[TR] use of overdrive

[Randall]

> My  later 68 TR250 and 71TR6 
> A-Types OD's are "Slurrers".

That was a deliberate modification by the factory, which I believe was to
lessen the driveline shock on IRS cars.  Considering how many TR6 have
broken where the diff mounts to the frame, it seems like a valid reason.

BTW, using a lighter flywheel also reduces the 'thump' noticeably.  Still
not what I'd call 'slurred', but the difference between a slap and a kick in
the pants.

Randall