Here's the content of a recent e-mail to me from another newsgroup.
Might be pertinent to our vans, especially if you are a Gadgeteer.

John Rodges
88 GL Driver

In the Australian electronics magazine "Silicon Chip" for April 2004 there's

a construction project called "A Smart Mixture Display for Your Car".
Basically it's an add-on gadget for the motor enthusiast, to track the car's
fuel mixture, rich or lean, by monitoring the engine's EGO sensor.

The "Silicon Chip" article is accessible on-line at www.siliconchip.com.au, or more specifically at http://www.siliconchip.com.au/cms/A_101150/article.html
though disappointingly only if you pay an amount for the download.  It comes
down strongly in favour of a particular EGO sensor, described as "NTK
manufactured Ford E7TF 9F472 DA " , and  includes a graph  showing the
response curves of the sensor at three different temperatures. A very similar graph can be found at  http://www.innovatemotorsports.com/news3.php)

Doesn't make much difference in practice. Just interesting to now have an
explanation for the decrease in meter readings as the firing progresses.