Speedometer Gear

I am looking for a speedometer gear for an old cable shift, push button, 904 trans. The current gear is a 17 tooth. I have been told that I need a 15 tooth.

I went from 6.50/13" tires to 185/70R13" which is supposed to be a direct replacement tire but me speedo is at least 10 mph off. My R&P ratio is 2.93, The new tire diameter is 23.7"

The gear is the push on type, held on by a clip. I hope someone can help me as I can't find any gears NEW.

I don't get on here very often so PM me and I will get back with you.

Thanks, Bill