Peakster, if you're getting EPA mileage @ 55/60 mph in steady speed driving, you're doing about as well as you can expect from a regular Metro. There's nothing wrong with your car.
I've owned 2 Fireflies, and they are
without question the most optimistically EPA / NRCAN rated cars I've had (had lots). Check my speed vs. mpg graph here for the story in a nutshell:
http://www.metrompg.com/posts/speed-vs-mpg.htm
The only reason I'm getting ridiculous numbers with my current car is because (1) I don't drive in real-world (ie average) conditions, and (2) I use hard-core hypermiler techniques (3) on top of numerous tested mods.
The engineers either tried awfully hard to spec the cars to excel in the government test cycles, or they just got lucky.
The Metros will not excel at highway efficiency without extensive aero mods, through which you may be able to gain a 15-18% FE improvement at speeds around 55 mph.
They have excellent potential in low/medium speed pulse and glide/ CODFISHing due to the low weight and low engine power.
Seriously though: get a scangauge. Watch eBay. The used SG1's are going for around $75 US these days. It'll pay for itself, and you can bring it with you to your next vehicle.