Speedo ratio Math wizz needed - JeepForum.com

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post #1 of 3 Old 09-04-2013, 07:48 PM Thread Starter
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1997 XJ Cherokee 
Join Date: Oct 2010
Location: Coquitlam
Posts: 121
Speedo ratio Math wizz needed

Hi, having trouble calculating my MPG due to larger rubber and stock gears.

1997 XJ auto I6, 3:55 gears 265/75R16. Can anyone tell me how many Ks or miles my speedo is not registering over a 100 kilometer or mile drive?

I was going to swap out the gears in the transfer case for the speedo just not sure if I really need to with those tires?

Thanks in advanced for your time. you`ll probably save me 2 hours of staring at the screen with a confused look, cheers

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post #2 of 3 Old 09-04-2013, 08:17 PM Thread Starter
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1997 XJ Cherokee 
Join Date: Oct 2010
Location: Coquitlam
Posts: 121
Found this super awesome site but still can`t find my answer..

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post #3 of 3 Old 09-06-2013, 01:21 PM
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1998 TJ Wrangler 
Join Date: Apr 2004
Posts: 26,172
Determine actual speed vs. speed displayed on speedometer.

Whatever percentage your speedometer is off is the same percentage your odometer is off.

For example, say your speedo shows 60 MPH, actual speed is 55 and you traveled 100 miles per your odometer:
55 divided by 60 = 0.916666667

0.916666667 multiplied by 100 miles per odometer = 91.6666667 actual miles traveled.

Another way to explain the calculation using the same 60 MPH speedo reading, actual speed is 55 MPH, and odometer shows 100 miles traveled:

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