Tire size and speedo accuracy

boardsurfer

boardsurfer

Active Member
I recently put in a gps and noticed my speedo is off by as much as 4 mph at highway speeds. I got to wondering if I'm actually going faster than the computer thinks I am, is my odometer inaccurate? And then is the mpg reported also inaccurate?

35s and manual trans.
 
boardsurfer said:
I recently put in a gps and noticed my speedo is off by as much as 4 mph at highway speeds. I got to wondering if I'm actually going faster than the computer thinks I am, is my odometer inaccurate? And then is the mpg reported also inaccurate?

35s and manual trans.
Click to expand...

In my '10 2dr that I had, when I put 35s on it, it threw off my speedo, and my odometer. Unsure of the MPGs reported though!

I'd suggest investing in an AEV Flashcal or Superchips type programmer.
 
boardsurfer said:
I recently put in a gps and noticed my speedo is off by as much as 4 mph at highway speeds. I got to wondering if I'm actually going faster than the computer thinks I am, is my odometer inaccurate? And then is the mpg reported also inaccurate?
JL Audio Cleansweep

35s and manual trans.
Click to expand...
yes, they are inaccurate. get a programmer to reset the tire size/gears.
you can use your GPS to check the accuracy after
 
Cool. Thanks guys. I didn't think I needed a programmer because they were mostly for shift points and an auto trans. I see that I was mistaken. I'll add it to the lengthy list of jeep stuff.
 
Yep, the 15 MPG's your computer says you are getting is wrong. There is good news however...since you went bigger your tires revolutions per mile are down and your computer thinks you are traveling at a slower MPH and traveling shorter distances than you really are. Since that is the case the MPG reading on you computer is lower than what you are actually getting. I went from 255/75r17 to 295/70r17 (approximately 32" to approximately 33.25" ) which represents about a 3.7% increase in tire diameter (to be more accurate I would need to look up manufactures data on exact size and Rev's per mile). Using a GPS I tracked my mileage and how much I fuel I was putting in over 2 month. My computer is currently reading 15.4 MPG's while my calculations put me at 16.1 MPG's, which is approximately a difference of 3.8%.
 
Last edited:
I was pondering that as well. Thanks for the explanation! That also means the odometer is showing a little less than I have actually traveled.
 
You must log in or register to reply here.
Top Bottom