I wonder if you could give me some further information about how the “on board computer” actually works on my Peugeot 308, specifically with regard to the fuel economy, range, and mileage displays. It seems to sometimes show rather strange figures, and I would appreciate a deeper understanding of how it all works. 

Charles Felgate, Suffolk

Well, there are some differences, as you might expect, between different manufacturers’ systems, but the principles are very similar. Let’s pretend that you are picking up a brand new car, with only a trickle of juice in the tank! What’s new about that? So you pull into a nearby filling station. Typically you will have three displays available. One would, whilst you were driving to the filling station, have shown a “current” mpg figure that changes with every change of engine power, accelerator input, and speed etc. In the filling station it will, with the engine then stopped, now be showing a blank for this figure. This display in any Peugeot will probably show you the “range” of the car, dependent upon the fuel tank level, and a calculated history of fuel consumption since the last reset, or from new in this case. Since the car has probably been stopped and started at the dealer, and done very little real motoring, this range figure will be meaningless, but probably very low. A third figure might be “distance to destination” which you can generate by inputting a known distance to destination, which will count down as your journey progresses. You would have no other control over this display, which updates whenever the fuel tank is filled, and thereafter continuously, as and when the running average mpg changes, and the fuel is depleted.

Pressing the appropriate button, or turning a rotary knob maybe, will then select another display, perhaps described as “memory 1” which, along with the other selectable display “memory 2,” is under driver control, when all memories and displays can be reset to zero by the driver, usually by a long push on the appropriate button, or with a reset button. So you might initially reset both to zero at the filling station, after which both memories will then begin to display identical figures, showing accumulated miles, average mpg since reset, and usually average speed. Sometimes these figures are displayed individually, and selected within any memory setting by scrolling with a rotary control. Many people will keep one memory for the overall cumulative figures for the car, throughout it’s life, and save the other memory for individual journey figures, or possibly for each tankful of fuel, resetting the memory at each refuelling; some manufacturer systems reset one of these memory displays automatically, after any four hour engine shutdown, so that effectively predetermines your use for that memory. If you have a choice, you might possibly reset one memory for a significant event, like a European touring holiday.

But how can “strange” figures sometimes appear? Well, the range and current mpg display after a fill-up, the next few miles may see some wild changes of the “range” display. If you were to set off up a long hill from the filling station, maybe in fourth gear, your tankful display might settle at a low figure of, for instance, 440 miles. At the top of the hill, and down the other side, now in sixth gear and using little fuel, the average mpg climbs fast, as does the displayed range figure. I have actually filled up once, just before a long downhill stretch of 30mph limit road, and seen range figures in excess of 2,000 miles! Only with the passage of maybe 30 or 40 miles of “average” motoring will the range figure become more meaningful and settled, along with a trip mpg figure that stabilises with the miles. The range figure is, of course, most needed when you’re down to maybe quarter of a tankful, by which time a reliable range figure will normally be available, adjusted continuously by inputs from mpg measurements and tank level. But it’s not impossible to set out on a journey of a few miles and see the range figure actually gaining, particularly if the motoring is of an economical nature, and the mpg displays high.

The only other situation when I think you might spot a “strange” reading is when the current mpg display looks suspiciously low. This can happen during a particulate filter regeneration, when extra fuel is being injected to burn off the stored particulates, which may last for a few minutes, after which it should return to readings you would normally expect for that speed and engine power. The fuel usage data is determined not by actual measurement, but from an ECU databank of fuel consumption for any particular combination of the engine settings that determine the rate of fuel injection, as dictated by the power demands. I hope that gives you some idea of how it all works Charles, but you must nudge me again if I’ve forgotten any key aspect. Regards,

Doctor Diesel