33.4 MPG, Down over 10MPG

I know it is cold, it is not the weather. I know that if I drive better I can get 60+ blah,blah,blah. I have read just about every post on here about low MPG (inlcuding the 12v battery issue) but none of them seem to be my poblem. I have an 07 prius with 67k on it. I have been driving it every day since July of 07, I drive 1 hour a day each way to work. I drive the same everyday and yes I drive very fast, 85+ on the highway and half the drive is back roads where I do 65-70, nothing hilly at all. In the last few months I have noticed a steady decline from my standard 43-44MPG to a current 33MPG. It has only JUST recently gotten cold at all AND the car is garaged at night and starts off not "freezing".

I thought perhaps I had found the answer with the 12v battery issue I had been reading about the last few days but last night had my battery tested at Autozone and he said the battery tested fine. I also had them check to see if any DTC's were present and he said there were not trouble codes (which I suspected since no indicator light was on).

I do my own oil changes each 4k, the oil level is fine, I just replaced the air filter last oil change. I run the tires at 42/40.

The dealer wants a diagnostic fee just to check it out but I would hate to pay a couple hundred just for them to tell me they can't find anything.

Any ideas??

Could a battery test good and actually be bad??
What about spark plugs, could they cause this issue??

 

Yes, it is 12.4 ish when in accesory only mode and goes to 14.4 when ignition on. I have also checked the battery terminals with a multimeter and everything looked fine. I only took it to Autozone so they could do a load test to see if perhaps it was storing a charge but didn't contain the amperage needed (i.e. it would discharge and the voltage would drop quickly) but the meter said it tested like it was a battery with 450 cold cranking amps. And yes, the car was off when tested.

Don't know about the transaxle fluid but ball joints are fine (I do my own oil change and brakes) just changed the brakes (new rotors and pads) about 7k ago. I would have to check the glide to see if there is any drag but nothing I have really noticed so far.

Just (about 1,000 miles ago) had the alignment done. They said there was really nothing wrong when they test drove it but I had them do the alignment anyway because I just got new tires. Yes, I know tires will affect it but the MPG drop started loooong before new tires (btw, it started back when it was 70 degrees and sunny). The tires I got are General AltiMax, I know they are not going to have as low rolling resistance as the stock tires but I had gone through too many stock tires be low RR tires wear so quickly (especially how I drive). And again, the mpg hit actually started before the new tires anyway.


The "way" I drive definately... definately affects my gas mileage and like I said I usually get 43-44, I know that could be higher. But my problem would be that without a change in the way I drive AT ALL what would have caused a 10+ mpg hit.

I will check the plugs tonight to see if they are at all fouled, I wouldn't think so either but I am running out of this to check that I can think of would affect MPG.

I have a scangauge coming in on Thursday so we'll see also what I can determine using that. Thanks for all your input.

 

I haven't really felt any extra drag but I will check the parking brake as well.

I know it probably isn't designed to be driven at these speeds. I would love any average MPG data from someone who drives similar to me, if any of them want to comment. I know I can get better mileage but I actually got this vehicle so that I *could* drive the way I do and still get o.k. mileage instead of getting 15MPG in a car that otherwise would be getting 24-25. 43-44MPG may be low for a prius, but I can (atleast I used ta could) get that MPG and still drive the way I wanted.

 

The tires are 195/60R-15 GENERAL ALTIMAX HP. Yes, I know they don't have the RR of the stock tires and they are wider than the stock tires. But this mpg drop started steadily 2-3 months ago. For a while it went to 40-41 and then in the 36-37 range for a few fillups and now the past 2 tanks have been 33.4 and 33.1.

 

Honestly I have not been on here is probably over a year. I thought I used to have my location in my profile, IDK what happened to it. I don't care if anyone knows where I am. I am in southeast michigan, I drive from Monroe, MI to Dexter, MI everyday. US-23 to I-94 route.

Again, driving conditions and style has not change and the MPG drop started long before I got new tires. Also, this was a few miles at a time steadily until I am where I am, it's not like over night it dropped 4-5 miles (right around when I got tires) it has been very gradual. I am hoping to maybe switch cars with my Mom who has an 07's Prius as well for a tank or two and see how we do respectively. I still think there is possibly some gradual issue that has been going bad over time which is why MPG has gone down. Like I said I have driven this car everyday for the last two years and have been STEADY at 43-44 the ENTIRE time until a few months ago. It has fluctuate slightly in the winter before but only down to 40-41.

Oh, and BTW for probably the previous year or so I have not had the original tires on the car anyway. I get flat tires CONSTANTLY and prior to buying this most recent set of 4 brand new, I went through 5 other replacements one at a time (70MPH down a pothole filled country road with PSI at 42 will do that). When I would replace the flats I would generally use whatever was cheapest at the place that was closest to where I got the flat tire.

 

No, I will try that when I get home tonight and see how hot it gets. When I read about the battery issue I thought for sure that was it. I expected the load test to fail miserably yesterday when I had it tested and am now at a loss.

I will check battery temp when I get home, plugs for fouling, and any possible sources of drag.

 

FYI - On the tire thing read this post:
http://priuschat.com/forums/gen-ii-prius-modifications/20573-tire-upgrade.html#post253313

This is actually almost identical to every bit of research and conclusion I did before buying my tires and is the reason I settled on the 195's. I don't think the new tires or the tire size really have much affect at all on mpg. BTW for a few weeks I did have some 17" rims with 225/45R17 tires on the car and they actually fit. Those tires DID affect gas mileage but the rims themselves were also nearly twice as heavy as the entire prius tire, they are off now anyway.

When I got home yesterday I checked battery temp, it was not noticeable warm to the touch at all. That, coupled with the battery checks saying the battery is good leads me to believe it is elsewhere. I did not get a chance to check the plugs yesterday but will do that asap.

Scanguage comes tomorrow so I'll see if that can at least confirm the horrible mpg rating I know I am getting. Maybe it will give me some other insight as well. I am switching Priuses with my mom on Monday so I'll see if her car does any better on my route and likewise with mine on hers.