Author Topic: Sorry fellas it's MPG again.  (Read 4077 times)

Sidot

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Sorry fellas it's MPG again.
« on: January 15, 2020, 10:23:03 AM »
A little advice if I  may ask please.
It's about miles per gallon again I'm afraid.
I have Jazz 1.3 CVT new car August 2018.
Collected 21.8.2018.
I keep a "petrol book ".....always have, mainly for MPG comparisons.
I'm a senior citizen and drive carefully and consistently trying to achieve maximum efficiency.
Last year at this time my mpg was 33.4.
This year it's just been measured on fill up at 29.0.
Nothing changed and car recently serviced.
Any ideas why the poor mpg ?
Thank you.

Downsizer

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Re: Sorry fellas it's MPG again.
« Reply #1 on: January 15, 2020, 11:16:21 AM »
These seem very low figures - I have been consistently getting above 50 mpg over 4 years and 38000 miles, but that's in a rural area with a number of long trips, eg 800 miles last week.  Are your journeys mostly very short, or with frequent stop starts?  If so, I don't think the drop from one year to the next is significant.

Basil

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Re: Sorry fellas it's MPG again.
« Reply #2 on: January 15, 2020, 11:57:20 AM »
What MPG does it show on the car display ?

peteo48

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Re: Sorry fellas it's MPG again.
« Reply #3 on: January 15, 2020, 02:05:09 PM »
Hi Sidot. I've got the same car as you. On other threads I've remarked on the huge range of mpg figures I have got on this car and my previous manual Mk2. As you can see from my avatar my current average mpg is a tad over 45 mpg. That said I have been as low as 35 mpg and as high as 57 mpg.

My typical drive is 1.7 miles to Sainsburys with a cold engine, an hour in the shop, then back home. I know that the car does about 28 mpg on that trip in cold wet weather. A few longer trips - 7 miles say - and the mpg goes up dramtically. A couple of times a year I do long trips - 200 plus miles - and that's when I get the best mpg.

I'd be looking at how many very short journeys in cold weather I did. If you don't do that type of motoring then your figures seem very low and I'd want a mechanic to look at the car.

ColinB

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Re: Sorry fellas it's MPG again.
« Reply #4 on: January 15, 2020, 04:24:08 PM »
A little advice if I  may ask please.
It's about miles per gallon again I'm afraid.
I have Jazz 1.3 CVT new car August 2018.
Collected 21.8.2018.
I keep a "petrol book ".....always have, mainly for MPG comparisons.
I'm a senior citizen and drive carefully and consistently trying to achieve maximum efficiency.
Last year at this time my mpg was 33.4.
This year it's just been measured on fill up at 29.0.
Nothing changed and car recently serviced.
Any ideas why the poor mpg ?
Thank you.
I wouldn't worry too much about the difference between 33.4 & 29 (that may be within the error band for the calculations), the real issue is that both figures are abysmal ! I just go by the car's computer, which typically gives me Trip A values around 55mpg over a mixed bag of in-town and open road travels, with Current Drive values as high as 70. Even allowing for the inaccuracy of the computer I'm happy with those figures.

It's tricky to try to diagnose your problem remotely because there are so many variables, but I'd recommend setting the infotainment screen to show the Current Drive to give an idea of just when you're using lots of fuel. It's surprisingly sensitive, and whilst not 100% accurate it does help in driving efficiently. Some thoughts:

Driving style. Obviously a heavy right foot is bad, but do you follow the green/blue lights on the instruments to prompt efficient driving? Not sure how that works with a CVT, but with a manual box you can get into 6th at a surprisingly low speed.
Geography; do you have lots of hills? I have a supermarket about 2 miles away up a fairly steep hill. The Current Drive display will show between 20 & 25 on arrival; on the return trip it's usually 80 or 90. So geography is important, especially on short journeys from cold.
Speed. I've been doing some longish trips west-east & return recently (A303-M3-M25-M20) with the cruise control set at the speed limit (so actual speed around 65), and I've found the stretch of 50mph limit through the M20 roadworks has a big effect on the Current Drive figures: typically 55-60mpg on entering the roadworks, 65-70 on exit a few miles later. Obviously if you can drive everywhere at a steady 40mph that'd be even better!
Weather. On my east-west trips, I find mpg eastbound is always better than the same route westbound. The only logical reason I can think of for this is the prevailing SW wind might be a factor.

Jocko

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Re: Sorry fellas it's MPG again.
« Reply #5 on: January 15, 2020, 05:11:32 PM »
Many short journeys where the car never warms up can badly affect mpg. So too does depending on your brakes instead of anticipation and slowing down without braking. I used to get those sort of figures with my 2 litre Volvo!

Sidot

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Re: Sorry fellas it's MPG again.
« Reply #6 on: January 15, 2020, 05:23:10 PM »
Thank you all for your replies and observations.
I have never trusted the on board computer and find that the only true way to measure mpg is full tank to full tank divided into miles covered.
I do agree that 29 mpg is abymal. However I am driving the same as last year and always try to keep the green light on. I'm a senior so no harsh breaking. All cold starts and urban driving.
My previous Mk3 manual gave better results.
Maybe I should drive it a bit more positively rather than trying for economy.

Jocko

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Re: Sorry fellas it's MPG again.
« Reply #7 on: January 15, 2020, 05:38:30 PM »
Maybe I should drive it a bit more positively rather than trying for economy.
Give it a try. The figures you are returning are terrible. The figure beside my avatar is my calculated average over 35,500 miles. The "fibometer" is really rather optimistic. Fuelly.com is great too, for recording your numbers.

Basil

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Re: Sorry fellas it's MPG again.
« Reply #8 on: January 16, 2020, 07:46:11 AM »
The on-board computer might not be that accurate but it should be consistently a bit optimistic so I just wondered if it was similar to your calculated figures ? This would confirm your manual calculations.

I use my 1.2 Mk2 manual mainly for commuting, 70 miles a day, mostly on dual carriageways and motorways and it never really drops below 55 MPG on the on-board computer, I accept the actual might be a bit lower.

Jocko

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Re: Sorry fellas it's MPG again.
« Reply #9 on: January 16, 2020, 08:02:58 AM »
This would confirm your manual calculations.
I wouldn't doubt the manual calculations. They are so easy to do. I have been calculating and logging my mpg since 1978. First on paper end now on a computer spreadsheet. Before that I shared a car then had a car with a broken odometer. The speedo worked but not the odometer!

ColinB

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Re: Sorry fellas it's MPG again.
« Reply #10 on: January 16, 2020, 08:07:04 AM »
I have never trusted the on board computer and find that the only true way to measure mpg is full tank to full tank divided into miles covered..

If you want the best accuracy you can get in calculating historic mpg then you’re probably right (although there are sources of error in the brim-to-brim method as well). But that doesn’t give you real-time feedback on fuel consumption, which is what the computer does. If you’re genuinely concerned about mpg - and that is what your original post suggested - then it seems unhelpful to ignore a tool that is fitted to the car specifically to help with that. As Basil says, you just need to keep in mind that the absolute values it provides are probably a bit optimistic, but you can still easily see when you’re using fuel heavily.

Jocko

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Re: Sorry fellas it's MPG again.
« Reply #11 on: January 16, 2020, 09:18:03 AM »
real-time feedback on fuel consumption
For best real time feedback you want to have the display showing "Instantaneous mpg". This shows you when you are in Deceleration Fuel Cut Off mode and using no fuel and shows when you are saving fuel and wasting it.
My Mk 1 does not have that function, but I have fitted a ScanGauge E which does. It plugs into the OBD port.



I have it set to show Instantaneous and Average for the journey. I aim to keep the top number greater than the bottom, whenever possible. The beauty of the ScanGauge is you add a correction factor each time you top up, so it is more accurate than the "Fibometer". If it says I have used 6.8 gallons since last top up and the fill says 6.6 gallons you make the correction. I normally correct to half the discrepancy, so in this case I would tell it top up was 6.7 gallons.
I know, that on one steep hill near me I get best fuel consumption, in 5th, with my foot flat to the floor. It shows 32 mpg. If I change down to 4th I get 27 mpg and 3rd (which seems the right gear to be in) 24 mpg.
Since fitting the ScanGauge my mpg has improved sufficiently to recover the cost, and then dome.

Basil

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Re: Sorry fellas it's MPG again.
« Reply #12 on: January 16, 2020, 12:32:21 PM »
I realise it's easy to calculate MPG manually Jocko but when you have to enter details manually there's always room for human error, did I mistype that odometer reading or number of litres, easily done. I was just suggesting that the on board computer removes this chance of human error would be a useful comparison tool.

The ScanGauge data is interesting, I would never have thought it would be more economical to drive up a hill in 5th with my foot flat to the floor, does the engine management system then limit the amount of fuel supplied to the injectors ? I know nothing about how modern cars work.

Going back to the original question, was the 4.4 decrease in MPG over the same distance and time period as last year ?

Has it been significantly colder this year ? Have you checked your tyre pressures ? Did you do any longer trips in the same period last year ?




madasafish

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Re: Sorry fellas it's MPG again.
« Reply #13 on: January 16, 2020, 02:21:46 PM »
I run my Jazz primarily in urban areas.. but teh OP's fourney length with a cold engine is going to end up withe teh exhaust never getting warm. Cue build up of carbon and ash deposits.
But teh issue is the catalyst. It will never get up to temperature despite being close coupled and will slowly block up. And teh plugs will be the same.

A 10 mile trip every week  with revs in excess of 3,000 is required to burn off deposits.

It is likely that consumption will worsen unless teh car is occasionnnally driven harder.


peteo48

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Re: Sorry fellas it's MPG again.
« Reply #14 on: January 16, 2020, 05:20:58 PM »
I'm convinced there is a lot in what you say madasafish. With this in mind I occasionally go the long way round to the shops giving the car a bit of a motorway run.

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