Many modern cars offer the feature of displaying instant fuel economy (eg. mpg, L/100 km etc.)
How does it calculate the value?
Is it a very complex formula involving some jaw dropping equations or just a preset value based on RPM, gear etc.?
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It calculates it on the fuel it's using at that moment(other options are available eg.averaging it over a period);it calculates it from the time an injector is open,multiplies it by the number of injectors.Injectors are virtually constant flow while open.
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Even carburetted cars, from VW first I seem to remember, sometimes had an 'economy meter' calibrated in MPG that gave an instant readout.
Even on the motorway the thing couldn't give you much idea of your real consumption, and in normal driving the needle simply veered between 2 or 3 mpg when you were haring away from the lights and 100 or so when you took your foot off the loud pedal. Useless as an instrument and distracting as a driver's toy.
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Carburetted vehicles had a little flowmeter fitted(like a little paddle-wheel) to measure the flow.
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My old 1984 Audi 80 had an "Econometer", similar to the analogue BMW ones.
But the Audi one did not even have numbered graduations, it had varying thickness lines where IIRC thin line meant poor fuel economy and thick line meant better fuel economy.
It worked by vacuum, would this be directly off the inlet manifold or off some part of the K-Jetronic system?
It was quite entertaining making it swing around but not a lot of use. It spent a lot of its time at the poor end of the scale! Happy days, when I didn't give a fig what mpg the car was doing..
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