Actual mpg differs from ECU calculated mpg - Ducati Supersport 939 Forum
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post #1 of 13 (permalink) Old 01-10-2020, 12:01 PM Thread Starter
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Actual mpg differs from ECU calculated mpg

I recently realized that the ECU calculated mpg isnít what Iím actually getting when I manually calculate miles travelled per gallons consumed. The ecu is saying I get around 43 mpg when Iím really getting 37. I have a RB EVO installed so I expected the 37 mpg number. However I donít see how it would affect the displayed mpg calculation. This problem effectively disables the range function. I have stock wheels/tires. Any ideas why this is happening?

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1997 Ducati 748 Monoposto
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post #2 of 13 (permalink) Old 01-10-2020, 12:25 PM
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I recently realized that the ECU calculated mpg isnít what Iím actually getting when I manually calculate miles travelled per gallons consumed. The ecu is saying I get around 43 mpg when Iím really getting 37. I have a RB EVO installed so I expected the 37 mpg number. However I donít see how it would affect the displayed mpg calculation. This problem effectively disables the range function. I have stock wheels/tires. Any ideas why this is happening?
It happens because the fuel level indicator doesn't provide granular enough data to measure fuel consumption per mile driven. Instead the ECU uses an algorithm looking at several metrics like throttle position, engine rpm, wheel speed, etc to calculate engine load and fuel consumption. On an stock motor these numbers are usually within 5% of each other.

Things that might affect the calculation are changes to engine efficiency:
  • Air flow - dirty filter, high flow filter or exhaust changes
  • Fuel composition or additives - like E15, or cold air additives in California
  • Spark delivery - fouled spark plugs or running hotter or colder plugs

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post #3 of 13 (permalink) Old 01-10-2020, 01:55 PM
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I wonder if the dash processor computes the fuel including the 5% speedo correction.
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post #4 of 13 (permalink) Old 01-10-2020, 02:51 PM
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I would run the TSU option and see how it works out. There could be a difference in tires, perhaps the TSU wasn't calibrated correctly upon delivery? You could also do a GPS check with your phone and determine if the speedo is off more than 5%. The odometer should be spot on despite the 5% difference with the speedometer.

Hopefully, some of these ideas will be a start.

For now, I'll manually check my mileage when I fill up and perform a TSU update as well. I am curious as well.
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post #5 of 13 (permalink) Old 01-10-2020, 06:28 PM Thread Starter
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It happens because the fuel level indicator doesn't provide granular enough data to measure fuel consumption per mile driven. Instead the ECU uses an algorithm looking at several metrics like throttle position, engine rpm, wheel speed, etc to calculate engine load and fuel consumption. On an stock motor these numbers are usually within 5% of each other.

Things that might affect the calculation are changes to engine efficiency:
  • Air flow - dirty filter, high flow filter or exhaust changes
  • Fuel composition or additives - like E15, or cold air additives in California
  • Spark delivery - fouled spark plugs or running hotter or colder plugs
Interesting, I have a CW exhaust and I have an RB EVO that is setup to target a richer AFR, perhaps it’s the primary factor in skewing the mpg. Additionally the o2 sensors are being enhanced in the closed loop section. Maybe I’ll try to run the TSU function but didn’t think that would matter given I’m running stock wheels and Rosso III tires. I have a blanking RB module that I’ll plug in the next time I have the faring off and tank up. I’m due for the 9k service, so soon. That will confirm whether it’s RB. Thanks for the info!

2017 Ducati Supersport S
1997 Ducati 748 Monoposto
2007 Moto Guzzi Norge 1200
2001 HD Road King Classic

Last edited by brucemswan; 01-10-2020 at 06:32 PM.
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post #6 of 13 (permalink) Old 01-10-2020, 10:53 PM
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My last bike had a setting for US or Imperial gallons. Not sure whether that's the case for the SS. But the imperial gallon is bigger than US and indicated a higher mpg until I reset it.
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post #7 of 13 (permalink) Old 01-11-2020, 07:27 AM
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My last bike had a setting for US or Imperial gallons. Not sure whether that's the case for the SS. But the imperial gallon is bigger than US and indicated a higher mpg until I reset it.
If this is the case, OP, 43 US gal = 36 Imperial gal.
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post #8 of 13 (permalink) Old 01-11-2020, 11:04 AM Thread Starter
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My last bike had a setting for US or Imperial gallons. Not sure whether that's the case for the SS. But the imperial gallon is bigger than US and indicated a higher mpg until I reset it.
I am traveling so canít access my bike but I just looked at the owners manual. Under Settings/Units/Fuel Consumption it looks like there is a setting for UK vs US gallons. Iíll sort it out when I get home. Seems like that should cause the calculation to produce a overly low mpg rather than high because it thinks itís consuming more fuel (UK gallon is larger than US) than it actually is for a given set of miles? Maybe I have that logic backwards, itís early morning and I gave up coffee a couple weeks ago, ugh! In any event, thanks for pointing that out Keith.

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1997 Ducati 748 Monoposto
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post #9 of 13 (permalink) Old 01-11-2020, 11:12 AM
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Think of it this way. The unit is measuring how far it goes on a given unit of fuel, say 1 cc. The Imperial gallon contains more cc's so the computer calculates that you will go further.

If you don't believe me, try switching back and forth between the two. When my Triumph ST1050 was delivered it was showing mid 60's mpg, but my pump calculations were high 50's. I discovered that it was 'normal' for Triumph to deliver bikes to the US set to Imperial gallons even though they had the facility to show US gallons. As soon. As I switched it over the indicated mpg matched my measured.
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post #10 of 13 (permalink) Old 01-11-2020, 11:19 AM
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Quote:
Originally Posted by Keith Gill View Post
My last bike had a setting for US or Imperial gallons. Not sure whether that's the case for the SS. But the imperial gallon is bigger than US and indicated a higher mpg until I reset it.
If this is the case, OP, 43 US gal = 36 Imperial gal.
The Imperial gallon is a larger volume than the US gallon. But the mile unit is the same. You will go further on a larger volume of gas than you will on a smaller volume. Hence mpg imperial is greater than mpg US. But tank since tank volume remains the same, the computer know that it contains less Imperial gallons than it does US. Hence tank range isn't effected.
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