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Discussion Starter · #1 ·
I was wondering. If bigger tires affect ones speedometer wouldn't it also affect the number of miles driven displayed on the odometer? Just upgraded some parts on my 2002 I6 4.0 manual and this crossed my mind.
 

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Discussion Starter · #3 ·
So It would display Less miles than I actually traveled. Right? Since my tires are traveling a greater distance per revolution.
 

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Re-gearing does too.

You can buy new gears for the speedometer cable to adjust for different tires and gearing.
 

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Discussion Starter · #6 ·
Cool. So how would I correct it? Will adjusting the speedometer do the trick or will I need to get the odometer adjusted as well. I've got 31s on now so I know that re speedo is off by 4-6 mph. Looks like I need to apply some my fancy physics to see just how far my tires actually carry me........ Or just use my TomTom gps! Lol
 

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Yes, figured this out using my GPS to get somewhere. Mine is off by about 5 MPH when I'm traveling at 55 MPH. So spedo says 55 and GPS says 60.
 

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Yes, figured this out using my GPS to get somewhere. Mine is off by about 5 MPH when I'm traveling at 55 MPH. So spedo says 55 and GPS says 60.
Yep...it also causes lots of confusion when calculating MPG by hand. Typically one just divides the miles driven by the gas used. With the speedometer being off by the bigger diameter tires, the MPG will calculate low. So ya need to add the difference back in. For instance, if the calculation is off by 12% (speedo vs actual)...add 12% of the miles traveled back into your MPG calculation.
 

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I am exactly opposite. 4.56 gears, 35's spedo will say I am doing 70 but I am actually doing 62ish? What gives?

Did you change the speedometer gear? You need the correct gear based on the gear ratio and tire size. There are tables around that indicate the number of teeth you need.
 

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There should be a tag on the bolts on the differential covers
 

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'97 TJ, 4L engine
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ID checkers due to speedo off

When I bought my '97 it came with optional big Wrangler ATs. I asked the dealer to check the speedo because I knew it was off. They pretty much blew me off by saying it was within the allowable range of error, something like 10% or +/- 5 mph. I just said the heck with it and have driven it that way ever since. I figure it's about 5 mph under at 60, so I do 65 on the meter. So with bigger tires my TJ speedo reads low which I once figured out to make sense. I don't know if my method of adjusting is good 'cause I get a lot of tailgaters sniffing around my butt. I always say they're like dogs and are checking my ID. ;)

PS - at least I'm not attracting the Po-lice to check my ID.
 

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Ive got 4.88's with 33's and in 5th gear at 3K RPM's its 40 mph off.
I just use my GPS, but if your looking to make it right quadratec.com has a chart that matches gear ratios with tire sizes for you to figure out what number gear tooth to buy on the spedometer gear. super easy and only a $35-40 part.
 

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Ive got 4.88's with 33's and in 5th gear at 3K RPM's its 40 mph off.
I just use my GPS, but if your looking to make it right quadratec.com has a chart that matches gear ratios with tire sizes for you to figure out what number gear tooth to buy on the spedometer gear. super easy and only a $35-40 part.
That's a very extreme case if you meant to type 40. You are racking up an incredible amount of miles over a cheap part. (I'm assuming that it overestimates your speed)
 

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Other effects of bigger tires?

Yep...it also causes lots of confusion when calculating MPG by hand. Typically one just divides the miles driven by the gas used. With the speedometer being off by the bigger diameter tires, the MPG will calculate low. So ya need to add the difference back in. For instance, if the calculation is off by 12% (speedo vs actual)...add 12% of the miles traveled back into your MPG calculation.
I'm being told that larger tires mess up the front end and the transmission. Is this true or just jealous non-Jeepers?
 
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