What MPG are you getting from your Mk1 Honda Jazz ?

[Jocko]

I run my tyres at 35 front and back. Even on the school run, don't be afraid to get it into 5th. I am usually in 5th or 3rd around town. Seldom use 4th. And try and make as much use of DFCO as possible.
The figure beside my avatar is my average miles per gallon since I bought the car. Four years and 41,000 miles.

[springswood]

The new rear coils seem to have improved fuel consumption. It's hard to be exact, my figures on Fuelly still vary widely (I think the pump cut off can make about 10 litres difference on what's in the tank) but my 3 month average is up to 45 so about 3 mpg more.

[DomnicZZ]

I run my tyres at 35 front and back. Even on the school run, don't be afraid to get it into 5th. I am usually in 5th or 3rd around town. Seldom use 4th. And try and make as much use of DFCO

Mines a CVT, so if I want to force an upshift I just lift my foot off the gas for a couple of seconds and gently press again. I notice it already uses the tallest gear ratio By 30MPH on flat roads.

I’ve run some stats on the school run yesterday using car scanner so I can better understand the vehicle behaviour. The middle gap in the graph is where I got out to pick up my kid. The gaps in the instantaneous MPG hopefully represent DFCO. I do not know the accuracy of the car scanner app yet, but will hopefully find out over time.

[Jazzmeister]

I have asked this elsewhere but this is a very busy thread so I'll post it here as well:
Does having a non standard wheel/tyre size have any influence on the onboard computers calculations?
My tyre is a 185/65/15, which is a 2.1% larger circumference than stock 175/65/15

[Jocko]

It must do. There is no way the ECU knows the tyre size.
My ScanGauge E allows me to calibrate its speed against the GPS. Knowing the speed calibration, it can calculate the distance correctly.
Also with The ScanGauge, you input a correction factor for its calculation of how much fuel you put in to fill the tank compared to the actual fuel added. The secret here is to add or subtract HALF the difference, and over a few top-ups, you get pretty accurate. Now the ScanGauge is generally within 0.1 gallons against the actual. So I leave it as it is.

[Jazzmeister]

It must do. There is no way the ECU knows the tyre size.
My ScanGauge E allows me to calibrate its speed against the GPS. Knowing the speed calibration, it can calculate the distance correctly.
Also with The ScanGauge, you input a correction factor for its calculation of how much fuel you put in to fill the tank compared to the actual fuel added. The secret here is to add or subtract HALF the difference, and over a few top-ups, you get pretty accurate. Now the ScanGauge is generally within 0.1 gallons against the actual. So I leave it as it is.

What data exactly does the onboard computer use to make it's calculations?
I saw someone mention somewhere on the forum that the OBC uses the amount of time the injectors are open to calculate the fuel usage, does it then compare that to the odometer mileage to come to it's finding on historic fuel usage?

[DomnicZZ]

My 2 cents on this bigger tyre impact.
Let's say it's 2percent bigger circumference, over 100 miles reported on the odometer, you would have actually travelled 102 miles. The ECU does not know this. MPG calculations are based on odometer reading, so in theory it shouldn't impact. In practice though, the car would have worked 2 percent harder over the reported 100 miles, so you may see a minor dip in MPG on the display. But since you know you've traveled the extra 2 miles. You can calculate to compensate for this dip. In real life though, there are more MPG impacting factors like traffic,  road surface, weather etc, so unless testing on a dyno in a controlled environment, it will be hard to detect.

[TnTkr]

And the odometer isn't 100% accurate with the factory tires either...

It is not sure if larger diameter increases or decreases the absolute fuel consumption as it effectively changes the ratio making engine running lower rpm. It depends where on the engine specific consumption diagram is the current operating point.

[Jocko]

Unless you do mpg testing using a 5th wheel and measured amounts of fuel it is all just approximate anyway. I calculate my mpg from when the pump first trips off, but who is to say that always happens at the same point. Fill up, calculate your mpg, and that's your number. Suppose it is +/- 0.2 mpg, who cares. You know if it is better or worse than expected, which is the main thing.

[TnTkr]

Unless you do mpg testing using a 5th wheel and measured amounts of fuel it is all just approximate anyway.

Exactly, that was my point too!

The accuracy of the fill-ups improves through time when calculating total miles and total amount of fuel.

I have a habit to fill up until the fuel surface is visible on the filler neck. In Jazz that makes only 1-2 liter difference, but in some other cars it may be even 5 litres.

[culzean]

In real life though, there are more MPG impacting factors like traffic,  road surface, weather etc, so unless testing on a dyno in a controlled environment, it will be hard to detect.

Exactly, which is why I cannot understand why people need to calculate their MPG to two decimal places... ( or even one decimal place )

[TnTkr]

Maybe the two decimals are needed to get better results than the neighbor 

[Jocko]

Exactly, which is why I cannot understand why people need to calculate their MPG to two decimal places... ( or even one decimal place )

Both Fuelly and spritmonitor automatically generates one decimal place. My ScanGauge E requests one decimal place on input.

[Jocko]

At 59 mpg today's top-up was the worst since the beginning of April. I knew that as soon as I started running my wife back and forward to work the fuel economy would suffer.

[peteo48]

Still outstanding Jocko.

Reckon you'd get close to 80 mpg with the Mk4 Jazz.