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Discussion Starter · #1 ·
I took my 03 Prius to the dealer for a 15,000 mile maintenance and to replace 1 blown and 1 about-to-blow tires. Before the maintenance, I was getting 45-48 mpg (51 mpg on the freeway). Since then (350 miles), I have not gotten more than 39 mpg. A mechanic at the dealership told me it was because of the new tires, which were the recommended Bridgestone Potenza. The BS meter was way off the scale. The dealership is not helpful. Any ideas?
 

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Tire pressure too low? Wrong size tires? Similar, but not exactly the same model number on the tires? Did they do an alignment? If so, did they do it right?
 

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Is the tread pattern similar to your other two tires? I had to have a tire replaced a week ago and the tire salesman told me I should not buy the more expensive tire because the tire tread wasn't similar to that of the other tires. He said that the medium priced tire was a better choice because it was more compatible and I'd get better mileage.
 

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I don't think a mere 350 miles is enough distance for you to conclude you mileage is down. Several tanksfull maybe. Isn't there a "settings" screen where you're supposed to make corrections for more accurate screen readings, especially when one changes tires? Maybe we all trust the readouts too much. Perhaps we should compute the fillup gallonage and odometer readings. I'll bet we'd all be in for surprises. I've been fooling with computers for just a few years, but I can tell you, they're sneaky. :wink:
 

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Which Prius do you have? the new one or the Classic?

Also, check the tyre pressure. That's always a cause for lower mileage. What type of tyres are the Bridgestone Potenzas? What's the spec for that tyre?
 

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If you only got 15,000 miles out of the tires, it is pretty much certain that the PSI was way too low. So it is likely still that way with the new ones, which will definitely cause lower MPG.

You should have an absolute minimum (measured cold) of 35 PSI in the front tires and 33 PSI in the rear tires.

And tires do have a break-in period. Regular MPG won't return until you have a few hundred miles on them, with soft rubber. With hard rubber tires, it can take 1,500 miles to reach break-in.
 

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They didn't do any software updates at the same time, did they? Isn't there a famous TSB that has the effect of reducing mileage? Don't recall if it was relevant to an '03 or not, but a search on the "tech" forum might turn it up.
 

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Oil level? Service departments habitually overfill the '04-05 models, and it hurts mileage. I don't know if this is so common on the '01-03 models, but it's easy to check.
 

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The sharp, clean edges on new tred will reduce your FE pretty easily. After a few tanks and a few thousand miles those edges should be smoothed out and your FE should return.

Watch your inflation pressure, as others have said.

I doubt that oil level has anything to do with what you are noticing.
 

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Discussion Starter · #10 ·
Decreased MPG

Thank you for your replies. The tread on the new (2) front tires appears to be very similar to the older (original) ones on the back. Tire pressure is 34/35. I've been operating the air conditioner about the same amount and the routes I drive are identical to before. I removed boxes of papers from the trunk before taking the car to the dealer; therefore, there has been less weight to carry. I use the same gas station. I haven't found a way in any manual to change the settings on the display. When I first bought the car, the tires were new and for a short while under-inflated but I was getting great mileage from that time up until now. (A friend from another dealership said that cars on his lot have under-inflated tires to make the ride seem smoother for new buyers.)
I am frustrated. It does not seem logical that 2 new tires would drop the mpg by 8-10 mpg - a 16-20% drop. I never thought I'd be nostalgic for 40 mpg readings.
 

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Fragility

Fragility....

It looks like that any minor misadjustment alters normal performance or it is just blackmagic in the whereabouts that makes to guess everybody.
Anyway, more precise diagnostics are needed when loosing 8 to 10 mpg's.
 

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I replaced all four tires last month. My MPG gallon went from 53 to 47. At the same time the temperature and humidity went way up. I remember from last summer that my MPG went down from 53 to 50 when the weather changed to very humid and hot. Now that the temp. and humid has gone down my MPG is back to about 50. At least in my case I believe the new tires can reduce MPG.
 

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Bump up your inflation pressure. How about using 40 front and 38 rear. That will stiffing up the ride a lot, and you should see some immediate MPG feedback. And don't forget what I said earlier. The sharp edges on the tread of new tires increases rolling resistance. The overall design of the tread may increase rolling resistance. Eyeballing the tread design means nothing with regard to how a tire is going to perform in terms of ride, grip and rolling resistance. Lots of other factors are there that can't be seen with the eye, such as rubber composition, sidewall flexibility, steel belting quantity and design, and the list goes on.

Tires, and tire material and design have a lot to do with overall vehicle efficiency and this applies to all vehicles. We notice it on the Prius because we have plenty of feedback from the vehicle about our performance. Before passing too much judgment get a few tanks of fuel and a few thousand miles on the tires.

Just as an additional thought, find out how soon, or how many miles, you can return the tires for something else. These tires may not be all that great for your needs in the end. Factory tires are notorious for not being ideal on the vehicle, but are there because some kind of rubber needs to be on the vehicle as it rolls off the line. I will not be replacing my tires with Integrities when the time comes. They are just too inexpensive and to ineffective as modern tires are concerned.

Just personal opinion, but I wouldn't touch Bridgestone Potenza's for the world. Too bad that was the tire that shipped on the 01-03 models. I'm not all that happy with the Integrity on mine. Terrible wet and snow performance and nothing to get excited about on dry.

And RM, it isn't Fragility of the vehicle, it is the characteristics of new rubber on the car. All vehicles have this issue, but we notice it because we are told what the car is doing while we are driving.
 

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A classic can't have its programming changed without replacing the ECM/ECU, unlike the 2G which can be flashed.

The 'screen adjustment' is for the NAV on the 2G. Not sure if it is also available on a classic NAV.
 
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