Another speedo gear question - JeepForum.com
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post #1 of 16 Old 06-29-2020, 08:25 PM Thread Starter
jerseyjeeper09
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Another speedo gear question

I know this has been covered at length on this forum, but I can't find a solution for my particular set-up. 98 TJ 4.0L, automatic with 33's. According to Quadratec, I should have 3:55 gears, but I have 3:07, and on every speedo gear chart I've seen, there is no speedo gear for this combo. Anyone found a solution ? My speedometer reads about 8 mph slow pretty consistently, I've checked it with gps and against other vehicles.
Thanks

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post #2 of 16 Old 06-29-2020, 09:06 PM
2jhanna
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Quote:
Originally Posted by jerseyjeeper09 View Post
I know this has been covered at length on this forum, but I can't find a solution for my particular set-up. 98 TJ 4.0L, automatic with 33's. According to Quadratec, I should have 3:55 gears, but I have 3:07, and on every speedo gear chart I've seen, there is no speedo gear for this combo. Anyone found a solution ? My speedometer reads about 8 mph slow pretty consistently, I've checked it with gps and against other vehicles.
Thanks

This is the very wrong way to phrase it. Are you 8mph off if going 2mph, and 8mph off going 70mph.

Figure out what percentage you're off. From that, it's easy to see what gear you need.

If you consider maybe your current gear has 32 teeth, and change of 1 tooth up or down will have approx a 3% result.

If your speedo is off, say 5%, you'll need a 2 tooth change.

If, your speedo is in fact always 8mph off, regardless of traveling speed, the needle on the speedometer needs to be adjusted.
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post #3 of 16 Old 06-29-2020, 09:28 PM
4Low2G0
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if the speedo is off 8 MPH all thru the range, then I would say the speedo is off.

Error from gear or tire size change is a ratio, and the error will increase with speed, more speed = more error.

Whats over that next ridge?
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post #4 of 16 Old 06-29-2020, 09:47 PM Thread Starter
jerseyjeeper09
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Never thought to check it from like 0-70, i'll do that with gps on my way home from work in the morning. But in the range of 50-60 mph, it's pretty consistent. That is to say, my speedometer says I'm doing 50 mph and gps says 58. Up to 60 mph on my speedo.
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post #5 of 16 Old 06-29-2020, 10:52 PM
TJ4Jim
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For every inch of tire with the 3:07 gears starting at 27" the gear is one tooth less so a 25 should be the correct gear.

If you come to a fork in the road take it.
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post #6 of 16 Old 06-29-2020, 11:23 PM
TheBoogieman
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You need a 25 tooth gear like TJ4Jim said.

Quote:
Originally Posted by jeepinmichguy_ View Post
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GOT IT BACK on 6/17. 03' Rubicon/Zone 4.25" combo lift with 4" lift coils up front/Zone hydro shocks/5.13 gears/35" Mickey Thompson MTZ P3 tires/Black Magic brakes. Jeep #17 & 19.
SOLD 99' XJ /4.0L-AW4/ 5.5" lift / 33" DC tires.
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post #7 of 16 Old 06-29-2020, 11:28 PM
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Quote:
Originally Posted by jerseyjeeper09 View Post
I know this has been covered at length on this forum, but I can't find a solution for my particular set-up. 98 TJ 4.0L, automatic with 33's. According to Quadratec, I should have 3:55 gears
3.55 was only an option on a 97' TJ or on most XJs.

Quote:
Originally Posted by jeepinmichguy_ View Post
TheBoogieman is a jerk.
GOT IT BACK on 6/17. 03' Rubicon/Zone 4.25" combo lift with 4" lift coils up front/Zone hydro shocks/5.13 gears/35" Mickey Thompson MTZ P3 tires/Black Magic brakes. Jeep #17 & 19.
SOLD 99' XJ /4.0L-AW4/ 5.5" lift / 33" DC tires.
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post #8 of 16 Old 06-29-2020, 11:49 PM Thread Starter
jerseyjeeper09
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Thanks for the help guys, ill get one ordered and give it a try
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post #9 of 16 Old 06-30-2020, 01:41 AM Thread Starter
jerseyjeeper09
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I can't seem to find a 25 tooth anywhere, 26 is the smallest. I guess this is why none of the charts have anything listed...
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post #10 of 16 Old 06-30-2020, 03:22 AM
Jonny Jeep
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I'm pretty sure I have a 39 tooth gear with 33s and 4.55 axle ratio in my TJ. My speedo seems to over rate speed by about 2mph in the 40-70 range but strangely is only about 1mph out at 50mph. Export MPH speedos go up to 110mph though. Top speed in UK was specced as 108mph with 0-60 in 8.8s IIRC.

39x3.07/4.55=26.3
a 26 tooth gear should be accurate enough for 33s and 3.07 gears.


Here's my 110mph speedo when I took it for it's annual inspection last week at 200k miles.
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post #11 of 16 Old 06-30-2020, 08:58 AM
2jhanna
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if you can only get a 26 tooth, but really want a 25 tooth, since the speedo gears are plastic, it's very easy to file a tooth down to nothing !!


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post #12 of 16 Old 06-30-2020, 10:14 AM Thread Starter
jerseyjeeper09
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It appears to be off by about 12%. At 30 mph(according to speedometer)the gps read 34, at 55 mph it said 63. So that would mean a four tooth change, down to 28, correct ? That’s assuming I have a 32 tooth at present... I’m sorry my math sucks! All these numbers are giving me a headache, I need a beer. I’ll be doing some interstate driving this evening so I’m gonna get more of a range to go by and compare numbers. Please let me know if I got something here wrong, I appreciate all the help
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post #13 of 16 Old 06-30-2020, 11:50 AM
2jhanna
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Now, you're on the right track. You can remove your current gear pretty easy, if you want to count its teeth.

If you're going to do some interstate, straight line, driving, you can also use your odometer. Get on the shoulder, reset the trip meter, and your gps (I used a phone app). Then drive at least 10 miles, pull over and compare reading.

This is easier that trying to watch speed.

I am multi-talented, I can talk and piss you off at the same time.

jhanna
denton county, tx
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post #14 of 16 Old 06-30-2020, 12:24 PM Thread Starter
jerseyjeeper09
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Good idea, I wondered if the odometer would be off by the same 12% as the speedo. I’ll definitely try that. I have about a 45 mile drive to work, mostly interstate and four lane highway so I’ll have a good sample size of speed and distance. Thanks again
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post #15 of 16 Old 06-30-2020, 12:31 PM Thread Starter
jerseyjeeper09
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I use a phone app as well, it’s called “gps speed”. I’ve used it on my motorcycle too and it’s off by about 3 mph but that’s Harley for you!!!
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