These days, my 2000 Camry averages around 10.2 l/100 km (= 4.35 gal/100 mi = 23 mpg) doing normal city driving.
A couple of days ago, I was road tripping up to Florence (464 km/266 miles round trip), where the official speed limit is 70 mph (for most of the way), but most people drive 80-85 mph. I usually set the cruise control at 75 mph. Fuel economy on that trip came out to 9.45 l/100 km (= 4.02 gal/100 mi ~25 mpg), just slightly better (~7.6%) than what I get tooling around the city.
Contrast that to a road trip out to Walterboro (157 km/98 miles round trip) earlier in the week, where the official speed limit is a more sedate 60 mph with most of the traffic going around 65-70 mph (I had the cruise set to 65 mph for most of the trip). On that trip, the Camry managed to get 7.7 l/100 km (= 3.28 gal/100 mi = 30.5 mpg) or nearly 25% better than my normal city driving.
On top of that, to go 65 mph, the engine has to run at around 2300 rpm. To go 75 mph, the engine has to run at about 2700 rpm (17% faster) which means that much more wear and tear on the engine.
At 65 mph, a 100 mile trip should take 1 hour 32 minutes. At 75 mph, you finish the trip 12 minutes faster but use up 25% more gas to do it (if you're in my car).
Just a few things to think about.