98 blazer questions on tire size

I have a 98 blazer with a 2'' lift and 31" tires. Since the tires are bigger, that means the odometer is going to read off? How much will it read off from stock. Secondly i have a mildly heavy foot, and have posted 230 miles off of 15 gallons(before compensating for the larger tires) i do an equal amount of highway and city driving. Is that within normal range. Lastly will installing a new cat, muffler system help or hurt fuel economy?

should be around 5mph on the slow side ( showing 50 mph on the speedo, but doing about 55mph ) you will lose mpg, it wasn't designed for those tires, a new cat or free breathing exhaust may help a bit
hth
Rick, Ore

Speedo will be just a bit on the slow side. If your truck has the std 3.42 gears it has to work harder to turn those tires. ZR2's use 31in tires from the factory and have 3.73's to help overcome that issue. Don't replace the cat unless your original one is rattling or plugged. A new high flow one makes a small bit of difference in sound, thats about it. A performance exhaust with a larger pipe size (2.5in is what most use) and a nice flowing muffler may help fuel economy slightly depending on how you drive. I only saw a gain on the highway since most of my driving is city.