RPMs are just being weird.
So I drive a Chevy HHR and today the rpms were just being
really odd, in neutral it could rev up to 4k, but in drive it
would go up to say 3500, switch to second, and then I cant
get it over 3k. Struggles up hills, and hard to accelerate
past like 50 unless going downhill
Most likely an exhaust restriction, like a clogged catalytic converter. I'm guessing you will soon have a check engine light on due to the O2 sensor.