in 4WD? This can also zap mileage.
  3. Is your air filter clean?
  4. Check the parking brake to see if it’s stuck.
  5. Are you using the right shift points? You should shift @ 3-4000rpm. Any higher and you are just wasting gas. Today’s modern manuels have a shift light that tells you when engine revs are high enough to shift up. Don’t rely on it exclusively, but it may give you better insight as to the ideal shift points for you vehicle.
  6. Are you doing mostly city driving ,highway or a mix? This will affect mileage.