Is RPM Running too high? - Motor Vehicle Maintenance & Repair Stack Exchange most recent 30 from mechanics.stackexchange.com 2019-07-20T01:14:25Z https://mechanics.stackexchange.com/feeds/question/42978 http://www.creativecommons.org/licenses/by-sa/3.0/rdf https://mechanics.stackexchange.com/q/42978 1 Is RPM Running too high? user26580 https://mechanics.stackexchange.com/users/26580 2017-03-20T05:31:29Z 2017-03-20T05:51:19Z <p>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.</p> https://mechanics.stackexchange.com/questions/42978/-/42979#42979 1 Answer by juhist for Is RPM Running too high? juhist https://mechanics.stackexchange.com/users/13825 2017-03-20T05:51:19Z 2017-03-20T05:51:19Z <p>You don't need to calculate the correct RPM. The correct RPM is whatever ratio the gearbox and the wheel size gives you.</p> <p>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.</p> <p>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.</p> <p>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).</p> <p>Or, it is possible that your rev counter is displaying the rotational speed incorrectly.</p>