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Old 09-21-2009, 06:17 PM   #1
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Jeep wrangler TJ Speedo Gear ?'s

This is really getting crazy, I have spoke with several people today on the phone about speedo gears and I have yet to get the same answer, what gives?

I have 3.73 gears and 33 12.50 tires, i know my MPH is off and probably the odo too. According to some charts i saw online i need a 31 to make it right, but a few people i spoke with today told me i need a 30 and a 29.. I dont even know what comes stock??
My next ? is im not really concerned about MPH, im sure its about 5 or so off, but my ODO is where im really concerned, is it showing higher or much lower?? Im wondering if this is why my gas milage per tank is so bad cause im really getting more miles then it showes, or could it possibly be less.. can someone explain this, and possibly tell me what gear i have and what i need? would greatly appreciate some insight on this headache....

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Old 09-21-2009, 06:23 PM   #2
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3.73 gears and 33s are why your gas milage is so bad. As for the gears I'd just research the net. Try to find some that others have tried and agreed on the results.

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Old 09-21-2009, 06:32 PM   #3
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This chart is known to be accurate and it indicates a 31 tooth is the correct choice for 33" tires and a 3.73 axle ratio. It has been GPS verified with all the shown combinations.

Even Quadratec has two different speedometer gear charts floating around their catalogs and website and only one of them is correct...

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Old 09-21-2009, 06:49 PM   #4
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gears

why are 3.73's so bad with 33's. what gearing would you recommend?
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Old 09-21-2009, 06:54 PM   #5
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why are 3.73's so bad with 33's. what gearing would you recommend?
Its not bad. Well just for gas. Jeeps don't get good milage anyway. When you gear them or put bigger tires on them they get even worse milage. Thats why most people don't by them for the milage. Poor milage is just a side effect of having fun.
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Old 09-21-2009, 07:14 PM   #6
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why are 3.73's so bad with 33's. what gearing would you recommend?
The factory considers 3.73 ideal for 30" tires, going to 33" tires just reduces performance and makes the engine work a little harder. Typical recommended ratios for 33" tires are 4.56 for a 5-speed, 4.88 for the 4-speed automatic, and 4.10 for the 3-speed automatic used 2002 and older as well as the new 6 speed manual transmission. For the 6-speed, it's a toss-up between 4.10 which would be more street oriented with 4.56 being more offroad oriented.
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Old 09-21-2009, 08:03 PM   #7
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Its not bad. Well just for gas. Jeeps don't get good milage anyway. When you gear them or put bigger tires on them they get even worse milage. Thats why most people don't by them for the milage. Poor milage is just a side effect of having fun.

we still taking speedo gears right?
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Old 09-21-2009, 08:18 PM   #8
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gears

so its either 4.10 or 4.56 gearing, which would Improve the gas milage alittle, and give me more pep, etc.. the only downside is im looking at probably $1000 atleast for gears and labor as i dont think i would attempt this myself?? I still need the speedo gear as it helps with ODO and shift points??
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Old 09-21-2009, 10:57 PM   #9
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Just bought new 33's same gears as you and got a new 31 tooth speedo from dealer for 56.00
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Old 09-22-2009, 12:17 AM   #10
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i run the 31 tooth gear, and average 14 to 17mpg with 33s and 3.73s
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Old 09-22-2009, 04:52 AM   #11
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speedo gear

sounds like a 31 it is until i can afford the labor, the gears really are not that expensive, i think i saw them for about $450
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Old 09-22-2009, 10:25 AM   #12
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288sunny....IF improved fuel mileage is your primary goal .....then here are a few thoughts FWIW: ' Suggest you get the speedo gear correct and then drive it some and recheck your mileage. Regardless of what that new answer is, you will not be able to improve it much, if at all, with a gear change. The gear change will help you get up to speed with the 33's better for sure and will lug the motor less off the line and hopefully save some fuel, but once you are rolling at your desire MPH you will be turning more RPM's and using more fuel vs the 3.73...so bottom line is you will gain some and loose some MPG.....if you spend more time at freeway speeds, then you will loose more than you gain mileage wise. I would strongly recommend that you talk with folks that have done the gear swap you are considering to see if they prove me wrong before spending the money on new gears. Good luck
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Old 10-13-2009, 09:50 PM   #13
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Quick question...I just swapped in 3:73's and 33's.

I came from a stock 1997 4.0 3:07 Auto TJ sport...what speedo gear did mine come with stock? I don't want to buy a 31 tooth if I already have one.
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Old 10-14-2009, 12:33 AM   #14
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Quick question...I just swapped in 3:73's and 33's.

I came from a stock 1997 4.0 3:07 Auto TJ sport...what speedo gear did mine come with stock? I don't want to buy a 31 tooth if I already have one.
To give us a fighting chance of getting the speedo gear's tooth count right, we'll need to know the size of tire it shipped with from the factory.
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Old 10-14-2009, 09:52 AM   #15
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Agh i didnt know they came with different tire sizes. I'll have a local run the vin and get back to you. Thanks
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Old 10-14-2009, 10:07 AM   #16
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I have 33" X 12.5 w/ 4:56 and finally found a 39 tooth on eBay for $60.. I verified my speed with a Garmin Hand Held GPS and it is right on the money..

As for gears; I too had 3:73s w/ 33" tires to begin with, but when I had lockers installed; i went ahead with 4:56 and it is night and day different between the 3:73s.. She took a little effort to drive before, but now she drives effortlessly.. I swear I'm getting better mileage as well since I don't have to floor it all the time to get it up to speed. Its not bad on the HWY either; she runs at about 2500 rpm in 5th at 65mph. I highly recommend the 4:56 swap as opposed to just running the 3:73s.. my $.02

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Old 10-14-2009, 07:49 PM   #17
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To give us a fighting chance of getting the speedo gear's tooth count right, we'll need to know the size of tire it shipped with from the factory.
Ok I had 225/75/15 from the factory...I appreciate the help.
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Old 10-18-2009, 12:58 AM   #18
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Old 10-22-2009, 07:15 PM   #19
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So would his odo being showing higher than true with the incorrect speedo gear?
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Old 10-22-2009, 07:26 PM   #20
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Ok I had 225/75/15 from the factory...I appreciate the help.
Sorry that's a 28" tire, my charts only go down to a 30" size... nothing any smaller.

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So would his odo being showing higher than true with the incorrect speedo gear?
Installing larger tires propel you faster and further than the speedometer & odometer will show. So he could have traveled 100 miles with bigger tires but his odometer might only show 96 miles.
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Old 11-02-2009, 10:59 AM   #21
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Quote:
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Sorry that's a 28" tire, my charts only go down to a 30" size... nothing any smaller.

Installing larger tires propel you faster and further than the speedometer & odometer will show. So he could have traveled 100 miles with bigger tires but his odometer might only show 96 miles.


that answers my question as well... the hole mileage thing was throwing me off.. wasnt sure if i was traveling 100 and showing more.. good to know that its showing less....
i had a hard time finding a 27 tooth gear for 31 and 3.07 gears so
would it be worth me replacing the RP in my dana 35.. seems like it would help with the larger tires.. i have 31's on 15x8 and only 3.07 gears.. can i just replace the ring and pinion and go up to maybe 3.73 or 4.11.. ??

can you just do the rear or do you have to do the front as well..
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Old 11-02-2009, 11:21 AM   #22
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You can upgrade to 4.11 (3.73 would be useless) But bear in mind it would cost abou $1000 in labor and parts since youd need to regear both axles or else youll have no 4wd

Im currently running 33" tires at 4.56 gearing and getting ready to put in a 39 tooth speedo gear.

3 weeks ago I was running 33" tires at 3.07 with stock speedo gear.

It only cost me $400 to trade someone who had a S35 and S30 with 4.56 gearing already in it with Detroit lockers front and rear. (plus $100 to pay someone gas and food to help do axle swap)

With that story, I would tell you not to regear your D35 since they break easily. What I would do is craigslist, jeepforum, midwestjeepthing, someone selling their axles. Itll end up being alot cheaper and youll most likely get some other added bonus like axles or trusses added. Ive seen several S35 and S30 combos going for $750 in the Northern part of the Midwest with 4.88 gearing and some type of locker. All this would be cheaper than having someone regear your axles.

If you do want to regear, buy the R&P from some person who just bought it but doesn't want it anymore, you get them for alot cheaper and most times theyre brand new. Then Regear it yourself, youll save $600 bucks.

I do believe I have 27tooth speedo gear, but I will never know. I had 3.07 gearing with 27" tires stock from the factory if anyone knows how many teeth that is let me know.

But I also do know that jeepforum.com has a thread where people simply trade speedo gears for free as long as trade with someone else for free, thats how I got my 39tooth speedo gear.
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Old 11-02-2009, 01:19 PM   #23
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cool, thanks Terrible, and welcome to the forums.
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Old 11-04-2009, 11:03 PM   #24
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You can upgrade to 4.11 (3.73 would be useless).
First thanks for the help above! I must have missed the email notification.

second, I have better power with 33s and 3:73 than I did with 28's and 3:07

The 3:73 arent worthless if you can get ones already geared like I did for 100 bucks then a few nights to swap it all out and finalize everything.

My 4.0 auto 3spd is great for city/highway with 3:73, I think 4:10 at 75 mph would be through the damn roof with the RPM's

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