Technical 'REAL' MPG Vs 'Trip Computer'???

[Most Easterly Pandas]

On this subject, we know the speedos over read a fair bit, but presumably the trip/odometer does record the true mileage as opposed to an artificially sped up number? I did a small fill up of 15 litres or so the other day and Fuelly calculated as 23.9 MPG in my 100HP. No spirited driving at all, just short town trips. It's a very low figure whichever way you look at it, but some of the blame lies in my larger rolling radius from the 195/45r16 tyres it wears giving under-reading mileage numbers.

The odo is done at real distance travelled as the +10% is added by the computer (will notice the difference if running a live data run on diagnostics).

 

[ivantate]

Our panda always seems to be with +/- 0.5 mpg on the computer with generally a max of +/-2. anymore and i have doubts of the pump accuracy or whether i filled the car properly.

the idea is generally a minimum of 2 out above or below. I have details of every tank but havent made the statistics for it them yet.

I think its the cold running that confuses the petrol computers whereas the diesel still knows exactly what it has injected.

 

[Most Easterly Pandas]

I think its the cold running that confuses the petrol computers whereas the diesel still knows exactly what it has injected.

You'd be surprised just how safisticated they are.

 

[Benz man]

Funny, my MJ's trip computer also over-reads the actual fuel consumption. It consistently shows 5 mpg less (0,3 l/100km more) than I actually calculate, and I track it very accurately!

Fuel consumption ranges from 4,1 l/100km (69 mpg) during Summer, to 4,3 l/100km (66 mpg) in the Winter. I fill it mostly at BP stations, tyres at 2,6 bar (155/80R13 Uniroyal Rainexpert), careful driving but overtaking a lot. On long journeys, I travel at 62 mph (100 km/h) - 2400 rpm in top gear. Temperatures here in Portugal range from 1ºC (once) to 40ºC (twice).

Another question: last year I tried BP's Ultimate diesel for 3 tanks, but I found NO difference both in performance and fuel consumption. Does anyone have a similar experience?

 

[richardllew]

I keep a good record of my fuel consumption, its getting back to normal after the very cold weather.
The attached file shows the display mpg against the brim to brim mpg.
Over the duration of the graph

overall mpg standard deviation
73.7 3.14 brim to brim
75.4 2.74 display

Hence the display has been a bit optimistic but has a lower deviation.
What causes the lower deviation could be clever software or alternatively brim to brim is variable due to air locks or inconsistent filling or cut-off of the pump.

I'm impressed with the performance of the display system. There are so many variables, how accurate are the pumps, or the odometer?
What the display system does do is enable the driver to know the effect of speed etc on the fuel consumption, obviously FIAT have gone one better with eco-drive etc.

 

[David Bliss]

Over a 20000 mile usage we have frequently done a check on petrol consumption. The actual consumption is always lower than that shown on the computer, by about 6 mpg. When the computer says 56-58 mpg we actually get 50 - 52. (Nothing to do with the readout but in winter the consumption drops to around 48 mpg, a noticeable reduction in cold weather.) We tend to ignore the computer!

 

[gavin]

I've let my trip run for 999 miles, and the average mpg is 62.9, '07 mjet.

 

[ivantate]

We are still under reading at the moment but I think there is 1 thing we can all agree on, consumption takes a dive in the winter. I reckon 2-5mpg.

Some of this could be due to the extra drag of rain and higher headwinds.

 

[Matthen]

Yes- mine consistently reads around 4 - 5 mpg higher than it actually calculates on Fuelly.

4-5 Gallons? Fuelly works it out in US Gallons remember...

 

[Wee Smurf]

I have kept a spreadsheet for a while which records everything that fuelly records. My MJ is working at just a smidgen under 60 mpg (although computer says 58 mpg, so under-read).

My driving is mostly commute in traffic in town type driving, so I'm pleased with the 60 mpg. Lowest mpg was 55 in the snow, but I would put that down to others being unable to drive in the snow causing me to have to sit in queues excessively. I had winter tyres fitted and had no problems, other than stupidly slow traffic.

 

[Stu DemonD]

4-5 Gallons? Fuelly works it out in US Gallons remember...

No it doesn't - not if set to UK.

 

[Most Easterly Pandas]

Fuely tends to be 0.5 mpg out to the spread sheet I used to use. I put this down to my rounding though.