Author Topic: Fuel Economy  (Read 35405 times)

guest806

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Re: Fuel Economy
« Reply #30 on: April 11, 2009, 06:56:41 PM »
I seem to recall that manufactures always had 56mph as the optimum speed to get the best fuel economy.
 8)

culzean

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Re: Fuel Economy
« Reply #31 on: April 12, 2009, 10:18:12 AM »
Acceleration - probably the biggest impact on fuel economy is acceleration, studies have shown that in urban driving accelerating slower and changing up earlier (as opposed to redlining it in every gear) can improve consumption by up to 40%. 

Speed - next biggest fuel user is speed (where drag, and therefore power requirement is proportional to the square of the speed - which is why you can do 100mph with 80 bhp, but will need 400bhp to do 200mph). Above 30mph wind resistance really begins to become the major source of drag (obviously this will have more affect on a LandDrover Defender with the aerodynamics of a housebrick than a sports car with a Cd of 0.2). So all things being equal, the slower you drive the less fuel you will use.

Aircon- whether to use aircon is another choice, on an aerodynamic car like the Jazz driving with the windows open above 30mph causes a big increase in drag (unlike on the LandDrover where it has little affect) and it is probably better to open the windows around town but use aircon on main roads.

Weather conditions - the air temperature has quite an effect on fuel used, the colder the air the thicker it is (more drag), the engine also takes longer to warm up on a cold day, and a cold engine needs a richer fuel/air mixture to run - this can drag fuel consumption down to 20-25mpg for first couple of miles.
Some people will only consider you an expert if they agree with your point of view or advice,  when you give them advice they don't like they consider you an idiot

Pine

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Re: Fuel Economy
« Reply #32 on: April 14, 2009, 11:10:47 AM »
I have been wondering about the effect on fuel consumption of using the air conditioning. I remember the salesman telling me to leave the fan on the low setting all the time (because the Jazz is prone to misting – which the salesman did not tell me). He also said you may as well leave the Air Con on all the time as well because the compressor is much smaller and more efficient these days that the effect on fuel consumption is a lot less than it used to be. I was reluctant to take his advice as on my previous car the use of air con had a marked negative effect on fuel consumption.

I decided to investigate:
At weekends we often visit relatives who live 15 miles away, a gentle cross country run.
On a recent journey I filled up near the start of the trip and reset the gauge. When I had returned home the computer had recorded 53 mpg (1.4SE CVT). The following week the weather conditions were similar. I filled up again and made the trip with the air con on and returned the same 53 mpg (my normal weekly consumption is 40ish). Now I know it was not very scientific as speeds and traffic conditions can vary but it seems that using air con on the Jazz is not that detrimental to fuel consumption.
As a result of this I have been using air con much more. In traffic I use it in recirc mode as I do not like drawing in the traffic fumes. I sometimes wonder if it is wrong to use recirc for extended periods; is it possible to use up the air in the car if no fresh air is coming in? A work colleague laughs at this as he runs in recirc nearly all of the time, he says air con is more efficient used like that.

JazzyB

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Re: Fuel Economy
« Reply #33 on: April 14, 2009, 11:22:53 AM »
I too have noticed that using the a/c does not have that much effect on the fuel consumption. The benefits far outweigh the extra fuel used. I wouldn't use the recirc function for longer than necessary. The inside glass will after a while mist/steam up as well as the air inside being uncomfortable to breathe.

culzean

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Re: Fuel Economy
« Reply #34 on: April 14, 2009, 05:28:51 PM »
While it may be true that recirc is a bit more more efficient, I wouldn't use recirc A/C on the Jazz because the aircon unit is very efficient anyway, has negligible effect on fuel consumption, and the Jazz it is far too prone to misting up in anything but the best of weather (If it rains and the windows start to mist up, the only way to clear them is to put on the aircon and the heater together to dry and warm the air coming into the car).

I believe, from experience that having recirc on all the time can also lead to you getting drowsy. The combination of pollen filter and the aircon heat exchanger will take most of the crap out of the air coming into the car anyway. It is a good thing to replace the pollen filter at least once a year as it blocks up pretty quickly and cuts down the amount of air coming into the car - spring is probably the best time.  You may be surprised how black the filter is when you remove it.
Some people will only consider you an expert if they agree with your point of view or advice,  when you give them advice they don't like they consider you an idiot

culzean

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Re: Fuel Economy
« Reply #35 on: April 14, 2009, 08:19:29 PM »
Bear in mind that airconditioning makes the air in the car very, very dry, and can lead to sinus problems and headaches if it is constantly used. It is probably better to save it for when the weather really requires it thanto use it all the time.
Some people will only consider you an expert if they agree with your point of view or advice,  when you give them advice they don't like they consider you an idiot

guest671

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Re: Fuel Economy
« Reply #36 on: April 14, 2009, 08:36:23 PM »
Hi, My new 1.4 Jazz which I bought new last November regularly gets around 52 mpg (according to the computer) although it gets 2 or 3 mpgs less when I use Tesco petrol.

This is still better than my last 2006 Jazz which only on rare occasions reached 50 mpg. So the new model is definitely slightly more economical which is exactly what Honda claim.

guest806

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Re: Fuel Economy
« Reply #37 on: April 22, 2009, 07:26:27 PM »
 ;D
Just did a fuel check again now that i had to fill up.  The MPG has increased from 31.6 previously to 44.7.  The computer advised 44.2.
Have added a pot of Dtox lets see if i can get any better.
 ;D

ThomP

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Re: Fuel Economy
« Reply #38 on: April 23, 2009, 02:30:08 PM »
Just another calculator between mpg and km/l...
http://www.vangeyn.net/mpg/
Storm Silver Metallic rules:)

Geoffers

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Re: Fuel Economy
« Reply #39 on: April 23, 2009, 03:28:08 PM »
Just another calculator between mpg and km/l...
http://www.vangeyn.net/mpg/
US gallons, so watch out!

guest765

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Re: Fuel Economy
« Reply #40 on: April 24, 2009, 06:00:27 AM »
I did a first fuel check since i bought the car in Jan 2009 its a Jazz SE on a 55 plate with 15000 on the clock.......i filled the tank to the very top, did a coast run and back then filled it up to the top again.......the numbers crunched out at 51 mpg.....some motorway high speed driving involved too as well as winding coastal roads.....I'm happy with that!!! ;D

guest806

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Re: Fuel Economy
« Reply #41 on: May 05, 2009, 08:42:53 AM »
O.k so with the D-tox in the tank, the first 100 miles was town driving and the computer read out was 39.1 mpg.  The rest of the tank was used to get from Kent to Lowestoft and back.  The Jazz did not quite make it and i had to fill up at Lakeside (Tesco).  The result was a decrease in mpg to 41.45.  The car did have more of a load and the D-tox was present so this will of had an impact.  I will post the next calculation but the one after is the one i am interested in as this will be a fill up at my normal shell garage.
 :-\
Reading the other posts, it might be changing the spark plugs give a bigger improvement.

guest593

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Re: Fuel Economy
« Reply #42 on: May 12, 2009, 07:07:50 PM »
I regularly got between 39 to 41 MPG, I tend to do short journeys roughly 25 miles per day, but over the weekend I drove down to the Lake District, approx 200 miles. Then did a return drive home via sedgefield, 290 miles. On the return journey I got as much as 57 MPH and today my short drives gave me a reading of 51.5 MPH.
I have read that giving your engine a service followed by a long drive at high speed is good for reducing your emissions but didn't know it could also improve your MPH.

culzean

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Re: Fuel Economy
« Reply #43 on: May 13, 2009, 08:50:21 AM »
Have you zeroed your trip mileage since you did the long journeys? The 'mpg' reading responds very quickly to changes in driving conditions just after trip mileage display has been reset but as the trip mileage rises the rate of using fuel (mpg) readout responds slower and slower (the calculations done by the computer must use the total mileage on the trip and the present 'rate' of fuel use (the flow sensor is sampled about every 10 seconds) to work out the 'mpg').  If you haven't reset the 'trip mileage' this means that you will still be getting a 'hangover' effect from the good 'mpg' readings on your long trip.  Personally I reset the trip to zero every time I fill the tank and experiment with different driving styles to see how they affect my 'mpg' display. 
Some people will only consider you an expert if they agree with your point of view or advice,  when you give them advice they don't like they consider you an idiot

guest593

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Re: Fuel Economy
« Reply #44 on: May 19, 2009, 06:00:42 PM »
Like you I too reset the trip mileage every time I fill up.
Update. I have refilled my tank and reset the trip milage and am still getting around 50/51 MPG doing city driving.

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