Interstate vs. secoundary roads?

I just finished a 2500 mile trip with a 2012 3.6 R Subaru towing a 2600 lb utility trailer.

Now I know this is not an RV but this somewhat relative.

Most of the miles were in California where the towing speed limit is 55.

On the very rough Interstate highways where I was able to maintain 55 with cruise control I was getting 24 mpg.

But on the very smooth state highways with towns about every 10 miles the mpg dropped to about 18.

Most of the state highways were much less stressful and a much smoother ride.

I drive 55 in California as I have out of state plates and do not want to be a selective enforcement victim.

IMHO the consistent speed of the superslab (interstate) will give you better mpg results every time. Stop & go traffic yields dreadful gas mileage as you have to overcome inertia to get your rig up to speed time & time again.

Years ago we were stuck in I-95 near Fredericksburg, VA and in that mile after mile of stop & go I could just about see the gas gauge going down.

On edit: Is the scenery on the interstates more boring? Yes, no question. I try to plan trips to be a mix of interstate and secondary roads to keep things interesting.

I found going out East last summer (over 2000km round trip) that the answer was a big no for me. The sweet spot for my TV/TT combo was abt 95km/h (60mph). Going over that point even a little bit made a huge cost in mpg (just going up to 65mph was enough to make mpg go downhill by 25%. Driving thru Mennonite country between Watertown and Lake Placid and across US2 from Lake Placid to Bangor was great, the mpg was much lower, and yes there were small towns along the way with the occasional stop sign or traffic light, but we found the days went much quicker because the drive was much more interesting than on the freeways. The 401 in Ontario and AUT20 in Quebec were a much more boring (and slower feeling)ride even though they shaved a few hours off the return trip, I wished we had taken secondary highways on the return as well.
I am in the process of planning next summer's big trip down the US East Coast (Ontario to Delaware, down the coast to Myrtle Beach, and back thru the mountains in VA/WV. I have been going out of my way to find scenic secondary routes, with the realization that keeping my speed below 60 will save me more money than the cost of an extra night in a KOA due to the extra time needed.

I always get better mileage on interstate; for best use your foot not the CC.

We traveled FT for 7 yrs. and took secondary/back roads as much as possible.

Hate traveling Interstates. Boring, idiots that don't know how to merge, road surface beat up, heavier traffic etc.

MPG difference.....never really noticed any. When pulling a 14K 13'4" wind sail we got 10.5--11.0 mpg regardless