1960 Dodge Dart Seneca wagon in NH

Somehow, I think that Chrysler corporation had the know-how and foresight to gear the specific vehicles properly when installing the 6 cylinder engine. I have friends that had that engine in various different vehicles and, yes, some of them were the Valiant and some were not. The cars had plenty of power in all road conditions that I was exposed to when in them. Wisconsin is a very hilly state and the engine handled them just fine. Well, until the engine had a ton of miles and was worn out. Even then it ran good but lacked the power that it had with lower mileage. It's pretty common knowledge that in-line 6 engines have good torque. I don't think Chrysler would have put those engines in heavier vehicles knowing that they would not be able to handle the job.

 

The 318 would be subject to less hard work, depending on how the vehicle was driven and what load it had to pull. I'm sure that if you took 2 identicle Darts and equipped one with a slant and the other with an 8, the slant would be more economical if driven at 40 mph down a steady even road.
In fact, they performed a test between a BMW M3 and a hybrid Toyota Prius. When both were driven pedal to the metal, the BMW turned out to be more economical: http://www.youtube.com/watch?v=JmxUsGiGp3w

 

Tuned properly, our 72 never passed the 30 mpg (Imperial) mark for fuel economy. It would not manage Sault Ste. Marie on a single tank of fuel, and that was being driven carefully. One of my friends' fathers had a 73 Valiant with the 318 and regularly managed about 32 on the highway, plus he had enough power to pass when necessary. The 72 was really struggling in Northern Ontario on some of the grades. As an aside, the steepest, although not the longest, grade on the entire Trans-Canada Highway is in Northern Ontario, not in the Rocky Mountains as you might expect.