The trick to maximum mileage,is both little throttle and low as possible engine rpm.That translates to Starting out in first,with minimum throttle,shifting to third at 2000rpm,and repeat to fifth at 2000rpm.If possible use a vacumn guage and never let the vac reading go below 5in,that will inforce a lite throttle.Unreccomended is turning the engine off to slow and sit motionless. If you remember that modern engines require a specific amount of fuel,every time the combustion chamber is filled,even at idle,thats no throttle.
"Many cars with manual transmissions have an upshift light which is pretty close to giving the right advice for upshifting."
I was with you 100% until this point. If the light comes on, you have gone past the best shift point. I have one and when ever I see it, I kick myself for bad driving, except when accelerating up hill. Uphill, you have to go a little past the ideal shift point or your speed will drop off too much during the shift. But even then, it just blinks for a second.
The big issue with lugging an aircooled VW is that basically all the cooling comes from the fan that’s run off the engine-- if you’re turning fewer RPM’s, the fan’s not turning as fast either and if the engine’s still working hard, it’ll get too hot. When I first got my old Beetle, I was used to driving my old three-on-the-tree truck and I was definitely lugging it so I spent the first few months trying to figure out why the thing kept overheating until I read the section in John Muir’s VW-bible entitled “How to drive a Volkswagen”.