Discussion Groups / British English / July 2008

The phenomena is not without parallel analogies.

Petrus, Pedro, and Peter. Saltpetre, petrified.

Molly is entirely correct for British and American English,
but I note there is no real discussion of the competing
linguistic factors which may have fuelled AmEng development
in this manner.

But yes, assuming you (OP) have some online access but not,
necessarily, equivalent access to reliable reference sources,
the general rule of thumb is that if it pertains to the SI (Systeme
Internationale, or "Metric") units of measurement then it is "-metre",
but if the pronomial in question is the device which does the
actual measuring then it is "meter".

One may measure a perimeter however one wishes, in inches,
feet, American Miles, Metres, Roman Miles, Imperial Miles,
centiMetres, nanoMetres, picoMetres, TeraMetres, whatever.

Molly may, however, be wrong concerning "tachometer".

Until recently these were, officially, called "tachographs"
and records were kept by way of a stylus marking a disc
rather like a the film memes of seismograph or flatbed plotter.

They now are known as "digital tachos". This is rather more
interesting than the willpower I can summon to look into it
right now this moprning but the American English for "speedometer"
is "odometer" and the nearest etymologically is "tachymeter"
which is actually the rev counter (rev = rpm = revolutions per
minute) for the engine and bears no relation to speed or
distance of motion.

The leaflets I have seen from the British Driver and Vehicle
Licensing Agency (DVLA) and Vehicle and Operator Standards
Agency (VOSA) only refer to tachos and digital tachos, which
is a continuation of the shortforms drivers and operators
themselves use (e.g., I don't ever recall using a tachograph disc,
only ever a tacho disc), so the extension to tachometer, albeit
one I have used myself, may not technically be accurate.

G DAEB
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