odometer off by how much???
JK Jedi Master
Joined: Mar 2007
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From: Midwest City, OK
(( 5.13-4.10)/4.10/Pi) * 50) + 50 = 54.00 mph
Which suggests that the reason that the Accupro, Hypertech and AEV programmers have a setting for gear ratio is because gear ratio does affect the speedometer, odometer et al.
JK Enthusiast
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From: Florida Space Coast...but I get around
(( 5.13-4.10)/4.10/Pi) * 50) + 50 = 54.00 mph. Sorry but I don't agree. By using your formula...if someone put on 42" meats BUT didn't change their gearing, the speedo would be correct? NOT! (and in real life I'm not loud) 
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JK Jedi
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From: texarkana,tx
If you take any wheel, brake caliper and disc/drum off of the rig, what you'll see on the outside of the axle, before the wheel mounting studs is a circular "gear" with square cut "teeth". I'd guess there's probably 100 "teeth". In very close proximity to this "gear" is what I think is a magnetic pick-up. This set-up is the wheel/vehicle speed sensor. It allows the computer to monitor each wheel's speed so it determines when, for instance, 1 wheel is slipping (going faster than the other side/other end) and will apply the BAS or ESP as it's programmed to do. I certainly don't claim to be an automotive engineer but this seems to be intuitively correct.
JK Enthusiast
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From: Florida Space Coast...but I get around
that is the "abs tone ring" isnt it? is that not what reports everything to the computer, or is there another sensor? with the tj and older jeeps, if you had say 5.13s and 37s it would be the same speedo as 32s and 4.10s. because the sensor was in the t-case. now it is at the wheels, so it makes sense that gearing will not affect it.
and program for the tire size (rolling circumference) to determine road speed. I really
can't see an easier (with the way it's set-up) way to do it.
JMOpinion
JK Enthusiast
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From: Roanoke, Va
The final gear ratio has NOTHING to do with whether your speedo is correct or not. The speed sensors at each wheel feed info to the computer as the axles rotate. The computer, "knowing" that the tires are 32" diameter calculates the speed per tire rotations. 1 rotation of a 32" tire covers 100.528". 1 rotation of a 35" tire covers 109.53". So the computer is giving the wrong speedometer speed 'cause it "knows" how far a 32" tire travels in 1 rotation but the 35" is going further. Your actual speed is higher than the speedometer says and your traveling further than the odometer says. In this example that difference is 9.375%.
JK Newbie
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From: Hokes Bluff, Alabama
I recently installed 35" tires, and swapped my stock 4:10 gears to 5:13's. According to my GPS, my speedometer reads exactly 3 mph less than my GPS. I checked the speedo to the GPS before the install; they were dead on. This makes sense because I have just a little more pep now than I did with the stock set up.
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JK Jedi
Joined: Apr 2008
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From: texarkana,tx
I recently installed 35" tires, and swapped my stock 4:10 gears to 5:13's. According to my GPS, my speedometer reads exactly 3 mph less than my GPS. I checked the speedo to the GPS before the install; they were dead on. This makes sense because I have just a little more pep now than I did with the stock set up.
JK Enthusiast
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From: Florida Space Coast...but I get around
JK Jedi Master
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From: Midwest City, OK
Last edited by Mark Doiron; Mar 4, 2009 at 02:43 AM.