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Mileage on the Toyota Prius

Has anyone else found their mileage going way down on the Prius? Mine is four years old and I am getting about 35mpg when I used to always get over 45mpg.

Winter fuels and temperatures do cause reduced mileage. However the difference you are seeing is a lot for just that. Also it is possible you have already factored that in.

I would start by making sure all the scheduled maintenance is up to date. That would include plugs and wires air and fuel filters etc.

Have you just changed tyres? The OEM tyres were low rolling resistance tyres and if you did not replace them with like tyres …

having said all that, I would suggest having the computer scanned to see if it turns up any error codes, even if the CEL is not on. Note: I doubt if a standard scanner will be able to read all the possible codes or it may not be able to read any. I would also suggest having the batteries checked. If they have lost some capacity, that would give you a hit on mileage.

Lastly, have you been measuring your mileage consistently?  How are you measuring it?  Some methods are subject to a lot of error, especially those electronic read out ones.

I just had the scheduled maintenance done. They said everything was in order. I have new tires, Michelin Primacy (supposedly for the Prius). I changed to a higher octane gasoline on the suggestion of the tire dealer. It has been cold here lately. I check my mileage every time I fill up and it has never been this low. I check it by dividing the number of gallons into the number of miles I have driven.

Are your tires properly inflated? Cold temperatures can reduce pressure inside of tires. You have swithched to a higher octane gasoline, that may affect your mileage. Put the gas with octane rating your manual calling for, I have tryed higher octane gas on my car and my mileage went down. Cold temperatures can affect mileage as well, it takes more time for engine to reach operating temperature. Do you have proper oil for winter? Oil that would work ok for summer, may be too thick for winter.

I did not find that tyre, but it appears to be a performance tyre, not a high fuel mileage tyre and that could be part of your problem.

You seem to be using the right way of computing the mileage so there are fewer things to look at.

There is no reason you should need higher octane than recommended by the manufacturer. In fact it is possible that higher octane may give you lower mileage or damage the car, but it is rare. Usually you just waste you money at the pump. With octane enough is just right, more or less is bad.

The most probable reason for the drop in fuel mileage during cold weather is because of the Mass Air Flow sensor. This sensor measures the air mass that enters the engine. And from that the computer determines how much fuel to inject into the engine. And the colder it gets, the more air mass there is for the engine to take in. So the computer’s going to inject more fuel.

Also, if it’s cold where you are, there must be snow/ice on the roads/streets. If that’s the case, you probably don’t realize how many times you spin the tires when moving from a stop. And if that’s happening, you’re going to use gas but not move as far.


It has not been that cold in Napa. We don’t get snow or sleet, in fact, we are praying for rain. I still am getting under 40mpg and nobody seems to have an answer as to why my mileage has gone down. I have had this car for four years, and it just happened during the last few months. I would appreciate learning if anyone else has had this happen.

I have seen this question be asked by so many Prius owners that I wonder if this is what happens when the batteries start to degrade and can’t hold the same amount of energy.