The Natural Philosopher wrote:

Wasn't there an earlier side valve A series?

There was a 748cc sidevalve engine used in the pre war Austin 7 but it
wasn't an A series - it had a two bearing very spindly looking crank
as well. A friend drove his to Le Mans the other year and it never
missed a beat while much more exotic machinery keeled over in the
heat.

You could certainly get 40-50mpg out of some Minis.

I got around low 40's out of my 1000 - but it was always driven flat
out. Seeing the speedo hit the zero stop from the wrong side down a
very long hill was quite amusing ;-)

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