Gave in... Bought a prius!
 

Well the motor started going out on the 96 civic and then basically everything else started failing faster than I could fix it so I've sold the green *******. Flew Wed from Memphis to Akron to pick up a 2010 Prius I. Has a lot of everything except leather, touch screen, moonroof, but who needs that. It has all the gadgets anyways and its a nice no NVH car, basically exactly the opposite of the Honda. So the car gets great gas mileage around town, but at 70mph it only gets 39.1mpg over 700mi all interstate (45F). The EPA rating is 52/48! What am I doing wrong?
Car has Cooper cs3's in the stock size inflated to 55psi and all the fluids are fine.

39 seems low, but Priuses don't like higher speeds and colder weather. At least that's been my experience.

Driving 70 mph isn't going to help anything, even if it is a decently aerodynamic vehicle. The EPA tests don't have to maintain 70mph for more than a few seconds. Consumer reports puts the average MPG at 44 in real life. You're not terribly far off.

The tires could explain the 20% lower than everyone else that you are getting. But from what I have seen, you should be getting 42-45 on the freeway at 70.

Non-LRR tires; cold weather; driving west.

Tires are probably holding you back. My '05 will usually get over 40MPG even going 80MPH (don't judge me!) unless there's a good headwind or bikes hanging off the back. That's with LRR tires at the sidewall max and half or full lower grill block. Cold doesn't seem to hurt much with long trips vs hot and using the AC. Short trips are definitely worse in the cold.

Sounds like I may have to sell the Coopers and get some energy saver a/s tires. Does tire size effect the cvt highway rpms?

It shouldn't, but your odometer and speedometer will be off and I don't know how expensive it would be to bribe a dealer to reprogram the car to correct it.

As I understand it, a Prius is not a normal CVT. Rather, the speed of the output shaft is a ratio between the large electric motor (orbital gears) and the engine (sun gear). The electric motor can spin up, to a point, to slow the gas engine down, and vice versa.

I wouldn't mess with the tire size unless you get a screaming deal on something close to the stock size. The CVT can already lower the engine RPM below what's required to maintain most speeds, so you would probably just be adding more weight with taller tires. Here's a little explanation of how the tranny works (I believe it's for the Gen1 Prius, so the exact numbers and ratios are probably different for the Gen3). Toyota Prius - Power Split Device

Financially, running the current tires till they need replaced would probably be best. You'll have a difficult time getting decent money out of the used tires, and the Prius is so efficient/gas so cheap that the slightly lower mileage won't make much difference. If you're just looking to put up big MPG numbers, then by all means, swap the tires. ;)

Haha I really really want to swap the tires. But I wont worry about it too much. I've done the LED light conversion to get the lights to look like HIDs and I'm pretty happy with it now. This thing is fantastic! Reading Vman455's post and my car slowly dying convinced me to get the Prius and it was a great choice. :D
Would a passenger mirror and wiper delete make a noticeable difference, from your testing?