I drove from Toronto to San Diego over the last 4 days and I noticed the following changes in my car:

- the engine sound at high speed began to change (it got higher ptiched)

- the engine wasn't 'pulling' like it used to especially up hill

- the engine has to reach a higher RPM than normal to reach the same speed as before.

I think that the valves have to be adjusted (tappet adjustment) but I don't know. I was driving at around 80 mph for like 5 hour streches.

Any ideas?