Zagreb - Rijeka is 1,5 hour drive. You can drive faster on Croatian highways. Limit is 130 km/h but if you drive 150 km/h you shall have 0 problems with police.
On Croatian highways is 10 % tolerance of speed, which means if you drive 143 km/h you are legal. But you can go faster. The highway police is dealing with 200+ km/h drivers.

Also, there is highway by the Zagreb airport and you have direct connection to Rijeka and Pula, if you want to drive.
But you can also take a flight from Zagreb to Rijeka (which has an airport on island Krk) and Pula.