Declining mpg on my Prius

I have a 2008 Prius that I bought in Dec. Then it was averaging 48 mpg. It has steadily been declining…today avg. 46.1. I’m doing everything right - slow acceleration, coasting whenever possible, etc., yet it continues to decline. Service tech couldn’t find anything wrong…HELP!

I think you are within a margin of error that will require a long period of checking. The computer on your dash isn’t as accurate as monitoring mileage and gas added over a 2-3 month period.

Check tire pressure. If it’s down, your mileage will suffer.

I agree. There is also what I would call a calibration period for most of these devices meaning they can learn to be more accurate. Even at their best they are not as accurate as real measurements. They are very good and good accuracy when you are comparing say driving at 55 mph vs 75 mph. Neither may be exactly right (one may be a little low and one a little high) but they should provide useful information.

In the REAL world, most Prius cars get 38-42 MPG computed by actually measuring the amount of fuel used…Dashboard gauges are always guesstimates.

Are you running the A/C? If so that could explain the reduction of fuel mileage.

When people bring their cars to the Toyota dealer with a not delivering as promised/declining gas mileage concern how is it handled? Lip service? Toyota must have anticipated the situation where people are monitoring their gas mileage very closly. Is there equipment available that will measure gas mileage down to the oz.per mile (just to give a spec).What is the dealers obligation here and how far does he have to go to prove “meets design intent”?

Changing temperature will change density of gas. Depending how the dashboard display determines gas flow could cause a change. Tires should be expanding if any small change other than leak so probably not the tires. At your next few fill ups fill the tank as far as possible and note mileage between trips. On the receipt make a little note about recent driving: mostly in town, mostly highway, mostly 1 person, mostly more people, etc. Check every month or so and see how it’s doing. If you can use the same gas pump that will increase reliability in mpg calculations.

You need to ask the dealer what the Dash is actually reporting. There are a number of excellent suggestions here but you really need to know what sensors in the car are being used and also if the monitor has a learning period (uses some statistics of past behavior to predict the present) or just gives instantaneous calculations based on some set of sensors at that moment. If that is the case. find out what the sensors are and write back here.

Less than 2MPG is not really that significant in 45MPG+. It is likely your AC compressor running and running the engine more often.

Thanks to all for your suggestions…I live in South Florida so you better believe I use a/c!

fill the tank as far as possible

I would recommend not filling as far as possible. Topping off the tank of a modern car can damage the vapor recovery system costing some big $$$. However the general advice about carefully checking the real fuel used and miles driven is right on. I suggest three manually checked tank fills. If they are all similar then you have a good measure. Try to fill the same amount each time, but don’t top it off. Using the same pump and stopping it when the pump first clicks off usually provides accurate data.

Where did you get this information? I’ve kept careful records of the gas consumption on my 2007 Prius. I record the dashboard readout at each fill-up, as well as the mpg as calculated from miles travelled divided by amount of gas added. I then reset the dashboard readout at each fill-up. At the end of 12 months, the average of all the dashboard readouts was 48.9 mpg. The average of the calculated mpgs was 47.8 mpg. I consider these numbers as pretty close. The dashboard readouts are hardly guesstimates.

I’ve had 3-different Prius (Pri). And my millage varied with gas stations. For a while, Virginia required all of its gas contain 10% corn alcohol. Using this gas would reduce my MPG by more than 7 or 8, as displayed by the car computer. Filling up in North Carolina brought my average right back up, all other conditions the same. Now North Carolina also has the 10%. But for a while, a local Shell gas station fill up would get me over 50-MPG. And now, this same station uses the 10% corn alcohol. I believe you may be experiencing a loss due to almost all gas now containing the corn alcohol.

Is there another name for “corn alcohol” is there a type of alcohol that comes from corn that cannot be aquired from other bio-sources?

Corn alcohol. There is not form of alcohol that is unique to corn.

Eddie was apparently referring to ethanol. It can be produced from various vegetable sources, including sugar cane (a good idea, used with great success in Brazil), or corn (a bad idea that has only helped to accelerate the already rising prices of many types of food). A much better idea is to produce ethanol from Switch Grass (the process is being worked on in the hope of making it more efficient) or from wood pulp.

Unfortunately, there is no lobby for Switch Grass, so we are probably stuck with the political boondoggle known as corn-based ethanol. Politicians of both parties have latched onto this concept in a crass attempt to win votes in the states that produce corn.

That’s a 4% change . . . and given the number of factors that could cause this change, I’d live with it. Keep doing the same things you have been . . . check the tire pressure, easy acceleration, anticipate stop lights, fill the tank in the AM for more dense fuel, make certain that your car is tuned up, air filter changes . . . and enjoy your 46.1 mpg . . . I think the 4% is insignificant. Rocketman

And, as has been stated, since the hot weather is causing the OP to use the A/C, the gas engine is operating 100% of the time when he/she is driving. It is unfortunate that Toyota (and the other hybrid mfrs.) are not more open with information about the use of the A/C causing a hybrid vehicle to essentially bypass the electric motor function.

Since the weather in many parts of the country is such that use of the A/C is virtually mandatory in the hotter months, hybrid owners will never see the advertised gas mileage during those months. And, in areas like the South and the Southwest, that could easily mean 1/2 of the year!

My '07 Prius with 20k miles gets BETTER mileage in hot weather. 50 to 52 as opposed to 46-48 in the PA winter.