Need your advice

Need your advice. Have a 4"inch lifted truck with stock 3.42 gears and installed 33.2 inch tall tire (old tires were 31.6 inches tall) Have done aftermarket parts on my truck like flash pak super chip, airaid intake and body spacer and magna flow exhaust. Truck runs good but amechanic told me I neede to change my gears so my engine would not work harder then what is working right now. What do you think my mileage right now is good for a lifted truck.