Why the drop in fuel efficiency?

- What fuel economy are you getting and how are you determining fuel economy? (trip computer or manual calculations) RE: 2009 Toyota Prius Touring Edition Mileage 29,900. Trip computermileage reading of 39.7 MPG for 121 highway miles @65MPH in Texas heat of 104 degrees this week w-AC set at 72degrees on recirculate.
..
- What fuel economy are you expecting and why? 48MPH combined.

- What are the approximate outside air temps? normally in the 90's, currently 100-106 degrees

- How long are your trips?city 5-15 miles. highway 120 miles.

- How much of it is city vs. highway? Roughly what's the average speed in overall and and of each segment? Is there a lot of stop and go driving? 50% city-highway. city 30-40mph in Houston stop and go driving- highway 65mph

- What region/state are you in? (if you haven't set your location in your profile) Austin area and Houston

- What's the terrain like of your drives? (e.g. flat, gentle hills, steep hills, etc.) flat


- Is your oil overfilled? (i.e. above the full mark on the dipstick)
no-1/4th inch below top dot
How old is your 12v battery? What is the voltage reading of your 12v battery after sitting over night? (Method Here)2.5 years. 12.4V after sitting overnight. When the voltage was measured, the battery charger indicated only four blue bars-after the two hour highway trip of 121 miles the day before. Does this indicate a battery problem?

Have you had your alignment checked? Unusual wear? no alignment performed-tire wear even on tires.


- Are you using the factory tires and wheels? If not, please indicate tire make, model and size (e.g. Goodyear Assurance Fuel Max 185/65R15). Trip measured with factory touring tires-Bridgestone Turanza EL-400. I have just replaced them today with Hankook H727 P205/55R16.

- What are your tire pressures? 42F-40R

- Make, model, year, engine and transmission of previous car? (e.g. 08 Honda Civic Si 2.0L 4 cylinder, manual transmission) What did you actually get on the same trips/commute? (Please give us actual numbers, not EPA ratings.) 2006 Toyota Prius Base w- 48-51MPG

- How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking? Normal driving-keeping up with traffic.

- Are you "warming up" the ICE (internal combustion engine) by letting it idle after powering on? No

- Are you driving using D or B mode? D always.

- HVAC settings? Are you using the heater, AC, auto mode, etc.? If using auto, what temp is it set to? HVAC set on Auto 72-74 and set on recirculate air.

- If reporting a mileage drop, did anything significant change on your car (e.g. accident, hit a curb or big pothole throwing off alignment, oil change/other maintenance/repairs, changed tires or wheels, etc.) or your commute? The mileage has never reached the combined 48MPG. It normally averages 42-43MPG in combined driving. I changed the engine air filter before the last 121 mile trip. I use Castrol 5W-30 oil and change it and the filter every 3K miles.

 

With the heatwave Texas is experiencing I would expect a MPG drop, especially if you are using the A/C a lot and at low temps like you are. The compressor could be working overtime and cause significant losses. Even the diesel TDI guys are observing significant losses with the A/C. What is your average MPG overall, not on just this one trip. The one trip doesn't tell us much about the terrain, elevation etc.

Again, this can cause the battery temp to rise higher than is comfortable so power may be reduced. You are also pushing the A/C system pretty hard. See this video for evidence of MPG drop when the A/C compressor is working hard. When the ambient temp is closer to the desired temp of your A/C system the mpg drop is only about 1-2mpg, not 10mpg.

The 12v battery is not in the best shape but it should be fine. 12.4v is what mine reads and I can still achieve 55mpg with my OEM wheels.

I don't know much about your tires but I do know they are not listed as low rolling resistance so that could shave off a few mpg pretty easily. The fact that you just installed new tires makes the problem worse because all new tires require a break-in period before you will see your best mpg out of them. They are also usually taller (larger rolling diameter) than the worn ones you replaced so the speedometer will be slightly off and since the new tires are turning less revolutions per mile, the MPG indicator thinks you are traveling less miles per unit of fuel. So you get hit with a false mpg drop and a real one.

You have achieved normal mpg in your old car and if your commute and conditions have not changed then something could be wrong with your new car but I suspect it has to do with the extreme heat and heavy A/C use combined with non-LRR tires. Even the H727 is not rated as LRR so you will suffer some loss compared to those running the Integrity or other LRR tires.

I'm not going to recommend it but you could try to drive with the A/C off, driver's side window cracked about 3-4 inches and the passenger rear window cracked about the same. This will allow air circulation and often times is enough to keep me cool in 100+deg temperatures. If you can try this for a trip or two you can observe how much the A/C is or is not affecting your MPG.

 

8-10-11

F8L, and thanks for your reply and your specifically detailed questionnaire.

Commenting:
My overall mileage in the last year is 42-43 MPG. Yes this matches Consumer Reports "overall MPG" of 42 MPG for the Touring Edition, yet in researching many Touring Edition owners' results here on PriusChat, I feel that my car should do better. Also, the Consumer Reports "150-mile trip" lists 48 MPG for both the base and the Touring Edition.
As an aside, our previous 2006 base Prius attained 50-52 MPG on the same 120 mile trip to Houston, in all weather, using the AC at 73-74 degrees, traveling 65 MPH.

The scanguage mileage surprises me. It was my understanding that the AC is electric, not connected with a power robbing belt, and therefore would not affect the mileage. Can you explain the AC system mileage drain?

Yes we are experiencing record breaking temps now. However, in comparison to our previous 2006 model, it, too, performed in high summer temps. It's mileage varied only slightly from 48-52,53 MPG, regardless of the temp. We live in the same location and commute to Houston as frequently as now, while driving the same route and speed.

Noteworthy is that I purchased a new engine air filter last week before taking the trip in question which yielded 39.7 MPG, and I checked the Castrol 5W-30 oil level to be 1/4th inch below the dot on the dipstick. We change oil every 3K miles.

I am curious about why the battery was not fully charged after the 2 hour drive for 121 miles. Shouldn't it have been fully charged? My Prius formerly served as a dealer loaner car to Prius owners in Chicago. I purchased it 2-12-10 with 6205 miles on it as a Toyota Certified Used Hybrid. Could the battery have been discharged and damaged while in service? The reason I am considering this possibility is that after three years, my 2006 Prius suffered a large mileage drop until I replaced the 12V battery. Thoughts?

Our mileage for the last year and for this test reflected driving on the original Bridgestone Turanza EL400 tires, filled consistently at 42F-40R pressure. The 2006 model carried Goodyear Integrity tires filled to 42F-40R pressure.

After 29900 miles the Turanza's were evenly worn out, so I purchased the Hankook Optimo H727 tires at a slightly different size, P205/55 16R. After researching tires from three different owner's databanks and PriusChat forums, I selected the Hankook's. As far as LRR tires went, the Touring selection consisted of the Turanzas, --why buy rapid wearing tires again--, and only a couple others. Each of those, by research, proved a poor selection for my needs.

I also understand that we must use the E10 ethanol fuel and that it results in lower mileage. That said, our 2006 Prius also used the same E10 fuel and it maintained 48-51 MPG under the same conditions.

Thanks, F8L. I still think the overall 42-43 MPG should be nearer 46-49 MPG considering our driving styles and patterns. Thoughts?

Cordially,
archiveman2977

 

The AC compressor is indeed electric, but that doesn't keep it from using energy. Cooling requires energy. Despite wishes to the contrary, there is no free lunch. All work requires an expenditure of energy, and air conditioning is no exception. Cooling your car requires energy, which ultimately comes from burning gas, therefor reducing your mileage.

The direct drive variable speed electrically powered AC compressor in your Prius is more efficient than the older ones, but it still can't rewrite the fundamental laws of physics.

Tom

 

My pleasure but I cannot take credit for the questionnaire. I think Cwerdna created it.

Your MPG is not terrible. At least not enough for me to suspect something is terribly wrong with your car. There may be some small things negatively affecting your MPG though.

Like Qbee42 stated, there is no free lunch so when you use electrical energy to run the AC then you have to pay for that energy in fuel. Even though the AC is not directly tied to the engine, it is tied to the electrical system and hybrid battery. The initial hit to cool down the interior in extreme heat is about 10mpg. After the cabin cools down the hit drops to around 1-2mpg. I did some testing today and it appears it could be as high as 3mpg if it's hot enough outside.

The fact you had another Prius and it performed quite differently leads me to believe this isn't a driver or commute pattern issue. This is good info for us trying to help you troubleshoot.

If the 12v battery is losing capacity then it will never recharge fully. My Optima yellow top battery has lost capacity and now it never charges higher than about 12.4v. That is enough for me to still achieve 55mpg when running my OEM wheels. 45mpg when running my 17s.

I have a sneaking feeling that this is your issue.

#1 You purchased a tire that is larger in rolling diameter than your stock tire. The difference is only about 1.75% larger but this equates to a 1.2mph difference on the speedo. Your computer then thinks that you are traveling less miles than you actually are and thus it will calculate a lower mpg. Your OEM Turanza EL-400-02 was a LRR tire and it was rated at 854 revolutions per mile. Your new Hankook H727 is not a LRR tire and is rated at 834 revolutions per mile. So as you can see, your OEM tires turn more revolutions per mile and this will make the computer think you are traveling further on the same unit of gas.

We all suffer from E10 so I'm not even going to focus on that.

I would suggest having your MAF sensor checked out to ensure there is not a build up of gunk on the sensor. It is very easy to clean and will ensure your car is reading correctly so it can accurately control your air/fuel ratio. This is a common issue on any car and could be why you have never achieved the MPG your other vehicle did.
Justin

 

Many things have already been covered by previous responders. I will add a few more.

Except in very extreme cases, the air filter has no impact on MPGs in nonhybrid gasoline (Otto cycle) engines. I don't have the same certainty for Atkinson cycle engines in hybrids, but believe that any impact is very small.

The dashboard battery gauge reads only the HV traction battery, which must save some room for regenerative braking for stops and mountain descents. Cruising on flat land, it will never fill to the top. This is also best for battery longevity.

Aging 12V batteries are a known cause of poor MPG, and the subject of numerous threads here.

It is worse than that. 854/834 is a 2.4% difference. And it is a double whammy, as you (OP, not F8L) are driving faster than you think, reducing MPG directly from the increased drag, in addition to the odometer reading too low, sandbagging your calculations. And don't forget that fresh tires also produce lower MPG until they are broken in.

If a GPS is available, use it to check speedometer and odometer accuracy. And do both independently, as these two gauges have different errors.

Roadside speedometer check stations might also be useful, but the ones here are just 5 miles long, too short for accurate measurements.

 

Thanks, fuzzy 1.

I started off using the generic calculation from 101tires.com and forgot to re-calculate after I found the true revs per mile for his specific tires.

 

And its a good thing you caught the major tire difference. My first reading completely glossed over it.

When my Prius was young, I got into a fuel economy discussion with an older National Park Service employee who significantly improved his truck mpg by increasing tire diameter something over 10%, effectively improving the overall gear ratio. Others told him, from experience, that it wouldn't work. He figured out that they had not adjusted for the speedometer and odometer changes. He went ahead with the tire change, adjusted for the meter changes, and was very pleased with the results.

Perhaps I should do the same with my next Subaru tires? That pig's mpg is hampered by a very uneconomical gear ratio. Prius owners, don't do this at home, as your cars are not saddled with the same Otto-cycle / gearing inefficiencies that hamper nonhybrids.

 

8-11-11

Thanks FUZZY 1 and F8L,

You gents have worked on my mileage problem concerning tire size, rotations, etc., yet have overlooked one thing.

The specific 121 mile trip just taken rolled on my original Touring tires, Turanza EL400 tires! I just bought the new Hankook's a few days ago, after the trip in question.

I wanted to be sure to create comparisons with the original tires, in hopes that all things remained equal, to facilitate an easier MPG troubleshooting scenario.

From the scanguage video of mileage dropping with AC on, and the explanations so far, I am heavily leaning on the outside heat factor while using the AC to have lowered my MPG reading to 39.7 MPG on the 121 mile trip. My wife told me that she set the AC to 70 & 71 degrees and drove at 65 MPH in 100+ temp that day. (she's a hot mama that appreciates and uses Prius' Eskimo Air!)

Notwithstanding this poor mileage single trip, I still am puzzled as to why my mileage overall is only 42-43 MPG vs. our previous 2006 Prius with overall mileage of 48-51 MPG under identical conditions. A suggestion of a MAF cleaning may help. Also, I think that the 12V battery condition is not the culprit, and does not warrant replacement, as another Prius owner reported 12.4V as mine, and achieves 55 MPG.

I would appreciate, however, a calculation for a speedometer reading using my new Hankook Optimo tires. P205/55/16R. What should my speedometer read to indicate a true 60MPH?, and what should I figure in to correct the new MPG figure shown on the MFD?

Slide rule time, boys...

Thanks,
Allen
archiveman2977








However, may I ask you to help me calculate a speedometer reading now with the new Hankook tires

 

Ahh, I had incorrectly assume you made that trip with the new tires.

The info is still good to have just in case you end up with further decline in the near future.

The AC does indeed drop mpg but the huge drop only lasts for a a few minutes but on a short trip that is enough to put a dent in your overall mpg for that trip. There is also the continuous drop of 1-3mpg while using the AC so that 121mile trip could have suffered from a 2-3mpg loss.

Because we do not know wind speed and direction, you could have also been traveling into a slight head or side wind which can drastically affect MPG if the wind speed and wind direction are are against you.

I cannot point a finger at the cause of the mpg difference between your touring model and the older car although tire size will play a difference. The wider tires on the touring will most certain reduce mpg compared to the skinny tire on the non-touring. I would check the basics like the MAF sensor first. It's quite easy to do.

The non-touring Prius model usually exhibits a 1-2mph error, like most cars do, on the speedo. So when you think you are traveling at 65mph you are really doing 58mph. This has been verified by many PC members using GPS and scangaugeII. I have observed this on my own car as well. The taller tire you are using now may correct this error such that it is dead on correct or very close at least. Use a quality GPS device to determine where you are at currently. The MFD mpg indicator may be more accurate in this case but you will be traveling faster than you used to drive by about 2mph so your mpg may drop accordingly. I wouldn't worry about it though since your old MPG average was probably artificially inflated albeit not by much. The GenII Prius' mpg indicator is only off by approx. 2% compared to the 4%-6% on the GenIII.

In summary, I think there is something odd about this car getting such lower MPG than your previous one. The odd tires size (new ones) will contribute to further losses but 42-43mpg on your OEM tires seems a bit low. You may want to have your alignment checked. Just because you don't see unusual tire wear doesn't mean that it is perfectly within spec.

 

Allen - When was the last time you did this trip in this car? Was it before your rediculous heat wave down there took place? I may have overlooked it in here, but I can tell you that those temps, plus the AC temp setting are going to sap your MPG drastically. If it's only happened this one time so far, it could be a possible hiccup with the bladder in the fuel tank for this particular trip? I assume you calculated the MPG manually vs the using the MFD to obtain.

 

8-12-11

Thanks again, gents for your input.

Yesterday afternoon, I conducted two test drives of 50 miles round trip.

Trip #1: 102 degrees, 60-65 MPH, AC OFF-WINDOWS CRACKED SLIGHTLY

MPG: 53.2 MPG!!!!!!

Trip #2: 104 degrees, 60-65 MPH, AC ON--SET FOR AUTO, RECIRC AIR AND 70 DEGREES--The auto fan never slowed from max speed during the one hour trip

MPG: 43.2 MPG!!!!!!

Conclusion: Huge mileage drop when driving in extreme heat with the AC set at 70 degrees.
Yesterday's experiments cleared up the actual relationship between extreme high outside temperatures and maintaining a very cool cabin temperature.

The experiments also cleared up my questions of owners achieving 55 MPG with a Touring Edition. The 53.2 MPG result comes close to their reading, taking to account the extreme heat outside, and the brand new, stickier, non LRR Hankook tires, which exact an MPG penalty, and driving with the AC turned off.

I will now temper the AC according to suggestions presented, allow for a new tire break in and will feel that our Prius Touring is performing as well as possible under adverse conditions. Today marks day #59 of over 100 degree heat this year, with the next week's temps also over 100 degrees.

Can someone calculate for me a speedo correction at 60 MPH indicated, with the new tires? Will the adjustment factor apply to the MFD MPG as well?

Thanks,
Allen

 

How about try the AC Auto to 78 deg F? It'll have minimal MPG hit and you'll be comfortable also.

 

I second this idea. During my testing I only observe a small 1-3mpg hit at most once the cabin has cooled.

Were both of your trips on the same stretch of road? I just want to rule out elevation changes. Even minor elevation rise can drop mpg quite significantly. My commute is very slightly uphill such that you cannot even tell you are going uphill but mpg drops from 53avg to 40avg..

 

8-13-11

Good suggestion of a test run @ 78 degrees. I will conduct this test in the next few days...

Thanks,
archiveman2977

 

104 degrees yesterday in Austin. 2009 Prius w brand new Ecopia 422 tires. 12 mile flat route (no wind) with no AC and windows cracked = 62mpg. Return trip with Auto AC set to 78 degrees resulted in 43mpg. I can live with the heat but don't know when I'll be able to see how these tires perform in the rain

 

Did you compare elevation and wind direction and win speeds for both directions?

 

Both were pretty negligible. Route was flat I35 from North Austin to Slaughter Lane. Winds have been 5-10mph and, in fact, the trip with AC on had the benefit. Otherwise, the mileage hit would have been even worse. Average speed both ways was 60mph.

Elevation? What's that? That's right, you're not from Texas . Just kidding.

 

LOL, true Texas is pretty darn flat but even mild elevation gains can't drop your mpg. Thanks for confirming. That is a pretty drastic drops from just A/C.

 

Small point.
Gasoline is around 114,000 BTU / Gallon
Ethanol is around 76,000 BTU / Gallon

Therefore, Ethanol contains 67% of the energy of gasoline.
E-10 (10% Ethanol, 90% Gasoline) contains 110 BTU / Gallon (97% of the energy of 100% Gasoline)