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My 2006 Ford S-Max (1.8 diesel) trip computer says 53 mpg but my brim-to-brim calculation (353 miles on 42 litres) only works out to 38 mpg. That's only 72% of what the trip computer reckons. Could this discrepancy indicate a problem? If so, what?
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My 2006 Ford S-Max (1.8 diesel) trip computer says 53 mpg but my brim-to-brim calculation (353 miles on 42 litres) only works out to 38 mpg. That's only 72% of what the trip computer reckons. Could this discrepancy indicate a problem? If so, what?
Think you'd be better starting a new thread rather than ressurecting a 16yr old one, but hey ho!.
38mpg is not great, but it isn't that bad either. The S-Max is a big old and heavy bus for a 1.8 diesel, so if you find its performance frustrating you are more likely to be driving it hard. In that case, your economy sounds reasonable, especially if a lot of town driving is involved. If you drive gently at or below the speed limit, it does seem a little low. But not to the extent of worrying about it if the car drives fine otherwise. Look at websites, including this one where owners review their own car including the actual mpg they get.
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I've been pleasantly surprised by the average fuel read-out on my Mazda CX-5. I calculate brim-to-brim mpg every time I fill and 99% of the time the car is a bit pessimistic.
An example from today: car says 40.7 mpg, calculation says 41.1 mpg.
(In case anyone is wondering, I do get substantially more on long journeys, and in warm weather.)
Edited by FP on 03/01/2019 at 15:10
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Most of these gadgets are simply a guide, some more accurate than others. My brim to brim figures are within a close margin of the computer figures. Even if the computer goes 'walkabout' and fuel gauge packs up, I still have a reasonable idea of fuel consumption and do not rely entirely on electronics.
Cheers Concrete
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Most of these gadgets are simply a guide, some more accurate than others.
I think they are remarkably accurate, as they don't 'know' how much fuel you have added, they just integrate what goes through the injectors many times per second. My Pug also calculates pretty close to the figures I get with pencil and paper.
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My 2006 Ford S-Max (1.8 diesel) trip computer says 53 mpg but my brim-to-brim calculation (353 miles on 42 litres) only works out to 38 mpg. That's only 72% of what the trip computer reckons. Could this discrepancy indicate a problem? If so, what?
Bear in mind that the trip computer could be displaying any one of three figures for the mpg:
- The current mpg, dependent on what level of fuel the car is using as its being displayed, or;
- The mpg since the last system reset (whether manually done or if the battery was removed), or;
- The mpg since the last fill-up. This would likely be the least accurate (if possible), as most cars cannot accurately record the amount of fuel deposited into the tank via the pump, and only have a low level warning. Most cars systems work by a fuel flow meter only.
I suspect you're looking at the second figure. At least your car's trip computer reads in mpg - mine can only do so in l/100 km or l/100 miles, though it can show both the first and second types in those odd units.
For a car of your size and engine, 38mpg at this time of the year is perfectly reasonable.
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The car has no,need to know the amount of fuel put in, all it needs to know is when it was refuelled, the distance travelled, and the fuel used. I presume there is some form of flow meter, which naturally has a degree of error, to measure fuel use.
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"I presume there is some form of flow meter..."
I believe that what is measured is the amount of CO2 the engine produces, which relates to the amount of hydrocarbons (fuel) consumed.
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"I presume there is some form of flow meter..."
I believe that what is measured is the amount of CO2 the engine produces, which relates to the amount of hydrocarbons (fuel) consumed.
I think it computes the amount of fuel used from the injection timing signals
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I believe that what is measured is the amount of CO2 the engine produces, which relates to the amount of hydrocarbons (fuel) consumed.
I'd love to know how you think that might work (anywhere near accurately) in practice, especially for a diesel ?
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They're not the only options, Andy - I get mesmerised by the Android app for my Peugeot, which "bluetooths" itself and gives an MPG readout for each journey (plus time taken, distance, locations). Haven't checked the running average(s) myself yet, brim to brim, but the figures seems plausible so far.
The computed figure on my old Saab 9-3 was in fact extremely accurate as a running average (since last reset, I think), when empirically monitored by the tankful.
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I was only stating the options that the trip computer was actually showing at the time, not what a driver has for any car.
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I was only stating the options that the trip computer was actually showing at the time, not what a driver has for any car.
Yes, but I believe your reasoning for the calculation of the third one was incorrect. My car allows me to select current mpg, mpg since last refill and mpg long term. The Citroen I am borrowing seems to have current mpg, and average mpg, not that it’s easy to tell as the beast is far from intuitive.
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How does it know how much you filled up with or if you have done at all? It'll only have a fuel flow meter on the tank outlet side, and (as many have shown) isn't generally that accurate.
My car has the same set up as your Citroen. I must admit it would be great if the car did have extra sensors that could determine the fuel economy between fill-ups.
Anyhoo - the brim-to-brim method is still the most accurate, and I suppose the debate then turns to whether we should go by the odometer or the satnav for distance. That's a debate for another day (and probably thread, and probably has already been covered here somewhere).
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