cartop v trailer

get slightly better mileage with surfski
Actually get slightly better mileage with just surfski (hull up) on the roof. Have a wagon, so the teardrop shape is piercing the low pressure area and helping with airflow. It isn’t much, 5% at most, but the numbers have been consistent over the years.

Plastic boat (hull side up) really isn’t a difference.

With a couple of boats with one in the j-rack, get a 10% hit on highway economy.

Speaking of GEAR
How your vehicle is geared also makes a difference. With stop-go-turny-townie type of driving, mpg goes down noticibly in even a small truck, regardless of load. I’m assuming the truck is geared fairly low in 1st and 2nd gears, of course.

On highway trips, once the thing is up to cruising speed it stays there for a long time with much lower fuel consumption required to maintain speed, other things being equal. It doesn’t need to keep going through the thirstier stages nearly as often.

So . . . what’s the answer?
Better mileage car topping or a trailer? I’m not sure the question has been answered or that anyone knows.

I also don’t know. More importantly, I don’t care. I would make the decision based on which method of hull locomotion is more convenient for me, everything considered.

That decision is easy for me since I never take more than three hulls. I car top and want nothing to do with trailers, which I think are klutzy and just something else break and maintain and cost money.

There may come a time when I can no longer lift my canoes atop my van. Then, the calculus may change in favor of a trailer.

I checked my mileage several times
while cartopping one and 2 boats. There was absolutely no difference in either of my vehicles.

To be fair, one was an F150 and the other a Tundra. Both got 15 mpg no matter what was carried or topped.

I have never used them with a trailer.

just mpg or cost benefit?
It’s pretty clear that with a super light trailer with god bearings you are likely to get better mileage on the highway. Not a lot. But the initial purchase and ongoing cost of ownership favors racks so much it will take a lot of miles to offset (and some trailer costs like maintenance are incurred due to miles, so it isn’t straight line).

Good point
The cost of some lightweight trailers would cancel-out a lot of fuel savings. That’s why I like the idea of cheap used trailers (at least for people who can re-build them into something that suits their needs).

On a related note, and this is something that was often mentioned by many different people here in the past, all this stuff can also be weighed in the context of a person’s driving habits. A person who routinely drives 5 to 10 mph over the speed limit can save more gas by not doing that than they can save by various modifications in how boats are carried. For example, I’m always amazed how many people with their modern pickup trucks that are advertized as being so fuel-efficient compared even to models of a few years ago end up getting the same MPG as I used to get with a 1971 IHC that had an enormous engine invented in the age of ancient technology. At least with larger vehicles, driving in a manner that requires you to get out of bed a few minutes sooner to reach the put-in on time is likely to save more gas than anything else you can do. This isn’t to discount the various other ways you can save fuel, but sometimes the simplest answer to the fuel-savings question isn’t even considered.