Rev limiter

@lkjgmggWhat do you mean by "hitting the rev limiter"? Are you able to rev all the way up to the purple lights in 8th gear as in:
🟢🟢🟡🟡🟣🟣
and reaching 351kph?

Or you're not able to rev so high, ending up with:
🟢🟢🟡⚪⚪⚪
But still reaching 351kph as you do so?

If the former, that's the max speed for your engine indeed.

If the latter, you're not hitting the "rev limiter". Your engine can rev higher, reaching a higher engine speed (RPM), but it cannot produce enough power to overcome drag. Drag increases with the square of speed, becoming exponentially harder to accelerate as you hit those high numbers. You can only aim at a more efficient aero package at this point – lift-to-drag or downforce-to-drag ratio, as in more downforce points with less corresponding drag.

Edit: Also bear in mind that engine speed (RPM) and wheel speed (your car speed) are linked. For any given gear and RPM, there's a corresponding speed. If you're, say, at 11000rpm in 7th gear and 307kph in your current engine spec, you'll always reach 307kph when you're at 11k rpm in 7th gear and vice versa.

Meaning unless you upgrade the PU, your speed will always be the same once you hit the rev limiter.

It's practically a single rod connecting the PU to the wheels. Once the engine cannot spin faster, the wheels will also not be able to spin any faster.