Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Vehicle's RPM do not match speed. Why?

  1. Feb 23, 2014 #1
    I've been observing cars RPM and noticed that automatic transmission American cars reach 60 mph at 2250 RPM while automatic transmission Japanese cars reach 60mph at 2500RPM.

    Why is that? Like, what determines RPMs? The rotation from what is being measured?
     
  2. jcsd
  3. Feb 23, 2014 #2

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The only thing more surprising the transmissions being different is if they were all the same. I would suppose the automotive engineers select or design a transmission that provides the power transfer ratios that match the engine to be used.
     
  4. Feb 23, 2014 #3

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    The tachometer measures the RPMs of the engine, which will absolutely have a different relationship with the car's speed depending on the engine, transmission, or even size of the tires.
     
  5. Feb 23, 2014 #4

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    The speed at which the engine turns and the speed at which the driving wheels turn is related by the combination of gear ratio in the transmission and the gear ration in the differential. Gear ratios in the differential generally fall in the range of about 2.5:1 to 3.5:1, but they can range as high as 4.5:1 for cars designed for quick acceleration without regard to fuel economy.

    The purpose of the transmission is to keep the engine operating at a band of RPMs where it is most fuel efficient. When the transmission shifts into its drive configuration, the gear ratio thru the transmission is 1:1, but with overdrive transmissions, this ratio could be as low as 0.8:1. Different cars have different engines, transmissions, and final drive gear ratios, which is why the engines do not turn at the same RPM for the same road speed.
     
  6. Feb 24, 2014 #5

    cjl

    User Avatar

    There's probably quite a bit more spread even than that - some cars might be doing less than 2000rpm at 60mph (especially high performance cars with big engines), while others might be doing over 3000rpm at 60 (cars with relatively small engines for example). It's not really a matter of where the car was made/designed, it's more just a design tradeoff depending on the engine and the desired characteristics of the car.
     
  7. Feb 25, 2014 #6
    Thanks, this clears it up.

    < than 2000 at 60??! I would love to have a vehicle like that. :!!)
     
  8. Feb 25, 2014 #7

    russ_watters

    User Avatar

    Staff: Mentor

    Get a diesel or a truck. Heck, my Kia Optima is at 2000 rpm at 70 mph. Not sure why you would care, though; it doesn't really mean anything.
     
  9. Feb 25, 2014 #8
    I should do that then. I'm assuming less RPM means more silence, but I may be wrong. It does mean something to me if it makes less sound. I like silent vehicles.
     
  10. Feb 25, 2014 #9
    You certainly don't want a diesel then.
    Quiet = refinement = expense.
    Turn the radio up and have done.

    Also depends what you mean by quiet, at 60mph cruise combustion noise has a decent amount of masking from road and wind noise.
     
    Last edited: Feb 25, 2014
  11. Feb 25, 2014 #10

    rcgldr

    User Avatar
    Homework Helper

    2006 -> 2013 Corvette Z06 7.0 liter engine - 1500 rpm at 70 mph in 6th gear. Dodge Viper, 8.0+ liter engines, maybe even lower. Don't know about the 2014 Corvettes, 6.2 liter engine, and now 7 speed manual tranny. There will be a 2015 Corvette Z06, but it will be more like the previous ZR1, using a 600+ hp supercharged 6.2 liter engine.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Vehicle's RPM do not match speed. Why?
  1. Why do flags flutter (Replies: 11)

Loading...