FWIW. When I triked my Interstate I lost about 5-7 mpg on the highway. Taller tires (larger diameter) will lower your RPM at any given speed. How much depends on how much larger the diameter is. I have no idea what would be the largest tire you could fit in there. My guess would be that you might be able to lower the RPMs at the most by 100-200 with tires alone. If your tires are due for replacement, buying taller would make sense. Replacing nearly new tires just to lower RPMs, not so much. Here is a link that will give you some info about tire sizes. You can do the math to see how much RPM difference you can get.
http://ejelta.com/tiresize/index.html?tiresize=150%2F80-17&minwidth=&maxwidth=&minratio=&maxratio=&minwheel=&maxwheel=&maxdelta=2 Stock final drive gearing on a Valk is 2.83:1. I don't know what gears you have in the diff, but if you can get to 2.73:1 (biggest ring gear that will fit into a 7.5" Ford which is what I think M/T uses) that would give you about a 100-200 RPM reduction as well. I did the math on that years ago but can't remember what it came out to. Having said that, the question is whether the cost of the diff gear change (if you can even find them) would increase your mpg enough to pay for itself.
If you were able to achieve a full 400 RPM reduction, you could run about 70-75 mph & be below the 3000 RPM "magic" number where the carbs start to pull fuel through the mains. Other than the right wrist, the biggest factor is added wind resistance with a trike. The faster you go the greater the wind resistance. The added weight, about 400 lbs or so is also a major factor. Hope this is of some use. Stan
