Fuel-Economy Gauge Accuracy?

[Pushrod Pete]

I recently purchased a 1981 380 SL which has a fuel-economy meter in the dash. The other day I noticed that it shows WORSE steady-state gas mileage in D as opposed to running a gear lower and spinning the engine faster.

This would make sense if I were lugging the engine, but even at 70 mph (the fastest I go on my commute, honest officer :D ) the gauge shows better fuel economy if downshift a gear.

Is that right -- should I be getting better mileage at 3200 rpm vs 2000? Is this thing "autobahn-geared" so as to render 4th gear useless for legal U.S. speeds?

- Pete

[jtripp]

Most if not all early fuel economy gauges were simply a vacuum gauge. It does not meter fuel flow.

Highest vacuum occurs with the throttle closed. Vacuum drops as the throttle opens.

The engine works harder in drive at highway speeds requiring more throttle. In a lower gear the engine isn't working as hard in spite of the increase in engine speed and the manifold vacuum will be higher.

Newer cars such as my '93 300SD uses a computer generated readout. It does not meter fuel flow. It is basically there to show how driving style affects fuel mileage.

It's just another distraction when one should really be keeping their eyes on the road.

[Pushrod Pete]

That makes sense. Normally, I'd test it by doing one tank in 4th and one tank in 3rd, but those kind of experiments are expensive these days... :eek:

I've been running in 4th despite the gauge, as I figger that even if my fuel economy is worse, I'd rather take it easy on a 25-year-old engine......

- Pete