I think it has more to do with the grade. Seeing lots of posts on FB with folks having 37s and towing large campers without any issue. They all seem to be in fairly flat parts, however. The rules change when you gotta climb even a small hillit depends whether what you're towing is a big heavy sail or just a heavy load that doesn't give you a ton of wind resistance. travel trailer is 8ft x 12ft sail. you could tow way more without engine struggling if you're towing a trailer that is 6ft x 2ft wind facing footprint (ie. open cargo trailer). you combine weight and the big sail and you're redlining every time you go up a hill. Forget gas mileage because you're burning about 40-50L/100km. to give you perspective, going 90km/h i average less than 20L/100, probably average 18L if there's no wind. If i go 110km/h, I'm more around 21/22 which is a 20% increase in gas. Step it up to 120km/h and you're lucky if you get 25L/100.