Trip computer and long term Fuel Economy Calculation

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Trip computer and long term Fuel Economy Calculation

Post by jerrytaff » Tue Jan 29, 2019 3:30 pm

On previous cars, where the trip computer has computed 2 trips, I have used one for current, and the other for the very long term. The long term calculation of mpg has been stable and reliable. On the PHEV the most recent trips are stored in the trip history, so I disabled the mpg reset on fill-up and have let it accumulate since I received the car. The mpg calculation from the accumulated mileage is now wildly inconsistent. :? see below

Before a trip yesterday, the computer displayed 25479 miles at 103.0 mpg.
Yesterday I did a trip of 117 miles at 70.7mpg.

Some quick calculations would indicate that, if the figures are true, prior to yesterday, the car would have used 24.74 Gallons. Yesterday's trip would have used 1.65 Gallons. My new mileage is 2664.9 miles and total petrol consumption 26.4 Gallons, so I would expect it to bring the computed overall fuel efficiency down to 101.0mpg.

However. although the car now displays the expected mileage, it computes it at an average of a mere 84.3mpg. For such a short journey (4.4% of miles driven) to have such a big effect the car would have had to average 17mpg yesterday. I prefer to believe the approximation of 70.7.

So, it is apparent that for the most recent journey to have had such a big effect on the calculated economy, it cannot be counting all the miles with equal weight. I suspect it is ignoring the older journeys altogether; Perhaps as they fall off the displayed history.

So, just a word of warning - even if the car can compute reasonably representative fuel economy figures short term, over a longer interval its figures are wildly inaccurate and cannot be believed; Especially on the PHEV when the variation between economy on short and long journeys can be orders of magnitude.
2018 PHEV 3 in Gravity Blue :D
Previous DS (formerly Citroen) DS5 Prestige BlueHDi S/S Auto 2.0, Jaguar X type 2.0 S Diesel

Posts: 176
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Location: North Kent

Re: Trip computer and long term Fuel Economy Calculation

Post by niroal » Mon Feb 11, 2019 1:52 pm


I let my full one to reset on fillups and it can be 5-10mpg out either way. I do a Fuelly comparison and have been running that for just under 17000 miles in 13 months & my overall average is 87.7.

Currently going for my 2nd at least 1000 miles on a tank on the trot which I will live with in this weather!!!!
Niro 3 PHEV Graphite

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