Gas mileage gauge working?

pearldrum · 04-04-2002, 07:33 PM

I was wondering if my gas mileage gauge was working or if my car isn't getting very good gas mileage. So, what are you all getting at different points, like idling, and different RPM's above and under 20 mph?

97XRAY · 97XRAY's Garage

Mine sits at the right side during idle, and hits left I drive hard. If I just start from zero on a level surface and go slowly through the gears I can avoid hitting the 10 mark (bottom scale)... but usually I hit max when accelerating through first, second, and third. In traffic at 55 to 65 mph I rarely exceed the 8.

Charles · 04-06-2002, 05:29 PM

drive on a flat road with the engine between 2K and 3K RPM, and the transmission in 4th or 5th. You should be able to maintain speed with the meter above 25 or so (less for a turbo). If the needle is in the far left, the most likely scenario is that your wheel speed signal isn't making its way into the economy meter.

Also, in this situation, you should be able to pin the meter on the right side if you let off the gas completely.

The reason is that the gauge is effectively dividing the absolute flow rate (gallons per hour, on this side of the Atlantic) into car speed (miles per hour). That cancels out to miles per gallon, which is pretty useful when driving.

When you are stopped, you are getting zero miles to the gallon (yet the engine still consumes gas). In this case, gallons per hour is more useful. I think the gauge switches to gal/h when the car is travelling at less than 10 mph.

If the wheel speed doesn't make it to the gauge, the gauge is essentially stuck in gal/h mod. And when you think about 25 mpg at 75 mph, that's 3 gallons per hour, which maxes out that scale.

The flow rate is calculated by calculating the duty cycle of the injector pulses. If your injectors are not delivering their specified flow rate, then the economy meter could be off regardless of speed.

I know this doesn't answer the original question directly, but I personally think it is better to understand what's going on under the hood than to try and compare numbers from wildly different cars. Hope that helps.