Speedometer Pinion Gear

Dana67Dart said

If GPS says you are going 60 but your speedometer says your going 50, your speedometer is 16.67 percent slow. If your current gear is 45 teeth you need a gear smaller by 16.67 percent which is 37.5 teeth so your options are 37 or 38 teeth, no tire size needed.

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I'm thinking that will confuse the issue more since the poster has the opposite issue and different speeds.
His speedo says 40mph, the gps says 30mph.
Plus he doesn't know the tooth count on the current gear.

So, since he does know his tire height, and his rear end ratio, why not just use the calculator and get the right part the first time. These are a little pricey at around 44 bucks each.