On Thursday, 17 September 2020 09:24:21 UTC+1, Dave Liquorice wrote:
On Thu, 17 Sep 2020 08:58:24 +0100, Martin Brown wrote:

The optimum cruise speed for a particular vehicle shape can be
determined experimentally if you do a regular particular run and have a
trip computer. For my car it is about 55mph.

But varies by gear. In mine for sixth ("top") an indicated 61 returns
the highest mpg. However around 50 in fith returns an even higher
MPG. Trouble is 50 on a motorway is seriously tedious and
concentration is difficult to maintain. Is there still 30+ miles of
average speed cameras set to 50 on the M6 south of Stoke for smart
motorway work?

There is quite a long bit of 50 mph on the M4 (Port Talbot and west).

I am OK with that when I can set adaptive cruise control and not have to keep checking the speedo.

Noticeably high mpg on that section. As it is auto, gear changes are rare - usually goes to 7 and stays there.