I think the fuel light comes on at around 63-64 litres, so I calculate I did close to 37mph. I'd done nearly 560 miles before I refilled! Not sure I could fare as well in an automatic, which is a pity as I'm planning to swap mine for an IS 300 sportcross... just waiting for the right one to come up for sale.
Anyway, it's a very pleasant car for a long journey, lovely sound system, although the old sat nav disk seemed to think I was driving through fields on some occasions. Not sure it justifies an upgrade disk though. An update every 10 years is probably adequate for Britain's roads.