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Low Fuel Economy


Grumpy Cabbie
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Had the Prius for about 2 weeks now and the average fuel economy is coming in at 50 mpg. I guess this isn't bad considering that my driving is mostly round town - and I mean stuck in traffic at traffic lights etc and not cruising along inner ring roads and my last vehicle was returning 23-28mpg. Trouble is, when I tested the car in the late summer I seemed to get about 55-60 mpg. Is this difference to do with the heating being on instead of the a/c? I have noticed that the car will suddenly run the engine for no good reason every now and then but doesn't or stops if I turn the climate control off. I thought the a/c would power the heating rather than the engine powering it, in the same way home/office a/c systems have a heating option.

I suppose this is my only gripe with the car and I think 50mpg is not bad considering what I used to get. The money saved in the last week is over £60 which isn't bad.

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Is this difference to do with the heating being on instead of the a/c? I have noticed that the car will suddenly run the engine for no good reason every now and then but doesn't or stops if I turn the climate control off.

The fact that the engine stops as soon as you turn the climate control off answers the question (I think your car would refute the 'no good reason' allegation! :) ). I have found that in stationary traffic I can usually identify why the engine starts up by switching off the defogging/climate control systems. If the engine doesn't stop then in most cases the HV Battery will be down to the last couple of bars, i.e. needs charging up again.

I'm monitoring mpg on my commute which involves a similar route each day. I've seen a big difference over the past 6 weeks or so, varying from 36 - 66 mpg (according to the car). There are several variables of which traffic congestion is the most important for me but clearly the outside temperature is having an impact too. The Gen 3 Prius is supposed to be better designed for running in cold weather but in my case I don't think it's a question of engine efficiency, rather the fact that the engine has to run more often in slow moving traffic to warm the cabin.

I may post up some figures in a couple of months once I've got a big enough sample set.

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Power is needed for the AC and the heater.

Where is this power going to come from if not the engine?

Well I hate to be a stick in the mud but the Prius is different to every other car out there in that the a/c compressor is powered by the large Battery and NOT the engine via a belt as is the way with every other car out there. The a/c runs cold without the engine running but I am not sure if the heat runs in the same way or whether it uses the engines coolant in the same way as a traditional car.

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Power is needed for the AC and the heater.

Where is this power going to come from if not the engine?

Well I hate to be a stick in the mud but the Prius is different to every other car out there in that the a/c compressor is powered by the large Battery and NOT the engine via a belt as is the way with every other car out there. The a/c runs cold without the engine running but I am not sure if the heat runs in the same way or whether it uses the engines coolant in the same way as a traditional car.

Qutie reassuring to see Grumpy Cabbie's comments, and the results he is posting on www.fuelly.com I'm experiencing low 50's mpg & that is on an 50+ mile / 70minute drive to work on a mix of country lanes, fast A road and crawling traffic.... so compared with other recorded consumption figures I'm not doing very well.... but as with Grumpy Cabby, my consumption is much better than my last car, so I'm pleased with the savings. Perhaps the reported figures are skewed because hypermilers are keen to show off their prowess, whilst more "normal" drivers aren't logging their results! :huh:

Regarding the aircon - the answer is that the engine is used to provide heating, whilst the electric compressor provides cooling.

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Is this difference to do with the heating being on instead of the a/c? I have noticed that the car will suddenly run the engine for no good reason every now and then but doesn't or stops if I turn the climate control off.

The fact that the engine stops as soon as you turn the climate control off answers the question (I think your car would refute the 'no good reason' allegation! :) ). I have found that in stationary traffic I can usually identify why the engine starts up by switching off the defogging/climate control systems. If the engine doesn't stop then in most cases the HV Battery will be down to the last couple of bars, i.e. needs charging up again.

I'm monitoring mpg on my commute which involves a similar route each day. I've seen a big difference over the past 6 weeks or so, varying from 36 - 66 mpg (according to the car). There are several variables of which traffic congestion is the most important for me but clearly the outside temperature is having an impact too. The Gen 3 Prius is supposed to be better designed for running in cold weather but in my case I don't think it's a question of engine efficiency, rather the fact that the engine has to run more often in slow moving traffic to warm the cabin.

I may post up some figures in a couple of months once I've got a big enough sample set.

I think you might be right. I too find the engine turning on when the large Battery gets down to about 2 bars. Also, the engine does seem to start and stop more when just started (but after the initial warm up cycle) and I'm wondering if it's to do with it it keeping the cabin temperature upto level. Once, its warmed right through the engine seems to run less often. I guess the computer program telling the car when and when not to run must be very complex as there seems to be so many variables. Am still pleased with the car still :lol:

Regarding the mpg figures you're checking, why not use the 'fuelly' website that I use. It's quite good for keeping track - and FREE ;)

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  • 1 month later...
Regarding the mpg figures you're checking, why not use the 'fuelly' website that I use. It's quite good for keeping track - and FREE ;)

Hi Grumpy Cabbie, I'm now on Fuelly - only just filled up for the third time so now I appear on their browse lists - 3rd from bottom of the 2010's at 48.6 mpg. I see your mpg is about the same - I think we both suffer from lots of slow traffic (although you've done far more mileage). If you reclassified your car as a 2010 (which is what I believe the Americans call the Gen 3), as things stand you wouldn't be bottom of the (2009) mpg stats any more :)

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The following document is often referred to when people ask about the Prius warm up.

http://www.techno-fandom.org/~hobbit/cars/five-stages.txt

That seems to refer to the Gen 2 car. Is the Gen 3 any different?

I certainly note that the ICE runs alot longer in warm up in the cold weather. It also restarts itself when in stopped traffic to keep warm (which it never did in the summer). All of this is to protect the engine by keeping it warm so can't complain though obviously it's using more fuel. Perhaps engine block heaters arent such a silly idea after all!

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I certainly note that the ICE runs alot longer in warm up in the cold weather. It also restarts itself when in stopped traffic to keep warm (which it never did in the summer). All of this is to protect the engine by keeping it warm so can't complain though obviously it's using more fuel. Perhaps engine block heaters arent such a silly idea after all!

In cold weather you could try running in ECO mode (if not already).

I seem to be getting a couple more MPG on my daily commute in the cold weather.

http://priuschat.com/forums/gen-iii-2010-p...erformance.html

And look at blocking the front grill to keep the engine warm.

http://priuschat.com/forums/gen-iii-2010-p...g-strategy.html

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HI,

Was starting to feel worried as I have same poor MPG.

I'm now at 5200m on my P3.

Commute is country A roads (not straight dual carriageway), driving around 50 mph for 35m and having about 50mpg, if I go on motorway it's more 40 to 45mpg for 70mph drive (displayed, so 63mph real).

Feels like a drag on the car, gliding not possible, speed going down about 1mph every second or 2 when there is no energy flow displayed on level road.

However, from time to time the car feel different, a lot lighter, usually for a few miles but last week it did for a lot longer and drove from Abingdon to London (so A road to Oxford then M40 at 70mph) and arrived in London with 61mpg, what a surprise! (I reset the trip computer just after leaving as I wanted to check if it was purely psychological or if there was a real difference).

I was planning to discuss this with my dealer and try to get the test car for a day to see if I can spot any difference.

Any further experience/troubleshooting would be greatly appreciated.

Thanks,

Stephane

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Commute is country A roads (not straight dual carriageway), driving around 50 mph for 35m and having about 50mpg, if I go on motorway it's more 40 to 45mpg for 70mph drive (displayed, so 63mph real).

Feels like a drag on the car, gliding not possible, speed going down about 1mph every second or 2 when there is no energy flow displayed on level road.

However, from time to time the car feel different, a lot lighter, usually for a few miles but last week it did for a lot longer and drove from Abingdon to London (so A road to Oxford then M40 at 70mph) and arrived in London with 61mpg, what a surprise! (I reset the trip computer just after leaving as I wanted to check if it was purely psychological or if there was a real difference).

Hi Stephane

As per my earlier response, I'm getting similar consumption for a similar daily commute. The only thing I would add is that taking it a bit slower doesn't seem to make much difference!

That said, I'd expect something better than 40-45 when cruising up the motorway. Letting off the accelerator slightly should be enough to push the mpg meter up to 100, but don't let off too far as you'll slow down (as you experience). Setting cruise control is (counter-intuitively) less economical, as it seems to keep the power on in order to maintain a constant speed.

As per others' responses, try logging your consumption in Fuelly.com and see how you compare over time.

Dave R.

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Hi Dave,

I had the equivalent of Fuelly on my iPhone, so it was easy to fill my account, still have 51mpg average but the graph is looking like a descending line... let's put the blame on the weather/light for the moment and see in spring but on the French Prius forum they have a poll and the average seems a lot better (far above what I ever achieved).

It would be nice if people not complaining about their consumption also had fuelly... are we grumpy or is there some Prius with a performance issue/difference?

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That said, I'd expect something better than 40-45 when cruising up the motorway. Letting off the accelerator slightly should be enough to push the mpg meter up to 100, but don't let off too far as you'll slow down (as you experience). Setting cruise control is (counter-intuitively) less economical, as it seems to keep the power on in order to maintain a constant speed.

As per others' responses, try logging your consumption in Fuelly.com and see how you compare over time.

Dave R.

Toyota have confirmed that cruise control is not fuel efficient (somewhere on their blog site).

Basically, the Prius sacrifices fuel in order to keep the car running at a constant speed.

The main source of heat in the Prius is the petrol engine so if you have the cabin temp up, the ICE is going to run.

The AC does run off the HV Battery as does the AC compressor but it needs the petrol engine for heat.

There are some heating elements in the front demist system but they aren't man enough to warn up the cabin.

I am still getting about 49-50 mpg (trip meter reading) when I go for a motorway journey.

Daily commute (20 miles) is about 53mpg (trip meter reading) using eco mode and tyres at 38psi.

The main hit for my mpg is the cold start when it is around 6 mpg for the first couple of minutes.

This is because the front and rear demisters are on and the engine needs to warm up.

Will be interesting to see my summer figures.

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