Fuel Mileage Drop

wait for another fill up or two of gas and see what happens. Every once in a while, my gas mileage will drop like that and it will pick back up. When I replaced all four tires, the mileage really dropped (my roommate says it was because there is more tread on the tires)

 

Check the air pressure on the tires, the dealer may have lowered them to the placard settings. Also, it might not be a bad idea to have the alignment checked.

 

Just to eliminate the most usual possibilities run the volt test on the 12V battery described here
Weird stuff happening? MPGs dropping? Test The Battery | PriusChat
and tell us all three numbers. You can just use the parking lights instead of the headlights.

 

FWIW, if the tires are set at the placard settings the Prius will not give the best mileage IMO. I suggest you check the tires with a pressure gage while the tires are cold. If they read what is on the placard, you might find that if you increased the pressure the mileage will improve. I run 42 front, and 40 back. Other's here do that or slightly lower. Some go even higher. Whatever you run, try not to exceed the maximum pressure for safety sake. All tires are marked with a maximum pressure on the sidewall. You will notice a firmer rougher ride with the increased pressure.

Also, if they knocked the alignment off a little when they did the extension shaft work, that will also affect the mileage. That is why I suggested you have the alignment checked.

You mentioned something about different tires or types of tires on the car. What type and brand is on the car now? Are they tires that have low rolling resistance?

Finally, please post all three values of the test that SteveLee suggested you run. That will help quite a bit in trying to figure out what is going on with your Prius.

 

jack the car up and make sure all the wheels spin freely. make sure the plugs they put in are right type and gapped correctly. otherwise, it's unrelated and a coincidence. does the engine sound normal? does the battery gauge appear to indicate similar to previous? how many miles on her? is it the original 12v?

 

If your battery bars in the display are green, running 35-40 mph in cruise, the system should go to all electric on the display and run for about a mile before engaging the ICE again to recharge the battery. This is a simple way to get an idea if the system is operating anywhere near normal. My son never goes over 50 mph and gets 58-70+ mpg on the real time display on level surface. Of course average is less, usually around 60 mpg. The display will show the electric energy flow steadily going to and from the battery. The ICE should kick off at most stop signs and lights at least until you start to accelerate more than lightly.

 

My guess is the dealer pulled the 12V line when they were working on the steering extension shaft (close to the airbag sensor) and that has reset the running mpg average. Did you lose your radio presets and have to reset the auto-up driver side window (if equipped)? I usually don't reset my mpg reading very often so I get a rather high (47.3 to 47.6mpg) average until a reset where it will drop into the low 40's for a while. The spark plugs they replaced should be the iridium tipped variety. They are factory gapped, and shouldn't be adjusted. Iridium is strong but brittle.

 

The plugs are not gapped?? That does raise the question of what did they put in? The correct plugs are gapped.

Edit: Ok, maybe he meant they didn't gap them, since they are factory gapped. I think the right ones are DENSO PREMIUM CLASS IRIDIUM SPARK PLUG.

 

I do not know your location but if it has gotten colder or raining and also using more heat from the heater or defrosters and more fan speed all will drop the MPG's.

 

^^^ This.

The workshop manual for almost every procedure on the Prius calls for the disconnection of the 12V battery (whether everyone actually does it or not is a different story, but almost every procedure calls for it).

The Prius stores what's called "historical data" to help improve the accuracy of the displayed MPG (in effect, to calibrate the fuel consumption calculations). This is basically long term averages of actual fuel used and miles traveled. This is used to provide corrections to the instantaneous fuel consumption calculations, which are in effect nothing more than counting the duration of fuel injector pulses to estimate (sometimes relatively roughly) the instantaneous fuel usage.

When the dealer disconnected the 12V battery on my Prius the displayed fuel consumption dropped significantly, however the actual fuel consumption (as measured at the gas pump) remained unchanged. It took about a month actually, for it to rebuild its "historical data" and the displayed MPG to gradually go back to it's previous accuracy. Here I'm talking about per tank MPG calculation. Pre 2006 models automatically reset the mpg calculations (but not the historical data of course) each time you fill the tank, so you have no other option than per tank averages. On later models you have the option of either resetting it manually when you fill and doing tank averages, or leaving it be and doing long term averages. The OP didn't say if he was previously using per tank MPG or if he was using a longer term average. If he was using long term averages then this even further confounds the issue.

I'm sure this is the root cause of so many of these posts (yes there are tons of them) of visiting the dealer for some minor operation and then losing 5 or 6 MPG. Eventually most of these mysterious mpg losses just clear up by themselves with time and miles driven.

 

With a mix like that and the pressure's you posted, it does not surprise me the Prius is not delivering good mileage.

I see eight pounds difference in tire settings which is incorrect. Furthermore the front tires should be equal to one another on both sides AND be two pounds higher than the back. I suggest you even out the pressures accordingly. Try 42 in the front left and right and 40 in the back left and right.

Considering that the brands of tires are not alike and that you actually have four different type tires on the car will definitely contribute to mileage loss.

I suggest you consider purchasing a full set of LRR tires. There is a list here on Prius Chat that is stickied that calls out LRR tires.

I would also suggest that you invest in a good alignment once you replace the tires.

One thing that does confuse me, in your original post you said the following.

Today your post illustrates that the tires have not been changed. The two post's conflict with one another.

Ron

 

I do not know your location, But beware that running different tires and treads, the car can run very odd. Especially at High speeds and in snow or rain. It may cause you to loose traction much more easily,it will squirm.

 

Uart pointed out that the mileage will drop if the battery is disconnected. I too can vouch for that. When I changed the battery in our Prius, I did notice a drop in mileage. Also, one other time the dealer disconnected the battery to do work on the Prius. Again, the mileage dropped. After a couple of tanks, the mileage returned.

It may be possible that the dealer disconnected the battery when they did the work on your car. Along with the other factors that have been pointed out, to me it seems that what you are noting could be normal for the car.

The MFD works off of current data and also historical data. Whether or not it is cleared when you reset is unknown to me.

My best advice is to adjust the air pressure as I have pointed out more than once. If you find that the overall mileage (not per tank figures) continues to drop then I would look at other alternatives.

Ron

 

You've misunderstood what I've said there. It's the MPG data from before the battery was disconnected that should be more accurate (this is the correctly calibrated data, the new MPG data is not properly calibrated yet.) This statement however assumes that you've been resetting the mileage data on the dash, otherwise you're further confounding the issue with long term versus short term averages.

Think of this "historical data" as calibration information to make the cars calculated MPG more accurate. It's a different thing to the long term MPG average that you can reset from the dashboard. The calibration information is only cleared when you remove the 12V power.

Think of the instantaneous MPG calculation (based on accumulated injector pulse widths) as only being approximate, say plus or minus 6 MPG or so. Over time the car builds up and internally stores some extra calibration data to make this more accurate. This calibration data is definitely not cleared when you reset your mileage data on the dash.

BTW. For some people the MPG is calibrated upwards, for others it gets calibrated downward. It just depends on whether the initial injector pulse-width based estimates are plus or minus.