Speedometer gear help

[DanEC]

I used a GPS app to check out my speedometer and it appears to be off even more than I suspected. It appears that 40 speedo indicated is actually about 46 mph and 50 speedo indicated is about 58 mph - roughly 15% off. Some of this may be due to the 255-60/15 tires I run in the rear but the difference compared to a 295-50/15 is only about 1.5% based on circumference. With a TL and 3.31 rear the speedometer calculators I checked all say I need a 17 tooth gear with the 7 tooth drive gear.

I didn't count the teeth on the gear ERA provided but based on the 3.31 rear I figure it was around a 16 tooth for a 295-50 tire. Nothing on my order indicates the drive gear I was provided.

I can't see the difference between a 16 tooth gear and a 17 tooth gear to account for a 15% correction. If I have a 15 tooth gear by some chance the math seems to work out fairly close.

I doubt many of you have 60 series tires but does a 17 tooth gear sound about right for a TL with 3.31 rear and a 27 inch tire?