[NC3] MPG on trip computer is hugely inaccurate

Technical Area Electrics & ICE
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  1. My model of MX-5 is:2014 NC3/Mk3.75
  2. I’m based near: Planet Earth
  3. I’m looking for technical help or recommendations on: Trip Computer MPG readout

The MPG reading on the Trip Computer on my 2014 NC3/Mk3.75 is wildly over-reading - it shows figures in the 50/60/70MPG range, whereas I’m actually getting around 26MPG (calculated directly from petrol added and miles driven).

  • The reading updates every minute, as per the driver’s manual
  • Changes in the reading are inconsistent (e.g. I could accelerate hard, but the next display will increase rather than decrease)
  • Resetting the MPG (by pressing in the stalk for >2 seconds) shows - - - for the first minute, followed by a high number afterwards
  • Disconnecting the battery didn’t make any difference
  • All the other functions of the Trip Computer (Trip A, Trip B, Temperature) work correctly

Has anybody seen this, or know what could be causing it?

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Something odd there.

I drove up to Northumberland and back and tootled about for a couple of days up there. 38.5 mpg on the trip computer AND the same when dividing the miles driven by petrol used.

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Hi P,
Are you sure you are not looking at ‘current’ MPG and not ‘average’ MPG? Current shows what you are doing now and will vary greatly from minute to minute just as you describe.
:heart:

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On the NC2 we only have one mpg figure on the display, that’s what I would class as an all time average.
When reset it does shoot up (on mine) to over 40 mpg but after a few miles settles to high 30’s. Currently sat at 34.6 after a few 30 mile distance jaunts around the Derbyshire dales recently.
Whether that is accurate, probably not but on use of the car and time of year (not so cold) I do believe I’m getting on average over 30mpg but not really done a proper fill and test.

So Phillip I think yours is definitely acting up.

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Thanks.

Yes, it is definitely acting up. Not only is it very inaccurate, but the reading is not consistent (as I said, I could accelerate hard for a period and the reading would go up rather than down). On my other (2015) NC, the MPG gauge is both consistent and reasonably accurate when checked against actual petrol added/miles driven.

I’m struggling to find any documentation on how the car calculates this MPG. All the other gauges (including fuel gauge and the other trip computer functions) are fine.

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If you can get access to an obd Bluetooth dongle then you can read the fuel parameters in the ecu using the torque app. I imagine mpg will be calculated from injector duty cycle and other parameters using some weird Japanese algorithm. If you compare one cars typical readings to the other you should be able to id the errant info.
My car used to display good numbers but when the injectors were changed for higher capacity units to suit the supercharger the mpg values went completely off.

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Thanks for the suggestion - I do have OBD2 dongles and Torque Pro.

Interesting that you mention a supercharger. The “problem” car has a 250nhp corten-miller supercharger on it (although, as far as I know, the original injectors).

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There’s your problem, even the 250 has bigger injectors therefore a similar duty cycle will use proportionately more fuel

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The map sensor is also changed so the original algorithm may be fooled with regards to engine load

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That all makes a lot of sense, thanks.

I need to go away and do a lot more reading about how the supercharging works! (I bought the car as I was seduced by the idea of having 250bhp, and without first researching how it was achieved. I must make amends).

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I suggest you find out who supplied the supercharger kit and ask them. It is likely that the air flow parameters are way different from those the fuel usage algorithm is calibrated for…