Your mechanic is right, but it is not really worth it just for 31" tires, considering it's going to run you $1000-1200+.
With 31's the difference over stock tires isn't that great. People run 33" tires with stock gears/4 cyl/5 spd all the time. With 33's you're on the edge of needing to regear.
With 31's the difference over stock tires isn't that great. People run 33" tires with stock gears/4 cyl/5 spd all the time. With 33's you're on the edge of needing to regear.