ive got RPM problems
 

Ive got a '95 Ford Probe 5 speed. its a 2.5L V6. when im doing 100 km/h = 60 mph my car is reving at 3000 RPMs, in 5th gear. is there any way to lower my RPMs? Could this be hurtin my engine or tranny?

Im not sure which trans your V6 has but I know the G-series mazda trans has several 5th gears available. I swapped a .71 5th from a Mazda 626 in place of my .8 5th in my Mercury Capri trans (G25MR). It dropped the rpms at 60 about 300-400. Cruises 60 at about 2500 now.

Larger diameter tires will change your final drive ratio. the only problem is that it changes them all, so slower acceleration could be a downside.