First thing you need to do is determine if your speedo inaccuracy is linear or not.

If 35 indicated is 48 actual,
is 25 indicated 34 actual?
is 30 indicated 41 actual?
is 40 indicated 55 actual?

If so, then a gear ratio change can fix the problem. Either put a different tooth count speedo gear into the transmission or buy an adapter.

If the actual speeds vs. indicated speeds are not linear as I posted above, changing the gear ratio will only "fix" the speedometer at one speed. You'll need to get someone to repair the speedometer first. Then decide if the gearing needs changing.

Also, I'm pretty sure your thoughts about changing to a lower number of teeth is wrong. You want to slow down the rotational speed of the cable. To do that you need to change to speedo cable gear with MORE teeth, not less.

'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.