Wow ... 42 mpg, that's hard to believe since these cars weren't rated anywhere close to that for even hiway mileage. Based on the info below, it looks like 30 mpg was the hiway rating for the 95 models. I'd suspect your calculation that yielded 42 mpg is off for some reason.
Nissan Altima Gas Mileage
My 94 model gets around 26 mpg in 70% hiway, 30% city driving, so I would imagine it would be about 30 mpg on pure hiway just as the rating shows.
I'd suspect the O2 sensor or the coolant temperature sensor. Also, the throttle position sensor could be off or bad which could cause it to run richer than it should.