ok, so I have an SS350 with the TH350 Trans.... the car had a 120 Speedo, the used Gauge Carrier I bought came with a factory 140. now my speedo is off. I see alot of avail gears with 17 tooth up to 23 tooth ect. how in the hell do I know what to go with? Or is going from a 120 to a 140 a common enough swap that someone possibly knows which replacement gear I need? any help is appreciated.