Hey I have a 1997 f150

I put new tires on it and from what I can figure I need a 15 tooth speedometer gear to correct the speedometer but I can't find one.

All I can find is a 16 tooth.

How much will this leave my speedometer off?

My tires are 305 70 16 the rear end is 3.08 the factory tires where 235 70 16

I also am trying to figure out what it will do to my odometer?

Thanks.