Author Topic: What MPG are you getting from your Mk1 Honda Jazz ?  (Read 201347 times)

culzean

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #135 on: January 19, 2018, 03:13:59 PM »
I've often wondered what causes the difference between computer mpg and actual. I'm fairly consistent at the computer being 10% out (it exaggerates my mpg by 10%). This was pretty much the same on previous cars I've had including a VW Golf.

But here we have a 15% and there are drivers who record a very small percentage difference. Does driving style have anything to do with it?

I think driving style does play a part,  the fuel used data is gathered from ECU 'injector open times' and the miles traveled is gathered from speedometer.  I know the average MPG trip readout updates about every 10 seconds with a snapshot of the MPG at that time ( you can see this if you zero a trip and watch how often a new figure appears on the readout,  as the miles on trip increase it gets more and more sluggish to alter the MPG reading no matter how you drive, must be that any change in instantaneous MPG is averaged over greater mileage so makes less and less difference as trip miles mount up). If you are a particularly aggressive driver it makes sense that things can happen in between those 10 second updates that don't get recorded, but if you are a smooth careful driver the 'snapshots' and the time between snapshots would be more likely to be pretty much the same. 

Jocko is the most careful driver here so maybe he can comment on readout vs actual mpg he sees.

More likely explanation though is that the speedo normally reads 'fast' by up to 10% (by law it cannot read slow) so this could fool system that you have done more miles than you have actually covered,  so amount of fuel used spread over more miles is better MPG, and that is what the readout normally shows 'optimistic'.  Makes you think that trip and total mileage recorded by car is also 'optimistic' so that when you have done 110,000 miles you may only have actually done 100,000  :o

I reset one of the trips every time I fill up to get most accurate readout of MPG.
« Last Edit: January 19, 2018, 03:22:06 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

guest4871

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #136 on: January 19, 2018, 03:14:46 PM »
Do you zero the computer at each refill? i.e. compare like with like.

Jocko

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #137 on: January 19, 2018, 04:17:10 PM »
I agree with culzean (must be one of the few times :) ), driving style appears to make a difference. I notice a difference in accuracy depending on my driving style.
My Mk1 has the most basic computer, only showing the average mpg (I fitted a ScanGauge E to show instantaneous mpg) and I zero it every time I fill the tank. This works for me as I don't use the trip gauge for anything else, in fact I only ever move odometer off mpg when I am checking the mileage for a top up.

John Ratsey

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #138 on: January 19, 2018, 09:35:36 PM »
My 2005 got an actual of 48.08 MPG.
The board computer said 55.38.
(370 miles driven)

Did anyone else have such a big difference?
What could be the reason for such a big difference?
One possible cause of a big difference is that the fuel tank wasn't filled up as much at the start as at the end. This potential error will get smoothed out over time if you keep recording future fill-ups. However, the difference you are seeing represents 1 gallon (~4.5 litres) whereas any filling difference should be more than one or two litres.
2022 HR-V Elegance, previously 2020 Jazz Crosstar

John A

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #139 on: January 19, 2018, 10:03:46 PM »
With all the cars and motorbikes I've owned and checked against a GPS then the odometer tends to be spot on, even if the speedo is up to 10% out. My mk 2 Jazz has the most accurate speedo of all the cars I've driven recently, only a few percent fast.

Jocko

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #140 on: January 19, 2018, 10:27:09 PM »
My speedo is the same. Very close to the GPS speed indicators on my Sat Nav and Dash Cam.

MikeG1944

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #141 on: January 20, 2018, 10:39:46 AM »
My speedo is the same; I've never had a car with a speedo so accurate. Spot on when tested with the SatNav. Not even 1mph out! :)

Kenneve

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #142 on: January 20, 2018, 11:11:51 AM »
It would seem that the Mk3 speedo is not as accurate as the earlier models, mine is approximately 2 mph optimistic throughout the normal speed range, as compared with the satnav

When the satnav is showing 30 mph the speedo is reading 32mph, with similar results at 50 and 70mph.
I tend to regard the satnav as the most accurate and this seems to be confirmed, by the roadside speed indicators where they flash up a warning, when you are over the limit.

MartinJG

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #143 on: January 20, 2018, 12:45:18 PM »
Size of tyres and tread can make a difference. As an extreme example, years ago I made the mistake of fitting a rear fat nobby tyre rear to my motorbike thinking it would give me more comfort in Australian conditions. It made no difference to comfort, the grip was terrible in wet conditions and the larger diameter acted like an extra gear ratio with the result that overall, I lost performance and MPG. 

culzean

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #144 on: January 20, 2018, 12:48:43 PM »
It would seem that the Mk3 speedo is not as accurate as the earlier models, mine is approximately 2 mph optimistic throughout the normal speed range, as compared with the satnav

When the satnav is showing 30 mph the speedo is reading 32mph, with similar results at 50 and 70mph.
I tend to regard the satnav as the most accurate and this seems to be confirmed, by the roadside speed indicators where they flash up a warning, when you are over the limit.

Due to slight differences in tyre diameters and also for wear (although tyre wear would make it read 'faster' anyway) the speedo is by law (certainly construction regulations) 'optimistic,  otherwise,  with some police areas (like North Wales) having 'zero tolerance' for speeding you could be caught if your speedo was reading low.

https://www.theglobeandmail.com/globe-drive/culture/commuting/why-you-may-not-be-driving-as-fast-as-you-think/article11487709/

There are a lot of variables with tyres, and even inflation pressure can change diameter, but satnav does not need those fudge factors as it is independent of the vehicle it is used in.
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

Jocko

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #145 on: January 20, 2018, 01:45:15 PM »
Yes, Sat Nav is spot on for speed, but remember, the display only updates at finite intervals, so when it is spot on if you are travelling at a steady speed, it is lagging if the vehicle is accelerating or braking.

peteo48

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #146 on: January 20, 2018, 01:46:36 PM »
Interesting side issue here about wheel sizes and the effect on mpg. Buy a Jazz with bigger wheels and the CO2 goes up and the mpg down. Noticed this particularly when looking at the latest Jazz but also the Toyota Yaris. Get big wheels on the Yaris Hybrid and you are then liable for the London Congestion charge - get small wheels and you aren't. Small wheels 75 grams of CO2 - big wheels 82 grams

I am clueless on this and I know there are some clever people on this forum. My thinking is  - the bigger the wheel, the fewer times it will have to go round hence less fuel.

I'm embarrassed to ask this question but why is that?

Jocko

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #147 on: January 20, 2018, 01:57:07 PM »
The bigger the wheel the fewer times it will have to go round - but the more effort that is required to turn it, hence marginally more fuel burnt.

culzean

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #148 on: January 20, 2018, 03:13:30 PM »
Interesting side issue here about wheel sizes and the effect on mpg. Buy a Jazz with bigger wheels and the CO2 goes up and the mpg down. Noticed this particularly when looking at the latest Jazz but also the Toyota Yaris. Get big wheels on the Yaris Hybrid and you are then liable for the London Congestion charge - get small wheels and you aren't. Small wheels 75 grams of CO2 - big wheels 82 grams

I am clueless on this and I know there are some clever people on this forum. My thinking is  - the bigger the wheel, the fewer times it will have to go round hence less fuel.

I'm embarrassed to ask this question but why is that?

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=175

larger diameter tyres have more rolling resistance.
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

John Ratsey

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Re: What MPG are you getting from your Mk1 Honda Jazz ?
« Reply #149 on: January 20, 2018, 05:37:39 PM »

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=175

larger diameter tyres have more rolling resistance.
Isn't the outer tyre diameter the same but the EX wheels are bigger and shod with lower profile tyres? Do the less flexible shorter sidewalls on the lower profile tryreshave more or less energy loss than the normal tyres? I can't figure out the answer from the explanation in that link.

I also wonder if the extra bells and whistles on the EX add to the weight which worsens the mpg. Are there also any extras which would worsen the aerodynamics?
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