I know my '05 Prius and my mileage in various driving situations like the back of my own hand. The 50 mpg or so I should be getting with my typical city driving has dropped to around 45. To someone like me (you know, a Prius owner )who notices every mile per gallon, this is a huge hit.

I did just get new tires on the car. My original stock tires were finally entirely bald. The new tires are Firestone Firehawk GT's. They are 15-inch tires, which I think is the same diameter as the stock tires.

One thing I did notice is that the installers managed to run my battery completely down during the installation - hopefully that's unrelated, and I assume they just left the car on or something and the A/C used up the battery. I have reset the mileage indicator since then so that the full recharging of the battery wouldn't cloud the mileage numbers, so that's not directly the issue.

I've inflated the tires to 38 / 36 psi from their original 35/32 psi, but it hasn't really helped. Has anyone else seen this big of a mileage drop? Is something else wrong? The weather hasn't changed significantly, my A/C use hasn't changed, and I can't think what else would cause this. Help!

 

New tires are alway good for a mileage hit. Just remember that the stock Integreties were great for high MPG, but poor for handling.
As your new tires break in you should gain a couple of MPG's back.

If your 12V battery was drained and now will not hold a charge, it can indeed lower your MPG. I would get it tested, good luck.

 

Most, if not all, hybrids come with special low rolling resistance tires as std equipment. If you don't replace with OEM tires, you will definitely take a mileage hit. OEM's look like Goodyear Integrity 185/65SR15.

 

Ok, let's start with the easy items:

• wheel alignment - do you have the numbers?
• Firestone Firehawk GTs - what is the max sidewall pressure?
• oil level - did they do that too?

Now one problem is low-rolling resistance tires, I only know of these:


The reviews at Tire Rack suggest this is a handling tire, not a cruising tire. Handling tends to come with little regard for mileage.

Bob Wilson

 

Thanks for the helpful replies folks. My 12V battery is nearly 3 years old, and it may be part of the problem. Is there any easy check I can do there, or do I need to take it in to the shop? That line of discussion makes me recall that, for the first couple of days after I got the tires changed, the 270V battery State of Charge in the morning was a couple of bars less than the night before. I found that worrisome, although it has been so hot here (100+) that I figured the battery fan may have run a few times, drawing down the SOC. Since then, I haven't noticed any such discharge, and the weather has also cooled a good bit.

As far as the oil level: the tire place didn't do that; I had just had the oil changed the week before at the dealership. I will double-check that this morning.
The tire shop did an alignment; I don't know what the numbers are, but they did say it had been well out of alignment.
The max sidewall pressure of the Firehawk GT is 44 psi. I will probably revert to my 42/40 psi soon, but I just felt nervous about going that high immediately. I know, probably for no good reason.
I look forward to wearing these things in. At least I have noticed a good bit better handling (apparently, you don't actually have to invoke the Vehicle Stability Control on every corner).

 

I would not rule it out but you can go by most car parts stores and they'll do a battery check for free. If you have volt-ohm meter, reading it in the morning would give a great clue.

GOOD LUCK!
Bob Wilson

 

After some further checking, the oil level looks OK, and the 12V battery looks OK. The only remaining possible culprits, I think, are the hybrid battery and the tires. I am thinking and hoping that it's the tires. I'm down about 10% on the mileage - maybe that will come up somewhat as they get broken in. My car-savvy neighbor tells me going to "handling" tires from "low rolling resistance" tires could well be a 10% mileage hit.

Thanks again for the quick help - good to know you folks are out here.

 

When a complex system was working well, and starts behaving badly right after you change something, the cause of the problem is usually: What you changed.

When buying tires, it is necessary to do some research and then to select "low rolling resistance" tires. Tire rolling resistance is a big part of the total friction.

 

I'm still interested in your alignment numbers. I've been doing some work in this area. Poor alignment for "low rolling resistance" tires is bad but for "handling" tires, it is murder.

I'm especially interested in toe and camber.

Bob Wilson

 

I don't have the numbers, but if I get back to the tire dealership this week, I'll ask. After the disappointing mileage numbers, I checked my paperwork and saw that the Firestone place would replace the tires within 30 days if I so chose. I had them switch to more of a touring tire; Bridgestone Insignias. My mileage numbers went up immediately. I don't think they will hit my 50 mpg commute again, at least not immediately due to the break-in of the tires, but I have gone up from almost 45.5 or so to nearly 48 mpg. Hitting 50 after the break-in might be possible over time.

The ride is also more similar to what I was used to. I think I will be a lot happier with these, although I probably can't take corners at 30 in the rain anymore (not that I drive that way .

Thanks again for the helpful posts.