As I understand it, the biggest threat to operating a transmission at low engine RPM and in high gear is if you regularly apply a pile of torque to accelerate.

But I doubt the engines we use are capable of supplying damaging torque in that situation.

I do know a guy who had to replace 5th gear in his 5 liter Mustang though, and the rebuilder blamed it on hard acceleration in 5th.

I'm pretty certain coasting in 5th gear at 1000rpm is absolutely fine for the car/tranny/clutch/engine etc.... It doesn't lug, it's not a problem. I think if you are not running your VX below 1400RPM you are not tapping the potential of the car. Sure I'm a noob to it, but still, 1400rpm is high for this car. I shift at 1400rpm.

I'd say coast in neutral unless you need to slow down. Being in 5th will slow you down if it is in fuel cut mode (above roughly 1200rpm). If you are at 1000rpm, it should go farther in 5th, but it will be injecting a bit (probably more than idle) of fuel to do so.
thats my guess.

my car goes faster in 5th gear coasting down my hill than it does in neutral. In neutral it gets up to 42 or 43mph. In 5th it gets up to 45. Strange.

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