MPGS Have Fallen from 48 to 39

oh boy. are the premier 'low rolling resistance'? if so, maybe they will need some break in.
also the new brakes might be dragging. who did the work? did they do an alignment?

 

We don't have ready access to your car to diagnose what is going on. But we have a few clues about what might be going on. Here is the milage questionnaire put together to establish a baseline so we might understand:

Regardless, I'm interested in seeing two, 10 mile, benchmark runs, 10 miles each direction:

1. Under 41 mph on cruise control - reset tripmeter after reaching speed
2. Over 49 mph on cruise control - reset tripmeter after reaching speed

The first one helps us understand if there is a problem with hybrid-mode operation. The second one tells us if there may be an engine issue. We'll compare it with this benchmark to understand the problem:

Find fairly flat routes where you can run 10 miles, both directions at a fixed, cruise control managed speed. If there is a beltway (i.e., circular route,) perfect, one 10 mile loop will do it. Otherwise make two, 10 mile runs in opposite directions to take out elevation and wind effects. Average the two runs, a photo works great.

Two things we've recently seen:

• Excessive cabin heat - this can inhibit hybrid mode where the engine does not shutdown
• Driving in "B" - useful descending tall hills, it also keeps the engine on all the time

You mentioned two alignments. Any possibility of getting a copy of the last one posted here? The reason is sometimes the rear-wheels can be off from frame damage that wasn't corrected.

An equally valuable source is to measure the tire wear if you have a say 5,000 miles on the new tires. If there is still an alignment problem, the tread-depth will vary. Typically this requires 4-5 measurements on each tire.

Bob Wilson

 

Always look where you've been.
I'm presuming that you check your fuel efficiency manually, and tank-by-tank, and I'll take your word for the 7-8MPG change.

1. Make sure that the tires are properly inflated and manually check your vehicle's gross alignment (let go of the steering wheel on the interstate and see if you get an immediate lane change.) Many people in this forum over inflate their tires, which is probably OK in this car as long as you don't exceed the maximum value on the sidewall of the tire.

2. You're a little early for brakes in a Prius. The G3 has 4-wheel disc brakes. Drive the car 10 miles and feel your rotors. You might be dragging a foot. If one of the rotors feels excessively warm (like: you can't keep your finger pressed on the surface of the rotor) then you need to have your brakes double-checked.

3. If the car has been dealer-serviced, that might mean a 10,000 mile oil change interval.
Do you check the oil about every other fill-up? If not, you need to check now to determine if you have some premature engine wear. I'd also look at things like the PCV valve, MAFS, and the throttle body......and check to see if the engine air filter has been changed recently. These are low probability items, but they're easy to check and don't cost much if you do it yourself.
Be careful mentioning "throttle body" to the dealer.
That's like running can openers if you have pets in the house.....and for the same reason.

Get back with us and let us know what's going on.

I have a company car, and I don't think I can beat on it hard enough to make it get only 38MPG for a whole tank.

 

Uh, you can but it would make you sea sick unless you did a lot of surface time in small boats. <GRINS>

Bob Wilson

 

LOL!

Actually?
I did.

Quick tip:
Never say "I don't get sea-sick."
I watched a senior chief get green and barf into a trashcan (we called them something else) about an hour after he was bragging about never having gotten sea-sick in a 28 year career.
We were in 40-foot seas and taking water down the bridge access hatch during a surface transit - and I got close to engaging in sympathetic sickness - which is different altogether than motion sickness.
Since I have a lot of time in surface ships, I'm less prone to motion sickness, as in....it hasn't happened......YET.

Submarines have a unique roll cycle (because they're shaped like cigars) and there's no external horizon to give your eyes a chance to clue your inner-ears into what all of the motion is about. Sharing atmosphere with 120 of your closest friends doesn't help, although most submariners are used to some of the more aromatic side effects of living in a sewer pipe with stern planes. Having been in the cargo hold of a few military transport aircraft, I would imagine that that's close to what you get albeit for a shorter time period, but most trash haulers are smart enough to fly over or around the kind of WX that threatens to make the wing tips touch----hurricane hunters and some patrol aircraft not being in that more intelligent grouping!!
Bubbleheads are also notoriously prone to sea sickness since submarines spend much of their time below the wave action, where even a hurricane's fury is reduced to a gentle rolling action if you go deep enough.


I beg the OP's pardon for chasing that squirrel.
We now return you to our regularly scheduled mechanical mystery.....already in progress.

 

I once took a ride in a car driven by a former Naval aviator . . . who flipped between formation flying and intercept combat maneuvers . . . in rush hour traffic. Cinched that seat belt on tight!

Bob Wilson

 

Hi Zbrink;
I upgraded to Michelin premier A/S factory 17 inch tires and was having a hard time getting close to my summer MPG mark ... Until dealer noticed that I had uneven rear brake pads, indicating that one had stuck and wore the pad all the way down. After new pads and rotors (too much Syracuse winter road salt on rotors) I was back to expected summer MPGs. I even set a new fuelly record of 49.0 MPG. So check your brake pads and ease of wheele spin if you can.

 

One of the good things about rolling a Prius is that the car is pretty easy to maintain.
Just lift the hood every now and again to make sure that you're not using oil------like every 1,000 miles.
The Prius' oil sump is fairly small (<4q) and so running a quart low and changing the oil every 10,000 miles can be sorta bad for it.
It's OK to be lazy and let the dealer attend to your car's maintenance needs, but don't let them BS you into thinking that they're the pros from Dover.
Example: Brakes and rotors.

Unless you drive like an idiot or you deliver the morning paper, you just shouldn't be blowing through brakes and rotors at 90K......and from your assertion that you've suddenly lost 10MPG (not 7-8!)....something is amiss.
Sometimes, people in places like Bastan, New YAWK!, Houston, and anyplace on the left coast will blow through a set of pads in 90K, but it's kinda hard to maintain a 48MPG tank average and drive like a teenager who has aspirations to drive in NASCAR.
As hard as it is to grind through a set of pads @ 90K, damaging a set of rotors while wearing the pads down to the chirpers and maintaining a 48MPG tank average is even harder.

<sniff-sniff!!!>
I smell........boat payment.
And.....they might have screwed up the job to boot.






Check their work.

 

Something with the car is wrong and I suspect it is rolling resistance because it shows up at 57 mph and 37 mph:

• @57 mph - should be 60-65 MPG, not 48.8 MPG
• @37 mph - should be 80-83 MPG, not 59.3 MPG

I would start with a survey of the tire and brake temperatures. Then we need to see what is going on in the transmission. Something is eating your energy and the tell is 'heat.'

Bob Wilson

 

Ha-ha! Asking your car to cool the great outdoors to 71° explains a lot.

 

I have a similar problem. My mpg has fallen from about 44 to between 35 and 38. This has happened before, but seemed to correct itself. This time I have been getting the lower mpg for 4 tanks of gasoline. We have been having temperatures in the upper 90's for several weeks and the dealer says that is why I am getting the low mileage.
Are most of you also noticing lower mpg when the temperatures are high?

 

sure, generally a few mpg's. but it's hotter where you are, and humidity really makes the a/c pump.

 

Have you recently started partaking in lunch hour autocross events??

 

Please repeat benchmark without air conditioner. My benchmark was without air conditioner.

Bob Wilson

 

Nope, no autocross events. In fact, I drive my cars quite gingerly. No jackrabbit starts or sharp braking. In the present weather, I'd say that the AC is probably on continuously. Anyone know how many mpg you lose due to the AC running all the time?

It would be unbearable to drive without the AC in our Houston 100+ temperatures.

 

Rear brakes can be fine UNTIL they're disassembled for the first time. Putting them back together properly is tricky: the caliper piston has a four-spoke pattern that MUST be positioned so that the pin on back of inner brake pad is between the spokes. And it has to STAY that way.

It's possible to even assemble it all correctly, and then before the brakes have decently "bedded" the piston may rotate, screwing things up.

Raising the rear and turning the wheels by hand with parking brake off will confirm or dispel this possibility. A slight drag is ok, but if they're fighting, you have a problem there.