Why is my hybrid mpg getting worse with time?



I have a 2007 Toyota Camry Hybrid with 35,000 miles. The MPG has decreased from 34.5 mpg for the first 10,000 miles, to 32 mpg by 20,000 miles, and now is 30 mpg at 35,000 miles. My father analyzed the online database for Toyota Hybrid mpg, and found that the median MPG got BETTER with mileage, from 35.4 mpg initially, to 36.1 mpg at 10,000-20,000 miles, to 37.9 mpg at 20,000-30,000 miles.

My MPG matched the reports of others initially at 34.5 mpg, but is now at 30 mpg, WAY below the 38 mpg reported by others.

Do you have any idea what might be going on? Many thanks for any suggestions!

Here is some more information that might be relevant:

I purchased the vehicle mid-January 2007. My driving routine changed in May 2007, but has been fairly constant from there. I am driving in Southern California (San Diego/Riverside County), with my typical commute about 20 miles each way. With errands this typically works out to about 50 miles per day, driving typically the same week to week. On average, I probably do one longer drive each week, to Anaheim or San Diego from Fallbrook, about 80 miles each way.

A plot of my MPG vs. time, with all fill-ups, is here:


A plot of my MPG vs. mileage, selecting only fill-ups after May 2007 that have an average of 50-80 miles per day, to eliminate unusual driving patterns, is here:


Thoughts we considered are:

- A/C Econo mode was turned off last summer and not turned back on. I always drive with the air compressor running, so there should be no difference for seasons, other than it having to work harder in our “extreme” weather. I’ve run for a few tanks with the econo mode on and have not found any difference.

- I installed a ham radio antenna on the car in October 2007. No difference in mpg noted.

- Driving habit change. When I first got the car, I was driving to squeeze every mile out of it possible as it was new and interesting, and eventually the novelty wore off. I have tried driving recently paying more attention to my driving habit, trying to squeeze every mile out of it again, and there seems to be no detectable difference.

- Bad Oil. Toyota of Temecula Valley put the WRONG oil (higher viscosity non-synthetic) in my car for its first oil change 3/23/07, which was drained and replaced within a few days. There was a really bad dip immediately, but it didn’t cause a permanent decrease in MPG. I would expect any MPG decrease from this to eventually go away with time, and not continue to make things worse.


Have you had any service done on the car recently? Spark plugs, wires, air filter, transmission fluid change?


The hybrids are designed to improve MPG for mostly city-style driving. Please check the estimated MPG from your window sticker, or check www.fueleconomy.gov. Your communting 80 miles a day, and I’m assuming on the highway, is different than the hybrid system was designed to work with. The electric motors work for low-speed driving, like up to 35 mph. After that, the gas engine kicks in to go faster.

But, even with that, it sounds like the hybrid system is not kicking in like it should. Have you talked to the dealership yet? It could be the hybrid system is out of whack. They can download information from the hybrid system to determine if it is operating correctly, and remember, they have an extended warranty on the hybrid system separate from the rest of the car for a reason. This system is extremely complex. If your a person that hates going to the dealership, you bought the wrong car.

  • Driving habit change. When I first got the car, I was driving to squeeze every mile out of it possible as it was new and interesting, and eventually the novelty wore off. I have tried driving recently paying more attention to my driving habit, trying to squeeze every mile out of it again, and there seems to be no detectable difference.

This is probably the biggest reason. I don’t have a Hybrid, but one of my jobs is working for a University whose members have a few Priuses (Prii?) between them. Some of them compete to see how many miles they can get from each tank of gas, and one doesn’t. The ones who compete always get much more out of each tank than the one who doesn’t.


There is a possibility that the car’s energy storage battery has developed a internal short in one of the cells resulting in a high rate of self discharge. This could put an extra load on the engine as it constantly trys to charge up that battery.
Have a qualified dealer check everything out.


We get this question very often, all makes and models. Few are as detailed as this one, but I’m inclined to diagnose each and every such query the same way – change in driving habits. Both jsutter and OP schester suggested this possibility, so simply add my support to that theory.

Most folks who describe a drop in mpg are hoping someone will provide them with a strictly mechanical solution – replace a sensor or adjust the veeblefetzer. Few are willing to concede that the car drives as it always has done; it is simply the driver who has altered his pattern. But that is my opinion.


Here is a list of maintenance that has been done to the vehicle. It may not be 100% accurate, but should give a good overview. It looks like my entries missed at least two oil changes in Q3 & Q4 of 2007. I didn’t miss the oil changes, just didn’t record them in this spreadsheet as that isn’t a primary purpose of what I am tracking.

3/23/07 Oil Change
5/14/07 Oil Change
5/14/07 Flat Repair
12/6/07 oil change & air filter
1/23/08 new tires, alignment
2/25/08 oil change

*edit: I have always changed my oil when the car says I need to - every 5000 miles.


My drive to work consists of 10 minutes (7 miles) from home to freeway, mostly hilly, top speeds 35-55
10 minutes on freeway at 70 mph, elevation gain for the first 4 miles, then down and flat from there.
10 minutes (3 miles) near city driving through Temecula. We are able to get up to speed of 45 before stopping at the next light.

The drive hasn’t changed, so I will take your advice and take the car to the dealership and see if they can figure anything out.

*edit: And the extra driving would most likely be considered city driving. My longer trips usually increase my mpg as I can sustain over 35 mpg on the highway always always. Trips to Anaheim are usually reported by the car at over 40 mpg. I haven’t checked lately if that is still true, so will try to do so on my next trip.


I will continue to try to raise my mpg by altering my driving habit. I have been doing this for the last few fillups among other things and there does not seem to be a difference in the data. The data among other owners points to increasing mpg with time, so it seems that there may be something mechanically wrong with the system if mine is constantly decreasing.


Good tip. I will take the car to a dealership (that knows the difference between oil types) and have them take a look at it.


Thanks SteveF. I have tried altering my habits recently to maximize mpg, but it doesn’t seem to have had any impact. I will continue to do such so that we may eliminate that as a possibility or further study it.


You did a good job of thinking of possibilities, but one you missed is winter. It was spring of 2007 when you driving routine was standardized, Spring means standard fuel is available an warm driving conditions. Winter means colder temps (that lowers mileage and may mean more short trips and means winter blend fuels that produce less mileage.


Good thought Joseph. Would the change in fuel mixture show as a gradual chance in mpg on an instantaneous (per fillup) mpg chart, or would it be more of a sudden change with a standard range for winter and summer?

Do you know when the change is made between summer and winter blends in Southern California so we could look at that on the plots?


Here is a guide from Car Talk: At the bottom of this page, click on Actual Car Info, Scroll down to Guide to Better Fuel Economy.
When accelerating and braking, imagine that there is an egg shell between your foot and the gas pedal, and you don’t want to press hard enough to break the egg shell. Of course, good sense says that if you have to, you will.

So many people will pass somebody, and accelerate hard, to get to the stop light, and stop, before that person. Is this rational?


One thought: Is it possible traffic conditions have changed in your area? If you’re freeways are a bit more jammed up than they used to be, causing more stop and go driving, that might account for some of the difference.

Another thought: are the new tires OEM?



There is a lot to cover here so the only thing I will suggest is a four wheel alignment if it is possible to do on a Prius. You should be getting good gas mileage with that driving in that area. I can’t wait to drive that way again.


What if other people’s mileage increases with time because, on average, hybrid drivers learn to drive in such a way that they always maximize their gas mileage? You had your best gas mileage initially while you were trying to maximize your mileage but then you say you stopped driving that way. I would try driving that way again, but for a couple months instead of a few tanks.

You said your route changed last May, and your best mileage was prior to that change (in the first 10,000 miles). Was that route more stop and go? Was there anything you did differently you can incorporate into your current route?

You noted that your current route is very hilly. You get TERRIBLE gas mileage when you try to accelerate or even maintain your speed going up hills. If you’re not going to create a hazard, let your speed drop a few mph.

You also said that you can get up to a speed of 45 mph between stoplights. Are you accelerating hard and braking hard at the next stoplight? Like others said, accelerate gingerly and let off the gas and coast before braking. I noticed when I drive our Prius at work, if you accelerate lightly it uses the electric motor, but if you accelerate hard it switches to the gas engine.

As for possible mechanical issues, listen to everyone else. You should also make sure the air pressure in your tires is correct by checking regularly, if you don’t already.


I am pressed for time and did not read every reply so I appologies if this is a repeat but I did not see any mention to tire pressure. You did put on new tires on 1/08 and a flat repaired on 5/07. At what pressure do you run your tires at and do you check them reguarly?


I don’t believe that traffic has changed. The short strip of the freeway rarely has traffic at the times I’m traveling (and I’m going opposite the mass flow).

The new tires are not OEM. If the tires were an impact, I would assume we would see an immediate impact on mpg, not the gradual that we are seeing both before and after.


An alignment was done when I got the new tires. The alignment did seem to be off quite a bit before hand, so that could be attributed to the gradual decline possibly. I would presume if it was contributing we would see a jump in mpg and no further decline with the new tires though. Does that seem logical?