CGRotorhead wrote:Got some new smaller tires and steel wheels for my trip down to San Diego this summer. I think these will help my power and mpg while towing my trailer.
The smaller tires will change your effective axle ratio, so you should see a power improvement.
Gas mileage is a different issue, especially on newer vehicles with electronic automatic transmission. Since the shift points are controlled by the computer, changing to smaller tires means your transmission is shifting in the wrong spot to maximize efficiency, so you can actually see lower mpg...much lower in some cases. As a result, if the vehicle is automatic and new enough, you would need to buy a programmer that would allow you to change the tire diameter for the computer. Of course, going smaller, even on newer vehicles, isn't as bad as going to a larger tire diameter. Very common problem on the post 2008 Jeep Wranglers.
The good thing is that if you have a manual, you control shift points, so increases in mpg can be achieved. You will need to test your speedometer after the change with a GPS to determine the adjustment that will need to be made for miles traveled to get a proper mpg computation, but that's a simple process since most phones can access an AP that will allow GPS speed monitoring.