4:10 gears will make a huge improvement in the acceleration of your car. You will need to correct your cars speedometer to keep it correct. We charge $400.00 plus the parts, gears are usually $165.00, install kit with new bearing is $80.00, plus $15.00 in gear lube. The $400.00 price we charge includes hooking up our LS1edit to your car and correcting the speedo for the gear change.
Most people don't see any reduction in gas mileage if they primarily do city driving. The car doesn't take as much throttle to get going, and you can go into the next gear sooner. Hwy driving will drop one or two miles per gal on average.
I wouldn't put sticky tires on your car with the tiny 7.5" 10 bolt rear, as they are known to break even when the cars are bone stock, especially with a six speed such as yours. If you don't dump the clutch coming out of the hole on a regular basis, you may not have any problems. Bob