Speedometer Driven Gear Help

[murguia]

So I replaced my stock 3 speed with a TH350 and my speedometer is way off. It climbs pretty quickly initially, but when I'm cruising 60/70/80... it tends to be off by about 18-20 mph

I removed the driven gear from my old 3 speed but its much smaller in diameter compared to the one in my new 350. This one is an orange/pink color with 35 teeth.

I tried to do the TCI conversion but got a little confused, so hoping you can all point me in the right direction, so I can buy a new gear to get me as accurate as possible.

235 75 15 tires
3.73 gears
TH350

Thank you