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How much can wheel size affect RPMs?

Discussion in 'The Hokey Ass Message Board' started by billj, Jul 24, 2011.

  1. billj
    Joined: Nov 1, 2003
    Posts: 184

    billj
    Member

    I went from 4:11 to 3:25 gears on my Shoebox with a 3 speed 1:1 final drive ratio trans. It still revs a little higher than I want at cruisin speed so my question is I have 15" tires that are 65 series low profile radials that I picked up until I can afford some 75 series wide whites radials. Any thoughts on what a taller tire will do to my RPMs? Think it will be a noticable difference?

    Thanks for any thoughts.
     
  2. George/Maine
    Joined: Jan 6, 2011
    Posts: 949

    George/Maine
    Member

    2000RPMs x tires 28"
    ______________ = 51 mph.
    Gears 3.25 x336

    For small tires you need higher gears
    I,m running 2.78 gears with 205/70/15s 2000prm =55 mph.
     
  3. gasheat
    Joined: Nov 7, 2005
    Posts: 713

    gasheat
    Member
    from Dallas

    Find a tire you think you like. Measure the height and plug it into the calculation. This will give you a good idea what you will achieve. Lots of sites have formulas for this type of stuff. Tall tires do make a big difference. Good luck.
     
  4. lakeroadster
    Joined: Nov 6, 2008
    Posts: 604

    lakeroadster
    Member
    from *

    Here is a good chart for comparing tires sizes. The other thing to remember is bigger tires effect your Starting Line Ratio, a.k.a. "launch ratio". Basically bigger diameter tires will make it more difficult to get the car moving.

    I put 33" diameter tires on my '65 C10, it's noticeable - I have to slip the clutch a bit to get it rolling.

    Hope this helps!

    John


    Yellow = Better Gas Mileage /// Green = Near Factory Ratio /// Red = More Power
    [​IMG]

    More info on starting line ratio: http://www.5speedtransmissions.com/startlineratio.html
     

  5. Hnstray
    Joined: Aug 23, 2009
    Posts: 11,743

    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL

    Thanks for posting that chart. Very helpful.

    Ray
     
  6. AnimalAin
    Joined: Jul 20, 2002
    Posts: 3,417

    AnimalAin
    Member

    For what it's worth, the chart above seems to be calculated for 65 mph.
     
  7. squirrel
    Joined: Sep 23, 2004
    Posts: 47,394

    squirrel
    Member

    Making some assumptions...let's say your current tires are 26" tall. The tires you'll be getting are 28.5" tall. That's 2.5" difference. 2.5 is about 10% of 26. So you'll be increasing tire size by 10%, so you'll decrease engine speed by 10% too.

    Assuming you're running about 3000 rpm at 70 mph with the small tires, you'll drop 10% with the big tires. 10% of 3000 is 300, so you'll go down to 2700 rpm.
     
  8. billj
    Joined: Nov 1, 2003
    Posts: 184

    billj
    Member

    As always, this site is very helpful. Thanks for all the info and the chart. I will wait til I get my taller tires and see how that works before considering working on the rear again.
     
  9. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 25,721

    Jalopy Joker
    Member

    search online: project33.com and click on calculators=tires/trans/rear gears/etc figured in
     
  10. Forddraggin
    Joined: Jul 15, 2011
    Posts: 302

    Forddraggin
    Member
    from Lebanon,TN

    Im running a 3 speed and have 3:73 gears i went to a 7.50x16 and it helped alittle but im going to change gears out sometime.
     

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