I went past a radar trailer thing today and noticed my Speedo is already 4 mph off where it should be and I'm still running the stocks! What will 31s do to my speedometer and what is the fix?

 

funny mine with 31s didnt make a difference

 

those trailer rader things are junk. I have tested my speedo with a friend on Interstate and I am dead on. Drove past one of those trailer things going 25 and the screen said 30. Theyre pointless; just another gimmick to help PO's give you tickets.

 

With 31's my speedo is 4mph under according to GPS

 

X2

 

With 31" tires you will need a 32 tooth speedometer gear. You can purchase these from quadratec.com for fifty bucks.

Here is a step by step guide I put together on how to change it. https://docs.google.com/fileview?id=...YjJhYjY4&hl=en

 

Alright so I'll be adding that 4 to the other 4 i'm over hahaha

 

Thank you so much! I'll definitely take a look into that! I need to get a little more money to do it but I'll definitely bookmark both pages!

 

Running bigger tires makes your speedo read under, thats physics. If your speedo IS reading 4 over, 31's will make it right on because 31's make it 4 mph under actual speed.

Listen to 1991Jeep_Man on the trailers, but keep in mind even the radar guns the police use are PROVEN to have inaccuracies of up to 5 mph and most radar guns shoot a stream 10-11`degrees wide, which can tag cars next to you. Things like precipitation, radiant heat from the pavement and barometric pressure can greatly affect a radar or laser system. If you ever get a ticket for going 5 or under, fight it, most judges will look at a citation like that and ask Rosco P. Coltrane why he spent the time to write a 5mph speeding ticket while Bo and Luke Duke went flyin' by in the General Lee...

I haven't tried it on an XJ, but a few vehicles I've been able to just pull the needle off and put it right where I want it. The key is to not pry the needle, but pull the needle shaft out from the back, prying off the speedo case. Prying on the needle will make it shatter.

I'm running 31's and I'm 4-4.5 mph under too.

 

This honestly just made my day.

 

you guys do realize that your speed will be off progressively the faster you go? right?

you wont constantly be like 4mph off at all speeds. the gap increases the faster you go.

example:at 20mph you may be 1-2mph off, but at 60mph that would increase to like 7-8mph off, and so on.

 

then you dont have true 31"s

 

(31/29) X speedo speed = actual speed


I might have the division back words.

Usally im flying past cars on the express way when I'm "doing the speed limit" on my speed o.

 

i have 33's i know when my spedo is at 55 im doing 65 actual speed

 

Well, know i know for sure 70 on the speedo is accually bout 80... Guess I gotta back it down to 65. lol