Fuel consumption without A/C
Discussion
Indeed! I have been doing the same in my '98 C2 Cab manual. I drove 2,500 miles around Europe top down all the way and up and down and around the Alps and averaged 26.5 mpg. Had the aircon been on that figure would have been circa 21. It makes a BIG difference. Soft top down is also worse by about 3mpg, I'd say. 70mph rather than 85 is worth another 3mpg.
So I should have had the top up in the baking heat and kept the speed down and returned 32.5mpg...
So I should have had the top up in the baking heat and kept the speed down and returned 32.5mpg...
AlexH997 said:
Past few days I've been driving with the A/C off and noticed my fuel consumption a has definitely improved.
Used to get 140 miles from half a tank but this time I've got closer to 175
Is that improvement to be expected
Something doesn't add up.Used to get 140 miles from half a tank but this time I've got closer to 175
Is that improvement to be expected
Assuming a 16.9 gallon tank then a half a tank is 8.45 gallons.
140 miles divided by 8.45 gallons is 16.56mpg.
175 miles divided by 8.45 gallons is 20.71mpg.
I've driven hundreds of thousand of miles in my Porsches for sometimes nearly 1000 miles a day checking fuel consumption every fill up -- that can be 3 in one day's driving -- and have not noticed any real change in fuel consumption using the A/C compared to driving with it off.
At idle with the A/C on and the cabin temperature stable -- that is the A/C is not working hard to cool a cabin heated from the car sitting out in the sun for hours -- the engine load increases by an amount that represents maybe 0.5hp added load. No way a 0.5hp increase in load could account for the much bigger difference in fuel mileage. By my observation it takes around 40hp to move my Boxster down the level road at a constant reasonable/legal speed. The difference in fuel consumption going from say 40hp to 40.5hp is not going to cause fuel consumption to change that much.
I suspect a change in driving style is the contributor to the change in fuel mileage or your method of measuring fuel consumption.
Rockster said:
AlexH997 said:
Past few days I've been driving with the A/C off and noticed my fuel consumption a has definitely improved.
Used to get 140 miles from half a tank but this time I've got closer to 175
Is that improvement to be expected
Something doesn't add up.Used to get 140 miles from half a tank but this time I've got closer to 175
Is that improvement to be expected
Assuming a 16.9 gallon tank then a half a tank is 8.45 gallons.
140 miles divided by 8.45 gallons is 16.56mpg.
175 miles divided by 8.45 gallons is 20.71mpg.
I've driven hundreds of thousand of miles in my Porsches for sometimes nearly 1000 miles a day checking fuel consumption every fill up -- that can be 3 in one day's driving -- and have not noticed any real change in fuel consumption using the A/C compared to driving with it off.
At idle with the A/C on and the cabin temperature stable -- that is the A/C is not working hard to cool a cabin heated from the car sitting out in the sun for hours -- the engine load increases by an amount that represents maybe 0.5hp added load. No way a 0.5hp increase in load could account for the much bigger difference in fuel mileage. By my observation it takes around 40hp to move my Boxster down the level road at a constant reasonable/legal speed. The difference in fuel consumption going from say 40hp to 40.5hp is not going to cause fuel consumption to change that much.
I suspect a change in driving style is the contributor to the change in fuel mileage or your method of measuring fuel consumption.
American Society of Automotive Engineers estimates 5-10% lower mpg with a/c on. Surprised me, but they should know. The around town versus cruising is a myth based on studies trying to work out losses via a/c versus extra drag from windows down - conclusion is more efficient to drive with windows down and a/c off up to about 30mph. That is obviously not the same as saying air con saps a lot of power in town and very little at high speeds.
QED :-)
QED :-)
Rockster said:
Something doesn't add up.
Assuming a 16.9 gallon tank then a half a tank is 8.45 gallons.
140 miles divided by 8.45 gallons is 16.56mpg.
175 miles divided by 8.45 gallons is 20.71mpg.
I've driven hundreds of thousand of miles in my Porsches for sometimes nearly 1000 miles a day checking fuel consumption every fill up -- that can be 3 in one day's driving -- and have not noticed any real change in fuel consumption using the A/C compared to driving with it off.
At idle with the A/C on and the cabin temperature stable -- that is the A/C is not working hard to cool a cabin heated from the car sitting out in the sun for hours -- the engine load increases by an amount that represents maybe 0.5hp added load. No way a 0.5hp increase in load could account for the much bigger difference in fuel mileage. By my observation it takes around 40hp to move my Boxster down the level road at a constant reasonable/legal speed. The difference in fuel consumption going from say 40hp to 40.5hp is not going to cause fuel consumption to change that much.
I suspect a change in driving style is the contributor to the change in fuel mileage or your method of measuring fuel consumption.
I used to have an opinion which matched yours, but I don't now.Assuming a 16.9 gallon tank then a half a tank is 8.45 gallons.
140 miles divided by 8.45 gallons is 16.56mpg.
175 miles divided by 8.45 gallons is 20.71mpg.
I've driven hundreds of thousand of miles in my Porsches for sometimes nearly 1000 miles a day checking fuel consumption every fill up -- that can be 3 in one day's driving -- and have not noticed any real change in fuel consumption using the A/C compared to driving with it off.
At idle with the A/C on and the cabin temperature stable -- that is the A/C is not working hard to cool a cabin heated from the car sitting out in the sun for hours -- the engine load increases by an amount that represents maybe 0.5hp added load. No way a 0.5hp increase in load could account for the much bigger difference in fuel mileage. By my observation it takes around 40hp to move my Boxster down the level road at a constant reasonable/legal speed. The difference in fuel consumption going from say 40hp to 40.5hp is not going to cause fuel consumption to change that much.
I suspect a change in driving style is the contributor to the change in fuel mileage or your method of measuring fuel consumption.
For starters, on smaller engined cars switching on the a/c is very noticeable in terms of low down performance drop. You won't notice it on a larger engine perhaps, but the fuel penalty is identical. The "problem" with aircon is that the load is generally constant even when you really don't need much power at all to keep the car on the move. I don't think that you would need 40bhp to keep a Boxter moving on a flat road just overcoming wind resistance and mechanical losses. It's probably a lot less than that. 40bhp is a classic Mini's engine, flat out, on a good day!
Aircon is a drag even at idle.
This is the main issue, it ups relative consumption when otherwise it would be minimal, so the power drain becomes a significant percentage of the engine's output under those conditions.
Whenever I drive a wheezy motor I often find myself turning the aircon off when tackling a hill, it is that much of a drag on performance.
996TT02 said:
Rockster said:
Something doesn't add up.
Assuming a 16.9 gallon tank then a half a tank is 8.45 gallons.
140 miles divided by 8.45 gallons is 16.56mpg.
175 miles divided by 8.45 gallons is 20.71mpg.
I've driven hundreds of thousand of miles in my Porsches for sometimes nearly 1000 miles a day checking fuel consumption every fill up -- that can be 3 in one day's driving -- and have not noticed any real change in fuel consumption using the A/C compared to driving with it off.
At idle with the A/C on and the cabin temperature stable -- that is the A/C is not working hard to cool a cabin heated from the car sitting out in the sun for hours -- the engine load increases by an amount that represents maybe 0.5hp added load. No way a 0.5hp increase in load could account for the much bigger difference in fuel mileage. By my observation it takes around 40hp to move my Boxster down the level road at a constant reasonable/legal speed. The difference in fuel consumption going from say 40hp to 40.5hp is not going to cause fuel consumption to change that much.
I suspect a change in driving style is the contributor to the change in fuel mileage or your method of measuring fuel consumption.
I used to have an opinion which matched yours, but I don't now.Assuming a 16.9 gallon tank then a half a tank is 8.45 gallons.
140 miles divided by 8.45 gallons is 16.56mpg.
175 miles divided by 8.45 gallons is 20.71mpg.
I've driven hundreds of thousand of miles in my Porsches for sometimes nearly 1000 miles a day checking fuel consumption every fill up -- that can be 3 in one day's driving -- and have not noticed any real change in fuel consumption using the A/C compared to driving with it off.
At idle with the A/C on and the cabin temperature stable -- that is the A/C is not working hard to cool a cabin heated from the car sitting out in the sun for hours -- the engine load increases by an amount that represents maybe 0.5hp added load. No way a 0.5hp increase in load could account for the much bigger difference in fuel mileage. By my observation it takes around 40hp to move my Boxster down the level road at a constant reasonable/legal speed. The difference in fuel consumption going from say 40hp to 40.5hp is not going to cause fuel consumption to change that much.
I suspect a change in driving style is the contributor to the change in fuel mileage or your method of measuring fuel consumption.
For starters, on smaller engined cars switching on the a/c is very noticeable in terms of low down performance drop. You won't notice it on a larger engine perhaps, but the fuel penalty is identical. The "problem" with aircon is that the load is generally constant even when you really don't need much power at all to keep the car on the move. I don't think that you would need 40bhp to keep a Boxter moving on a flat road just overcoming wind resistance and mechanical losses. It's probably a lot less than that. 40bhp is a classic Mini's engine, flat out, on a good day!
Aircon is a drag even at idle.
This is the main issue, it ups relative consumption when otherwise it would be minimal, so the power drain becomes a significant percentage of the engine's output under those conditions.
Whenever I drive a wheezy motor I often find myself turning the aircon off when tackling a hill, it is that much of a drag on performance.
I often drive the three cars I have with a/c both on and off because I am not enamoured of the effect of dried air on my throat and nasal passages on a long run
I have never seen any significant difference in economy. A suspicion that it's better with it off but certainly not outside of normal variations.
Edited by anonymous-user on Friday 11th October 21:25
Fuel consumption will also increase noticably if you use the air con before the engine has warmed up, typically on short winter journeys.
That was one of the reasons I sold my C4s cab which had the well known excessive misting up problems affecting Porsche, but never BMW cabs in my experience, due to infinitely better hoods......
That was one of the reasons I sold my C4s cab which had the well known excessive misting up problems affecting Porsche, but never BMW cabs in my experience, due to infinitely better hoods......
Rockster said:
Something doesn't add up.
Assuming a 16.9 gallon tank then a half a tank is 8.45 gallons.
140 miles divided by 8.45 gallons is 16.56mpg.
175 miles divided by 8.45 gallons is 20.71mpg.
Mines a 997.1, I filled the tank the brim and it's now at half tank. Based on the figures you've quoted my fuel consumption is terrible. I thought I was gettig more than that. my daily trip is mainly non stop A roads around 56mph.Assuming a 16.9 gallon tank then a half a tank is 8.45 gallons.
140 miles divided by 8.45 gallons is 16.56mpg.
175 miles divided by 8.45 gallons is 20.71mpg.
So 25mpg should be giving me around 216 miles on a half tank? 25 x 8.45?
Another issue that could give conflicting outcomes is the efficiency of your ac system
A freshly regased and health system would cause the occupant to select a higher ambient temp as on full chill it would be too cold whereas a somewhat depleted system would require full chill setting there by demanding constant ac pump clutch engagement and consequently a full time engine load as opposed to partial one.
A freshly regased and health system would cause the occupant to select a higher ambient temp as on full chill it would be too cold whereas a somewhat depleted system would require full chill setting there by demanding constant ac pump clutch engagement and consequently a full time engine load as opposed to partial one.
AlexH997 said:
Mines a 997.1, I filled the tank the brim and it's now at half tank. Based on the figures you've quoted my fuel consumption is terrible. I thought I was gettig more than that. my daily trip is mainly non stop A roads around 56mph.
So 25mpg should be giving me around 216 miles on a half tank? 25 x 8.45?
Isn't a 911 (997) fuel tank about 63 litres, equating to 13.8 gallons?So 25mpg should be giving me around 216 miles on a half tank? 25 x 8.45?
(or is that just the 4S)?
andyglos said:
Why buy a Porsche if fuel consumption is such an issue? Wouldn't a Prius be better? Besides which, any benefit in fuel consumption is likely to be outweighed by expenditure on a/c servicing/repairs if it is left unused for any length of ime.
Calm down who mentioned fuel consumption being an issue? I said I noticed an improvement in fuel tank range when I wasn't using the A/C and curious why that was. OP, IMHO your " test " cannot be relied upon to give a true result. There are too many variables, and maybe it would be impossible for any private motorist to get a definite result. It would need multiple testing under the same conditions. Perhaps a motor engineer could calculate the results though and say why it s better with aircon off. We all know it is, but why and how much exactly. I for one would not use the Americam Society of Auto Engineers data for our UK or Irish situation, cars, drivers, roads there are just so much different.
andyglos said:
Why buy a Porsche if fuel consumption is such an issue? Wouldn't a Prius be better? Besides which, any benefit in fuel consumption is likely to be outweighed by expenditure on a/c servicing/repairs if it is left unused for any length of ime.
As previous re fuel consumption, but re "unused" issue, my '92 Landcruiser is used with a/c on for roughly 6mths a year and never used for the rest of the year, and, fingers crossed, 21 years and a zillion miles later it is still working perfectly.996TT02 drove a lot of miles in his 'test'. His mpg difference is really quite substantial. I was sceptical before about a/c but I'm not now. It has made a big difference to my fuel economy. I do enough journeys that have a similar balance to get an idea. It does a lot more miles to the tank when it's off. Off means off, by the way, not just not cooling. The compressor must be switched off.
I have also experienced a clear difference in underpowered cars on hills.
I appear to get more mpg in France despite probably driving faster. I think it is down to more than just less stop start than in England and am inclined to say there is a little less rolling resistance on the tarmac... I have experienced this in underpowered cars with changing to narrower tyres and the accompanying increase in top speed. It is quite material, again perhaps surprisingly so.
I have also experienced a clear difference in underpowered cars on hills.
I appear to get more mpg in France despite probably driving faster. I think it is down to more than just less stop start than in England and am inclined to say there is a little less rolling resistance on the tarmac... I have experienced this in underpowered cars with changing to narrower tyres and the accompanying increase in top speed. It is quite material, again perhaps surprisingly so.
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