odometer off by how much???
03-01-2009, 04:04 PM
#21
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You should get a bit more basic. Find a piece of fairly straight road , set your odometer to zero at a mile marker and run for 10 miles to get a rough estimate of how far your odometer (and speedometer) is off as a %. My Rubicon is off by a bit less than 6% on 315/70/17 tires and stock 4.10 gears.
03-01-2009, 04:06 PM
#22
JK Jedi
Thread Starter
ya, that would work. I think it is probably true that wheel sensors tell vehicle speed now. Which would have no relation to gear ratios. I doubt it uses them all for it though. It likely uses one off of a tone ring or whatever. Maybe it uses them all, who knows. I bet my jeep is wondering why my RPMs are so high and my speed so low. lol. I prolly need to save up for a procal, and get it comming. So much to buy, so little moneys.
03-01-2009, 05:28 PM
#24
JK Jedi
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Im J/K man. Thanks everyone.
03-01-2009, 07:00 PM
#26
JK Jedi
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03-02-2009, 03:33 AM
#27
JK Jedi Master
You're not alone in that. Before making my post with the question, I did a lot of reading in the service manual and the parts breakdown. The service manual says that the odometer (et al) receives its information from "the Vehicle Speed Sensor (VSS)." But, searching around the parts manual, I could only find speed sensors at the wheels. Somewhere on the web I found a reference to the JK's speed sensor on the transfer case extension housing, so I double-checked the parts breakdown, the R&R procedures, and the wiring diagrams, and there is no mention of a speed sensor on the transfer case. But, there was a thread here a year ago in which someone stated that he'd called the dealership service dept in his area and they say that the speed sensor is on the transfer case. I'm so confused. 
03-02-2009, 08:28 AM
#28
JK Jedi
Thread Starter
Mike is on the right track, I think. I know the mygig uses canbus data for speed info, and it takes an adapter to get vss from the canbus for an aftermarket radio. I bet is picks up vehicle from an abs tone ring or something on a wheel(s) and then processes it. There isn't a VSS in the transfer like the old days. I wish it was like the old TJs where a $7 speedo gear is all you need to compensate for tires/gears. Mike was right I am sure, that gear ratio will not effect speedo. gear ratio on these new jeeps only effects RPM. I bet I would get way better MPG if I adjust everything.
03-02-2009, 09:20 AM
#29
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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.
03-02-2009, 09:22 AM
#30
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I just changed my gears from 4.10s to 5.13s and I need to know how much my odometer is off. The computer is programmed for the 32" tire 4.10 gears combo, and my current setup is 35" tires and 5.13s. I plan to get the procal... BUT right now I can't (full time college student on a super tight budget), and I need to know my true mileage so that I can keep up my preventative maintenance. Thanks for the help!!!