i found out the hard way that mine was off that much. the state trooper gave me a warning cause i was able to show i just bought it that day

 

x3, I then went to 4.56's and 33's and was 7 over what I was actually doing. As many has said those trailer radar contraptions are junk. Check with a GPS. I swapped to 39 tooth gear (INCREDIBLY EASY!!!!) and now I am dead on.

How so? Gears are gears, they don't magically grow or shrink in ratios based on speed. When I had 31's I was off 4 mph at any speed and same with the 4.56's and 33's I was off 7 mph at any speed. Now the resolution at which your speedometer/computer is reading maybe off but that is going to be off even if your jeep is completely factory (hence why most speedometers dont even read below a certain speed or why pressure gauges are suppose to be used for the pressure setting they are reading)

 

True, but it is a percentage difference, not numerical. If you were going 1 mph, your speedo would not say you are going 6 mph.

If you went from 29" tires to 31" tires, it is an increase of 6.89%. So, you're speedo is going to be off by 6.89% no matter what speed you go...

10 mph = 10.689 mph
30 mph = 32.067 mph
60 mph = 64.134 mph
80 mph = 85.512 mph

 

My jeep only goes about 65 geared the way it is so i just push the gas and it goes who cares ahow fast your going lol...

 

Touche, Good sir.


And to OP. A new speedo gear which is located in the tail housing of the transfer case is the fix. Depending on your gearing and tire size will depend on what tooth count you need ( I have a 34 tooth laying around if that is what you need). You can check here for which gear you need http://www.quadratec.com/jeep_knowledgebase/article-46.htm

 

You would be wrong, the mph difference increases the faster you go

7 mph off at 30 isn't 7mph off at 60

 

Go to https://tiresize.com/speedometer-calibration/ this is a handy website when figuring tires.

 

https://www.quadratec.com/c/referenc...ar-tooth-chart