u/warrior-of-ice

I have been driving a 1.5L 6spd Honda Fit for a couple of years now, mostly highway to work. I go early so I drive calmly and listen mostly to podcasts. Recently I started listening to heavy metal in the car after i installed aftermarket speakers, and i realise that i usually forget to shift and would drive highway speed at gear 4 instead of 6, or i would go 40mph at gear 2 in certain sections of the road where I’m going downhill. This also happens if i open the windows and the wind is loud enough to mask the engine noise.
Car has a small engine rev gauge so not that easy to see sometimes.
Am I stuck with windows up and podcast/calm music only or is there a solution?
Thanks all!