Speedometer jumping around help

yeah, car is a '76, standard dash. tires are a little bigger than stock out back and brand new (originally came with D78-14, 25.5" tall, went to 225/70R14, 26.4" tall), so I'd expect it to be a bit low (like the odometer is acting)

if I change the speedo pinion gear to get that "correct", then I assume it will throw the odometer off even worse, since they're both driven off that gear. any advice on what to lube the speedometer with?