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  • Speedometer and Large Tires

    I'm 16, dont know to much about Jeeps. my cj has 31" tires and my dad told me becasue of the larger tires the speedometer is inaccurate. He says i should add on about 10 mph to the speed. but i've been driving it and i think he's wrong. I'm sure that there is a difference but only a small amount. If anyone knows what im actually doing please let me know, and if there is a way to correct this. Thanks.
    Last edited by CJ71985; 06-18-2005, 02:28 PM.

  • #2

    my cj has 31's and it's off about 5 mph.
    that's using the radars on the side of the road, if they're accurate.



    • #3
      I have 31''s, on my cj, and it throws of the speedometer by 10%.


      • #4
        Can't you get a different speedo drive gear to compensate?Mine had 33's on when I bought it but the speedo is dead on. DP


        • #5
          You should be able to source a different drive gear that goes on the end of your speedo cable to compensate for your tires.

          In order to determine what your actual speed is vs your indicated speed, try this link:
          You will find a table that should help you figure out what you actual speed differential is.

          You will need your axle ratios and the actual diameter of your tires (not what it says on the sidewall). A 31" tire will be closer to 30". A tape measure will tell you the best. Measure from side to side, not top to bottom.

          There are a ton of conversion equasions out there. If you are into the mathmatics of speed, check this out:
          There are several links there to figure out all kinds of cool stuff!


          • #6
            I checked mine with GPS. My speedo was dead on nutz with 33" tires. Course thats about to change when I swap out 3.31s for 4.56!


            • #7
              To get the proper speedo gear size, you will need to know your gear ratio of your axle. The site has a speedo gear calculator in the tech info section.


              Go here to get your axle ratio if you do not know.