post a link to the datasheet of this speed indicator
what makes you sure that
????
If it is an anlog input it needs an analog signal and not differerent frequencies
The only thing where frequency come into play is PWM.
Your whole code does
not
PWM
it changes frequency. PWM is something completely different than changing frequency
You are writing in small fragments about different things you seem to not know much about them. This is the worst possible combination for proceeding in any project.
So as a fundamental change
read this and
take it to your heart
Be the change you want to see in the world
best regards Stefan