[Gmecm] ALDL MPG Information

[Russ Harms]

> In the 80's Jaguar used a purely analog version of this idea for the 
> in-dash display. It didn't even receive data from the computer,  rather it 
> was wired directly to the injector harness. Aside from  normal car-to-car 
> variance in manufacturing tolerances, it was  acceptably accurate. You 
> could theoretically do the same thing with  your micro-controller box by 
> tapping the injector harness, VSS and  TACH leads to accomplish the 
> calculation thing without ever touching  ALDL at all.
>
> -Gary

The 80's vintage Zemco trip computers were primarily designed for carbed 
vehicles with a mechanical flow sensor that "T"ed into the fuel line. Zemco 
sold an optional EFI interface box for use with injected engines, the EFI 
box just tapped into one injector lead and changed the varying pulsewidth 
into a varying frequency to simulate the mechanical flow sensor.
The Zemcos were pretty accurate when calibrated over a few hundred miles, 
the longer the distance and the more fuel burned the more accuracy.
The calibration factor, IIRC, ran to six digits and was arrived at by 
comparing actual distance and fuel burned with the indicated values.

Russ Harms