Here's the follow-up to my previous post. I set trip meter B on the '06 HCH to 0 just before starting out. Here's what the MPG meter estimated:

• Outbond journey -- 507.3 miles, 53.4 MPG
• Start of return journey -- 572.1 miles, 53.1 MPG
• Back home -- 1018.1 miles, 52.4 MPG

Fortunately we only had to use the A/C for the last ~25% of both the inbound & outbound trips, and not at all during our stay. We kept to within 5 mph of the speed limit at all times. I suspect the lower MPG on the return trip was mainly due to heavier traffic -- we left on Saturday and returned on Friday. Also, we hit a patch of heavy rain around Homestead.

I was surprised that 64.8 miles of in-town driving didn't result in a larger MPG drop -- as anyone who has ever been to Key West will confirm, the place is very congested. And road construction where we were staying only made matters worse.