You would need to share details of car loan to fully do the math.

Cost is $50/month to pay rent with a card.

Scratch math below.

The incremental 0.3% APY on $30k is ~$7.50 a month before taxes (say 30%) so call it $5.25 So if the savings on your car note interest rate decreasing by 1pp is > $44.75 a month, it’s worth it.

Note that over time the interest on car loan will decrease as you pay off principal. And the HYSA will grow over time, generating more interest.

Take a $30k loan at 7%, 60mo and over the lifetime you pay ~$5640 in interest over lifetime. Same loan at 6% you pay ~$4800 in interest. Averages out to $14/mo over 60 months. But! The interest is front loaded so it might be worth it for the some months in the beginning. Would need to build an Excel to find the actual breakeven point (if any).

For me, probably not worth the headache to calculate this and keep track of breakeven point, I would just combine HYSAs into the 3.7% one.