How to recalibrate your speedometer after changing tire sizes or gears

Most people would say 27 because 31" tires on the chart are normally close to that. I'd personally never change a gear without pulling what I have and counting the teeth, measuring how far off the speedometer is now with GPS and then doing the math.
 
Most people would say 27 because 31" tires on the chart are normally close to that. I'd personally never change a gear without pulling what I have and counting the teeth, measuring how far off the speedometer is now with GPS and then doing the math.
I know its wrong based on my mileage and speedometer vs speed on navigation apps. It used to be off by around 10% and then I got these tires and its more like 5%. I just figured I might as well do it since I'm going to change the fluid in the case soon.
 
I know its wrong based on my mileage and speedometer vs speed on navigation apps. It used to be off by around 10% and then I got these tires and its more like 5%. I just figured I might as well do it since I'm going to change the fluid in the case soon.
Keep in mind that the difference between 27 and 28 is about 3.5% so you're not going to get it exact. I think the fluid level is below the gear sitting level so it's not going to spill out or anything. If you're within a five percent already you really should find out what you have, those gears aren't cheap. It's probably stock but that's allot of work only to overshoot the wrong direction 5%.
 
What’s the consensus on rounding up or down? I went from stock 3.73 on 215/75/15 to KO2s on 285/70/17 and can’t figure out if I need a 31 or 32. I’d rather be doing 60 and have it display 65 than other way around.
 
What’s the consensus on rounding up or down? I went from stock 3.73 on 215/75/15 to KO2s on 285/70/17 and can’t figure out if I need a 31 or 32. I’d rather be doing 60 and have it display 65 than other way around.
Haven’t you just answered your own question?
 
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What’s the consensus on rounding up or down? I went from stock 3.73 on 215/75/15 to KO2s on 285/70/17 and can’t figure out if I need a 31 or 32. I’d rather be doing 60 and have it display 65 than other way around.
If I'm thinking about this the correct way, wouldn't that then mean your odometer would be going higher faster?? In a 2-hour trip, you'd then have 10 extra miles on the odometer that you never really drove. Then your TJ looks older than it really is ;-)
 
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What’s the consensus on rounding up or down? I went from stock 3.73 on 215/75/15 to KO2s on 285/70/17 and can’t figure out if I need a 31 or 32. I’d rather be doing 60 and have it display 65 than other way around.
More speedo gear teeth will slow the reading on the gauge down. So you would probably want to err on the 32 tooth side of things. Plus as the tires wear down your speedo will become more accurate.
 
1 tooth is roughly 2-3 mph at highway speed so it would be close either way. Maybe more accurate on 32 over time but no big deal. Talking minimal difference long term.
Agreed, 285/70/17 calculates out at 32.7 I think I was good either way and close enough based on tire wear and general margin of error.
 
If I'm thinking about this the correct way, wouldn't that then mean your odometer would be going higher faster?? In a 2-hour trip, you'd then have 10 extra miles on the odometer that you never really drove. Then your TJ looks older than it really is ;-)
You are correct, but the other way has potential for speeding tickets. From the factory most vehicles are calibrated to read about 2mph higher than actual on the highway. So technically most all vehicles have more miles on the clock than they actually drove.
 
I have 2000 TJ with the 2.5 and 5 speed manual - I upgraded to some newer jeep wrangler take offs (245 75 r17 - alittle over 31 inch) and when testing gps speed, speedo shows 55 and gps says about 61-62, per the chart (think I have the stock 410 gear ratio) - I think I need the 37 tooth - does this sound correct ? Thanks
 
I just changed out my speedo tooth gear from the stock one to a 36 tooth on my 2002 TJ thanks to this forum. I have the 4.0, 32RH auto trans, 4.10 gears and 285/70/17 M/T . My speedometer is faster than GPS by 1 to 1.5 MPH so I'm happy. I love the wealth of knowledge that resides here! Check this chart out: https://www.quadratec.com/c/reference/jeep-speedometer-gear-tooth-chart
 
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Is this still accurate? 4.56 with 35s 37 tooth? PO put in 35 tooth and I'm about 4mph off
97 Sport 5speed
 
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It'll get you close which is good enough for most people. If you are thinking of getting a new gear to get closer I think it's better to do the math. A GPS reading at highway speed and a reading from your speedometer, divide and multiply a few numbers and you know what gear to buy. Tires vary allot, especially 35s, so you can be off a bit using the chart.
 
Excellent info but I must be missing something or just not smart enough to catch on. Bought an ‘02 TJ that has 33’s on it. Build ratio and the tag both say 3.07 however that’s just a dash on your chart. What am I missing?
 
Excellent info but I must be missing something or just not smart enough to catch on. Bought an ‘02 TJ that has 33’s on it. Build ratio and the tag both say 3.07 however that’s just a dash on your chart. What am I missing?

Likely re-geared when adding the 33s? Might need to check for yourself the actual ratio and see what gear is in it now
 
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Admittedly I have not checked speed with GPS but I’m fairly certain it’s not correct at highway speeds.
 
Excellent info but I must be missing something or just not smart enough to catch on. Bought an ‘02 TJ that has 33’s on it. Build ratio and the tag both say 3.07 however that’s just a dash on your chart. What am I missing?

It’s because you need a 25 tooth and that doesn’t exist (as far as I know). A 26 would get you close. Or you can get an electronic correction device that can get it to 100% perfect using the existing gear.
 
Admittedly I have not checked speed with GPS but I’m fairly certain it’s not correct at highway speeds.

Going from 27" tires to 33s is an increase of 1.222x. The same increase (numerically) in gears would put you at 3.75. You SHOULD have 3.73s in there, or something like that. Someone correct me if I'm wrong