Why the drop in fuel efficiency?

8-21-11

Thanks, usbSeawolf2000, for the suggestion to redrive the same route of 50 miles, while maintaining the AC inside temp of 78 degrees.

I drove the same 50 mile route today, temp 102 degrees, at 65 MPH, with the AC set on recirculate at 78 degrees and with the two passenger vents closed. Surprisingly my two vents kept me cool.

The result: ...drum roll please...49.6MPH

 

Very nice!

 

50 MPG, that's what I am talking about!!

 

For close comparisons, even Texans need to adjust for elevation changes. By my rule of thumb for elevation changes, even two feet per mile will cause a 1 mpg difference between the climbing and descending legs. My electronic topo suggests that the I35 / Slaughter Lane crossing is at about 640-650 feet. 12 miles away, the I35 / US290 interchange is at about 685-700 feet. This doesn't show the structure of the aerial roadworks, which must also be accounted for over short distances.

But your 19 mpg difference is not close, so the 2 or 3 mpg attributable to elevation difference is small potatoes.

 

Interesting results, it certainly made a big difference.

In summer I also like to run the AC with a relatively high set point. I'm usually up around 24C (75F) and I find that it's still quite comfortable. Around here the heat is also very humid, so I think that just the reduction in humidity that the AC produces can help a lot by itself.

 

http://priuschat.com/forums/other-cars/95536-car-driver-mileage-no-its-your-gallonage-really-counts.html

 

D'ohh!! Thanks for pointing out my stupid error. Yes, of course, *E10* (not ethanol) contains 97% of the energy of gasoline.

 

Not sure if this is the way to do this, but read the suggestion to answer these questions. I have an '01 Prius with 158,000. Last summer/fall, out of the blue, mileage tanked. Normally in the high 40's summer and low 40's winter; driving habits/use/gas purchase place, etc. did not change. In California, we get ethanol, no choice. Just got a new hybrid battery, they put nitrogen in the tires (new to me), cleaned the mass air flow filter and said I should get great mileage. It's still low 40's. Here's my answer's:-

What fuel economy are you getting and how are you determining fuel economy? (trip computer or manual calculations) both

- What fuel economy are you expecting and why? see above

- What are the approximate outside air temps? now, 60-95

- How long are your trips? drive for my job: can be anywhere from 2-3 miles to 20-30 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? half/half; 60-65 on the highway; 35 in town

- What region/state are you in? (if you haven't set your location in your profile) NO. Calif

- What's the terrain like of your drives? (e.g. flat, gentle hills, steep hills, etc.) moderate hill (up/down same hill every day) & flat

- Is your oil overfilled? (i.e. above the full mark on the dipstick) not now

How old is your 12v battery? What is the voltage reading of your 12v battery after sitting over night? (Method Here) 1-2 yr; will check the votage

Have you had your alignment checked? Any pulling or abnormal tire wear? no; no pulling or abn wear

- 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). Bridgestone 175/65R14

- What are your tire pressures? 38 psi; nitrogen

- 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.) na

- How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking? not doing anything special, just driving; not braking hard

- Are you "warming up" the ICE (internal combustion engine) by letting it idle after powering on? yes

- Are you driving using D or B mode? D

- HVAC settings? Are you using the heater, AC, auto mode, etc.? If using auto, what temp is it set to? heater in the winter; no AC

- 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? no change at all

 

Bennie, right off the bat I see you are allowing the car to warm up before driving it. If that is what you meant then that is one part of the reason for low MPG. If you are burning fuel and not going anywhere then your MPG is going to drop for obvious reasons.

You also mentioned Bridgestone tires but you didn't list the model.

 

Bridgestone Potenza RE92; also, just started letting the car warm up a week ago, thinking that might help. It didn't. Prior to this, just started and drove

 

Unfortunately cars generally don't make as effective use of the 97% as they do of the 100% and the Prius is one of those cars.

 

Ah, yes. Here's an example of the problem of mpg math from the School of Barbie: Tested: Speed vs. Fuel Economy - MSN Autos

It appeared years ago but I just can't get over the trauma.

 

Yep, Consumer Reports tests speed vs MPG & comes to false conclusion about Honda Insight 2 - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com took issue w/CR's findings about small vs. large drops in mpg.

In the case of the Insight that went from 51.9 mpg at 55 mph to 36.5 mpg at 75 mph, over a 100 mile trip, it would use 0.81 gallons more at 75 mph than 55.

The Mountaineer, with its "small" drop (23.8 mpg to 17.8), would use 1.42 gallons more.

 

Thanks, bennie.

The tire likely not helping. I don't think they are low rolling resistance rated. The other problem is short trips. Those 2-3 mile trips really dent mpg. Even a 20 mile trip is pretty short and won't boost your mpg significantly. Could you try taking a freeway trip over 20 miles? If so then reset your mpg trip meter then drive on the freeway for awhile at 65mph and observe your mpg. Let us know what it is. This will help determine if it is your car or the short trips causing the low mpg. Just watch out for the chickens in the road at Yuba City.

 

Yeah letting it warmup (in park) for about 30 seconds doesn't hurt the fuel economy at all, and it reduces the strain on both the engine and the battery.

I don't think the RE92's are a low rolling resistance tire. They're probably costing you a few MPG's

 

If you are not moving and you are burning fuel I would argue that it does affect fuel economy. It is likely not much but you can't argue with the math.

 

Yeah but it pulls very little power from the ICE for the first 30 to 60 seconds while the cats are cold. I know that if I drive slowly on a flat road while cold it runs on the battery and the ICE just idles, how does that help anything? At least when you're warming it in park it uses the ICE to pre-charge the battery.

After some testing I've come to the conclusion that it makes practically no difference to MPG while it reduces the strain on the battery, so I see no reason not to do it. I can tell for 100% certain that if I drive straight from cold that my SOC will often drop 2 or 3 bars in the first two minutes and then my FE is down for the next few miles because of this. I may even miss out on good EV-glide opportunities because of this. Now if on the other hand I warm it up (in park) for just 30 seconds I usually don't lose any SOC bars and my MPG during the first few miles is better!

In any case, there's plenty of hypermilers who use the warmup technique who are getting over 60 MPG, so it's a moot point when it comes to diagnosing the problem of people getting sub 35 MPG mileage.

 

30s really isn't that long which is why I think you don't notice the drop. The majority of people I observe warming up their cars do it for a much longer period. In fact, the start the car then run back into the house for some reason then come back out 2+ minutes later. lol

I've never noticed the SOC drop that you describe. I cannot go into a full stealth glide right after leaving my house but I can coast well enough that instead of getting less than 40mpg cruising during warmup, I can coast with minimal throttle input and observe 100-300mpg. For those who don't immediately hit the freeway like I do, there could be a benefit to the warm up technique because what you lose during warmup is less than what you gain by being able to stealth glide sooner. This is especially true for city only driving.

In the end, I'm not one of those guys getting 60+mpg regularly so what the hell do I know.