If they were the EXACT same size, you shouldn't have had a problem...however, even the difference in size between a WORN tire and a NEW tire, even that little bit of circumference difference, from...

16.5 sounds low to me.

We have a '99 XC, synthetic lubricants throughout, 68k miles, 1/3rd city 2/3rds highway (but at 75mph+ here in New Mexico, not exactly a fuel efficient speed), altitude...