My 2011 Hyundai Tucson (2.4 L 4-cylinder, 5-speed manual transmission) runs between 3600 and 3800 RPM when I'm in 5th gear going 70MPH. It feels and sounds too high. How do I calculate the correct RPM? Thanks.
You don't need to calculate the correct RPM. The correct RPM is whatever ratio the gearbox and the wheel size gives you.
However, if you really need to calculate the correct RPM, first you need the wheel size. E.g. 195/65R15 would be pi*(195*0.65*2+15*25.4) = 1993 mm. If you're travelling at 70 MPH, it's 70*1.6093/3.6 = 31.29 meters per second. So, it's 31.29 m/s/1.993 m = 15.7 wheel rotations per second = 942 wheel rotations per minute.
Now you also need the final drive ratio and the gear ratio. Let's assume the gear ratio is 0.8 and the final drive ratio is 4.0. The engine is now rotating at 942*4.0*0.8 = 3014 RPM.
In your case, you said that the engine runs at between 3600 RPM and 3800 RPM. So, in your case, most likely the final drive ratio number or the gear ratio number is higher (or then there was an error in my math).
Or, it is possible that your rev counter is displaying the rotational speed incorrectly.