1964 4 spd speedometer gears

1964 Belvedere with a original 64 type 4 spd,4.10 gears with a 28" tire.Speedometer is way off.What do i need to do to make the speedometer accurate?

A miracle. Seriously, they didn't make a gear to go with that ratio, you need an adapter. I've heard Imperial Services makes one, but can't swear to it. Here's a chart of tire sizes vs rear ratio, this will let you calculate the adapter ratio.
Mike

I couldn't find the right gear for my pushbutton 63 trans so ended up with a GPS heads up speedometer that puts the speed on the windshield. It was half the price of the gear adapter which would only be good for that tire and gear.

Jim

Steve at Imperial Services makes an adapter that changes the ratio.They are pricey.When I had the 4.88s in my 65 the speedo would show 80 in first.I have 3,91s now and know that the speedo is off by about 20 %.I just go by that.Rocky