The difference?

["jonmac" <jonmac@ndirect.co.uk>]

>Dave's done 110 in Nigel (And had it up to 9500 rpms.)

It's been a long time since I was confused by hood/bonnet, wing/fender,
trunk/boot blah blah - but ...
why is rpm trans-Atlantic pluralised? Revs or revolutions per minute means
rpm, so why does another 's' go on the end of rpm? This issue is causing me
a considerable loss of sleep.

John Mac