How to recalibrate your speedometer after changing tire sizes or gears

Chris said:
Just what I've been looking for! So when you say GPS, can it be done with a smartphone?
Click to expand...
Chris,
Not sure if anyone has replied but yes, smart phones will work. I have android and the latest version of Google maps shows the posted speed limit at the bottom left of the screen and beside it is your current speed.
 
VWilson said:
Chris,
Not sure if anyone has replied but yes, smart phones will work. I have android and the latest version of Google maps shows the posted speed limit at the bottom left of the screen and beside it is your current speed.
Click to expand...

Yep, that’s how I ended up doing mine 👍
 
  • Like
Reactions: MillenniumFalcon
I recalibrated the speedometer with a new gear and it was working fine and matched up with my GPS. Then I was out at lunch and noticed I'm 15mph low and it bounces a little. Would the housing rotating or backing out some to cause that? I'll pull it back out tonight to take a look, but it's odd that things were working.
 
I pulled the gear out checked the teeth and put it back in. I got the same result. 10-15 mph slow.

It was spot on when I switched it. These gremlins are keeping me from doing the fun mods.
 
Sorry if this has been covered elsewhere, I'm on mobile only right now and can't search as efficiently. Contemplating going from 235/75/15 to 31x10 instead of going to 30x9.50r15.

If I go up to either size, will this recalibration definitely be necessary? Not sure how much difference an extra 3/4"-1" of tire makes...

Don't think it matters but I'm very close tl stock suspension height with aftermarket wheels from Quadratec (4" backspacing). I've got factory 3.73 gearing.
 
astjp2 said:
Yes if you want it accurate, there is a chart of speedo gear vs. tire size
Click to expand...

Hey thanks for the reply, should have been more specific - trying to decide if it's worth the trouble or how "off" everything will be if I go from like 28.9 to a true 30". Are we talking like 5 mph off? I know it can affect the odometer too...Read the guide a few more times though and I may just end up doing it, just salty about the extra trouble and $$ on top of the tire/installation cost :)
 
Last edited:
I went from the OEM 30” tires to 31” BFG KM2’s. I checked to see how much it was off using my gps and a speedometer app on my phone. the gps, app and my speedometer all matched, so I never changed the gear.

I never thought to check the 30” tires so I dont know how off, if any, they were.
 
  • Like
Reactions: Not_Invented_Here_
Hi all. Mine won't come off/won't move. Any tips here?
IMG_20200129_155246_1.jpg
 
Is this the correct speedo gear "shaft size" for a 2000 Sahara NV3350? The reason I ask is they have a short or long shaft. I have 33 bfgs which are actually 32.5s so I'm gonna do a 32 tooth.

Screenshot_20200309-192419_eBay.jpg
 
When putting my 32 tooth gear back in the transfercase. I line it up with the 32-45?? part of the housing and the speedo dosent work. If I put it on the 31-28?? part of the housing. It works. Will it hurt anything if i leave it on 31-28 even though it's a 32 tooth?
 
Last edited:
So my diff is marked 5.73 (it's from an XJ) and I am running brand new 33 Duratracs. At GPS calibrated 40 MPH by speedo is reading 45 MPH. I haven't pulled/counted the current speedo gear. Any ideas?
 
bigshark88 said:
So my diff is marked 5.73 (it's from an XJ) and I am running brand new 33 Duratracs. At GPS calibrated 40 MPH by speedo is reading 45 MPH. I haven't pulled/counted the current speedo gear. Any ideas?
Click to expand...
Those gears aren't exactly cheap, no point in guessing. Just pull the gear you have and go from there.
 
pagrey said:
Those gears aren't exactly cheap, no point in guessing. Just pull the gear you have and go from there.
Click to expand...

My earlier post was not well informed. I mis-read the stamp on the rear end. It's a 3.73. Pulled the gear when I went back to look at the stamp again. It's a 29. Will be moving to a 31 to re-calibrate.
 
My new gear is coming today. I'm a little confused. You say to make sure the gear number range for your gear is in the 5 o'clock position, but also say the metal U clamp fits in the slots in the plastic housing. Don't the slots determine which number range is in the 5 o'clock position?
 
JLANEF said:
My new gear is coming today. I'm a little confused. You say to make sure the gear number range for your gear is in the 5 o'clock position, but also say the metal U clamp fits in the slots in the plastic housing. Don't the slots determine which number range is in the 5 o'clock position?
Click to expand...
Theres multiple slots so it can be clocked differently.
 
Top Bottom
Back