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?

The wiper delete, I don't know--although it is worth noting this is the only exterior difference between the (50mpg highway) Prius and (53mpg) Prius Eco. Removing each mirror takes around .3 ft^2 off the frontal area, probably around .005 off the Cd, and looks a lot cleaner.

http://i1047.photobucket.com/albums/...psd2tpfjqm.jpg

Both mirrors off, pizza pan wheel covers, no rear wiper, 1.5" lower, foam grill block, and some underbody paneling is good for Cd .21.

Is this by calculation, estimation, measurement or just a WAG?

Simon

Well looking at my five year cost of ownership chart over the civic with oil consumption factored in (basically an always 6mpg hit), i've got some distance to make up! So i've ordered the grommet for the rear wiper delete, block coolant warmer, iphone compatible smart programmable socket. Already have a scan guage. Hopefully this will help my 5am 18F commute I had yesterday especially since it's supposed to hit the single digits next week :( . I can fine tune my target temp by using the scan guage so we'll see.
Also did some weight reduction: Removed the carpet mats, trunk mat, trunk tray, spare tire and kit, owners manuals, no wiper fluid lol. *Feeling optimistic*

Grill block would definitely be on my to-do-soon list with temps that low. Blocking the whole thing should be fine.

I just took a trip home. Checked my tires and they weren't all at 50psi, so I rechecked all of them and aired them up, 43F temps, driving SW, and with the small weight reduction, and the passenger mirror folded in I got 45.6mpg trying to maintain 75mph for 200 miles. Definitely feeling much better about the car now. :D

So does anybody still make rear wheel skirt covers for the 2010+ Prius? I'm planning to get these to install and have a small tire in the rear. Looking at the VW XL1 I saw that it had matched its mm of tread per pound front to rear with a set of staggered tires.
With the weight of the XL1 @ 1753 lb, 45% F bias (@ 788.85 lb) with a 115 mm tire, 55% R bias (@ 964.15 lb) with a 135 mm tire.
This works out to:
Front: 115 mm / 788.85 lb = 0.1458 mm/lb
Rear: 135 mm / 964.15 lb = 0.1400 mm/lb

The prius has a 3042 lb curb weight stock with a f/r (%), 60.2/39.8.
Front: 195 mm / 1831.284 lb = 0.1065 mm/lb
Rear: 195mm / 1210.716 lb = 0.1611 mm/lb

Going for a 195 up front and rear options:

Option 1: 175/65/15 Michelin Energy Saver a/s
Option 2: something small too that will fit on the oem wheel but with the rolling resistance of the infamous re92
The new prius eco has enasaves, so enasaves or energy saver a/s? hmm...
Any Thoughts?

Thanks. Did not even think about coastdown. Pity there is nowhere arounfpd here suitable.

Simon