1. Swapping transmissions or components is unfortunately not an option.
2. Ride height is not a concern.
3. Drag ratio of car isn't a concern.
4. Drag ratio of tires is a concern. However, so is handling. I've used 175/70 R14's snow tires and find them to be too floppy. The 185 or 195 size seems to be the most realistic compromise here.
Right now with 185/65 R14's I get anywhere between 30 and 40 mpg. Once I got 44mpg but that was an exceptional case for a temporary job where I did a lot of travel along a long flat stretch of policed road with a 35mph speed limit. I was also running stock tires at 44psi.
Once upon a time in a former life I ran a with a set of 195/45 R15's. The reduced 22" OD provided much zippiness that was great for in-town driving, and autocross. However, 3,500 RPMs at highway speeds wasn't great for things such as fuel economy. The speedo was way off too. Radar signs would say that I was doing 75mph, but the speedo would be indicating a lot more than that.
I'm basically looking to do the opposite of this... bigger OD tires on 14" stock rims for taller final drive, less acceleration and potentially better fuel economy.
Has anyone actually tried this? If so, with what sized wheels, and what were the effects? For example, that little Honda engine only makes about 80 ft-lb of torque. Will the oversized OD tires render 5th gear useless for anything but downhill driving?