Author Topic: MPG for those interested.  (Read 172671 times)

AlanTR

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Re: MPG for those interested.
« Reply #255 on: December 05, 2018, 12:39:11 PM »
We have done approx. 26000 miles in our Jazz CVT and with 60/40 town/country driving - we average 48.5 to 51 mpg winter/summer with normal careful driving. These figures are as per dashboard display. Acceptable performance which does not make me feel like switching from Honda any time soon! By comparison our other car a Mk 2 Hyundai I10 (1.0 litre, 3-cylinder - an excellent manual small car) averages around 43-44 mpg (90% town driving).

peteo48

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Re: MPG for those interested.
« Reply #256 on: December 05, 2018, 01:51:46 PM »
Just a little example of what a cold snap plus a succession of very short journeys can do. This included a few minutes in the drive with every single thing on from the air con right through to heated rear windscreen one very cold morning as we waited for the mist inside to clear. Over the 14 miles since I last filled up the computer shows the mpg at 26.7 mpg.

This will rise over the tank as longer trips factor in but it shows that, in any car, short trips and cold weather destroys mpg.

andruec

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Re: MPG for those interested.
« Reply #257 on: December 05, 2018, 03:42:14 PM »
My MPG is being dented by all the damn' traffic lights around here and the need to battle my way along minor roads. It's either that or get good MPG on the motorway but burn 50% more fuel.

Supposedly the main road I used to use for commuting should reopen before Christmas. It can't come a day too soon  >:(

orcadian

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Re: MPG for those interested.
« Reply #258 on: December 25, 2018, 03:22:59 PM »
As a newbie to the forum and to the Jazz, I have certainly found this long thread interesting and well worth reading. Although I’m in the fortunate position to be not overly concerned about fuel economy, fuel efficiency does interest me and i’m certainly interested in the question of the inaccuracies of dashboard displays and the reason why these errors might be present.  The cynic in me feels that although legal it follows the VW saga, which has been proven not to be.  I understand that the speedometer cannot legally read under the actual speed over ground but with simple software adjustments on our modern vehicles, especially with satnav, the odometer needn’t be so erroneous.

We had a 2013 Pug Partner Tepee for a couple of years and under even good long run conditions the 12500 mile countdown to service always threw the spanner light well under 9000 and that was on the odometer, the actual distance being somewhat less than that figure.  We had a trip to Austria with that car and using our Garmin separate satnav and brim to brim fill ups for 3000+ miles, the actual mpg (diesel) was a shade under 54 whereas the display indicated 59.2.
My Daimler SV8 with a 4 litre Supercharged engine, weighing in at over 2 tons and with 370 willing horses achieved a properly calculated 25.1 mpg on a recent trip to Germany, which might alarm the tree huggers but that IS fuel efficient.
By the way, I was not born cynical, it only started when I was two 🤓

Regards,
Ian




culzean

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Re: MPG for those interested.
« Reply #259 on: December 25, 2018, 09:33:31 PM »
While the display of MPG May not be entirely accurate it is a good guide to how your driving style affects MPG. The thing is the smoother your driving the more accurate the readout because if the mpg follows the same format as the industrial 'rate' meters I have had experience of they have a sample rate - the Jazz one will update the mpg reading about every 10 seconds ( you notice this more if you reset the trip), and it uses the injector ' on time' from ECU and the speed information from the ABS pulses, there can be a small error in both, it possibly uses the instantaneous information available in a register of both quantities at the 10 second mark - so a short burst of acceleration within that 10 seconds will no doubt go unnoticed by the computer.  It is also interesting that when you zero the trip the MPG indication is very sensitive as it is averaging the rate of fuel flow over only a few miles, as the trip mileage gets larger ( above 50 miles ) driving style or whether car is going up or down hill makes less difference to readout as the present rate of fuel usage is averaged out over more miles, and above 100 miles it is hard to make MPG reading change at all.  The bottom line is the display is only a guide and the smoother you drive the more accurate it will be..

Not every car has GPS info available and if you drive straight off after starting some GPS receivers take time to get a signal, so that would be another inaccuracy , and sometimes GPS signal can drop out in cities or dense forests.
« Last Edit: December 25, 2018, 09:38:17 PM by culzean »
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

orcadian

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Re: MPG for those interested.
« Reply #260 on: December 26, 2018, 09:50:01 AM »
Culzean,
Many thanks for the comprehensive reply, very informative for those who didn’t know what parameters the display  figures were derived from.  I think my issue and perhaps even the original posters issue is that when something has a tolerance, then some readouts would be high and others low (obviously not the speedo though!) so statistically if the sample was large enough then it would eventually tend towards a 50/50 split.

The technology is clearly there for the manufacturers to bring the displayed readout somewhere closer to reality, but that is obviously not in their interests.
Hope you all had a good one,
Cynically yours,
Ian
« Last Edit: December 26, 2018, 07:11:58 PM by orcadian »

peteo48

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Re: MPG for those interested.
« Reply #261 on: December 26, 2018, 10:58:39 AM »
I've found virtually every one of my recent cars - ie those fitted with an mpg indicator - have exaggerated  mpg by between 5 and 10 %. In recent years it's always been about 4 mpg. In support of Culzean's arguments driving style is most definitely a factor. Most of my driving these days is short stop start stuff with a lot of acceleration and then deceleration in an engine that only gets up to operating temperature as we pull into the supermarket car park!

My avatar shows an average mpg of 43.8 (actual as opposed to indicated) and this is over just under 2,000 miles. Trip B which I never change is showing an mpg of dead on 48. Looking back in my records, the odd long trip where you are cruising for most of the time brings the computer much closer to the actual in percentage terms.

Being a bit OCD I faithfully do the brim to brim method and record the results on Fuelly.com but, to be honest, I would get a decent working figure by just looking at the computer figure and deducting 4 mpg.

mikebore

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Re: MPG for those interested.
« Reply #262 on: December 26, 2018, 12:01:54 PM »
Similar to my experience, except

1. My Fuelly number is 46.1mpg over 36,000 miles (fair amount of long journeys).

2. My computer to Fuelly difference is more like 2-5 mpg, but usually close to 4. When I get a big or small difference I suspect variability in top off procedure when filling, and pump to pump cut off variability.

orcadian

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Re: MPG for those interested.
« Reply #263 on: December 26, 2018, 12:31:37 PM »
Interesting Peteo that you can do it accurately over a period of time, OR just deduct 4 mpg from the computer readout which is exactly my point - the manufacturers could do that!
Wonder if anyone has ever got MORE mpg than the display indicated? It would not surprise me if no one had.

Ian

ColinB

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Re: MPG for those interested.
« Reply #264 on: December 26, 2018, 12:42:21 PM »
I've taken to monitoring the "current drive" page on the big screen rather than just Trip A or B. On a longish drive on fast A roads or motorways it's not unusual to see an MPG for that drive of over 60, but for a short journey around town only around 25. Those figures are probably optimistic, but I don't expect absolute accuracy but rather an  indication of whether I'm driving efficiently. Plus you can easily see the range reading on the same page and watch that fluctuate wildly (recently I saw it veer from 390 down to 370 and back up to 470 all in the space of around 10 miles whilst driving at a reasonably constant speed !)

John Ratsey

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Re: MPG for those interested.
« Reply #265 on: December 26, 2018, 12:48:52 PM »
The computer on my hybrid Jazz was about 5% optimistic while the Mk 3 Jazz was at least 7% high. In contrast, my current HR-V is only about 3% optimistic. I have to wonder if Honda put bigger a fudge factor in the Mk. 3 Jazz's computations so that people would get closer to the claimed mpg and more than the Mk. 2. The HR-V, in contrast, had no recent preceding model over which it needs to show an improvement.

I've never checked the Odometer for accuracy. There's no real reason for it to have any error greater than that caused by tyre wear which will cause an apparent increase in distance of up to 2% in which case new tyres could have up to 2% worse apparent mpg than worn ones.
2022 HR-V Elegance, previously 2020 Jazz Crosstar

orcadian

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Re: MPG for those interested.
« Reply #266 on: December 26, 2018, 01:25:34 PM »
That’s a very interesting point John about no recent previous model comparison, so we are gradually converging on ‘codology’ by manufacturers, which surprises none of us methinks.  This is obviously not aimed at just Honda but many people still believe these readouts and often brag about them on other less well informed forums.

Ian

peteo48

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Re: MPG for those interested.
« Reply #267 on: December 26, 2018, 05:31:14 PM »
I agree, Ian, people do, wrongly, believe the computer figure. This obviously works in the manufacturers favour.

Is it actually, technically, possible to produce an accurate figure via an onboard computer? I don't have the technical knowledge to answer that.

orcadian

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Re: MPG for those interested.
« Reply #268 on: December 26, 2018, 06:24:16 PM »
No Peteo, I don’t think it’s possible to get a consumption figure to be exact for many of the reasons already outlined by previous posters but as most folk have discovered, if the computer figure is always higher, then it’s certainly possible for manufacturers to get closer to reality.  Don’t hold your breath til they do it though!  I can’t believe some smart Alec Philadelphia Lawyer hasn’t tried yet.

It’s only a bit of fun for me and as many have correctly said it does indicate how you are driving and the CHANGE in overall consumption on your own particular vehicle is more important that the fictitious readout.

Ian

andruec

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Re: MPG for those interested.
« Reply #269 on: December 26, 2018, 07:27:45 PM »
Just got back from a trip to see my Dad. 424 miles, mostly motorway at ~60mph with cruise control. Also mostly at night so headlights on. Two bars and a claimed 54 miles left according to the dash display. 33 litres put in.

Just over 58mpg.

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