Adjusting Speedo
Thread Starter
JK Enthusiast
Joined: Dec 2009
Posts: 115
Likes: 1
From: Crawfordsvillle, IN
Adjusting Speedo
I just put MT ATZ P3's on my 09 that are 315/70/17 and my local dealership is unable to adjust my speedometer. Right now it is running 4 miles an hour slow at 40 mph. This is based on my GPS unit. Other than going out and buying a programmer how can I get this adjusted? And if the dealership with their programmer can't adjust up to the tires I have what programmer will be the least expensive?
And before someone says it I DON'T "want to deal with it"
And before someone says it I DON'T "want to deal with it"
JK Jedi Master
Joined: Mar 2007
Posts: 14,800
Likes: 374
From: Midwest City, OK
The dealer should be able to program it for 32s (which is what a Rubi comes with), which would get you pretty darned close (and probably right on as the tires wear down). Also, the Rubi 32s are pretty large. A set of Rubi 32s was larger than the Dick Cepek FC II 33s I used to run when compared side-by-side. Standard procedure with a programmer (ProCal, Hypertech, etc) is to measure the height of the tire from the ground to where the tire makes its sharp break into the tread--not to the bulged out top part of the tread in the tire center. You might measure that and ask the dealer if they can set the tire height to that.
By the time you're done with that, though, you'll probably have spent more than for buying the programmer. ProCal is modestly priced. Minor pain in the rear to use because you have to reference a chart, then set dip switches. Hypertech asks questions, you answer, and it walks you through the setup. But, not all programmers do all things. If you decide on one, ensure you get one with the features you want (and take any MPG/HP/Torque gain numbers with a grain of salt).
By the time you're done with that, though, you'll probably have spent more than for buying the programmer. ProCal is modestly priced. Minor pain in the rear to use because you have to reference a chart, then set dip switches. Hypertech asks questions, you answer, and it walks you through the setup. But, not all programmers do all things. If you decide on one, ensure you get one with the features you want (and take any MPG/HP/Torque gain numbers with a grain of salt).
JK Jedi Master
Joined: Feb 2008
Posts: 12,907
Likes: 185
From: Maine
When I set my tire diameter in the ProCal, I measured it this way to remove all other variables:
Inflate the tires to your chosen normal pressure.
Stick a piece of chalk in the tread.
Roll the JK enough to make two chalk marks on the pavement.
Measure between the chalk marks and divide by 3.1416.
That will be the diameter the computer needs to know.
Inflate the tires to your chosen normal pressure.
Stick a piece of chalk in the tread.
Roll the JK enough to make two chalk marks on the pavement.
Measure between the chalk marks and divide by 3.1416.
That will be the diameter the computer needs to know.
JK Junkie
Joined: Nov 2013
Posts: 3,703
Likes: 8
From: The Dirty South
When I set my tire diameter in the ProCal, I measured it this way to remove all other variables:
Inflate the tires to your chosen normal pressure.
Stick a piece of chalk in the tread.
Roll the JK enough to make two chalk marks on the pavement.
Measure between the chalk marks and divide by 3.1416.
That will be the diameter the computer needs to know.
Inflate the tires to your chosen normal pressure.
Stick a piece of chalk in the tread.
Roll the JK enough to make two chalk marks on the pavement.
Measure between the chalk marks and divide by 3.1416.
That will be the diameter the computer needs to know.
Best to do this with the tires warm? Finally a use for pi other than eating.
Last edited by 14Sport; Sep 13, 2014 at 03:46 AM.
Thread Starter
JK Enthusiast
Joined: Dec 2009
Posts: 115
Likes: 1
From: Crawfordsvillle, IN
Thanks to all for the help, looks like I am buying a programmer Monday. Right now using my numbers from my GPS I am off driving 1.1 miles (GPS) for ever 1 on odometer. Have a 2500 mile road trip coming up, would not like to have my odometer off by 10%.