Need the correct speedo gear

I have a 1991 5.0 with a T-5 trans (factory),3:73 gears, rear tire size of 255/40/17. From my knowledge it already has a 23 tooth speedo gear but the MPH is way off (reads much higher per Sneecer). I will have to pull the speedo gear out and confirm that it's indeed a 23 tooth unit.

Can anyone please comment on my tire size and which speedo gear I should go with?

Thanks,
Eric

sweet - thank you!