Forgive me for being stupid but I have a question. I am looking to learn here not get flamed, anyway here it goes. How come you do not have to recalibrating for different gears, and if he is running a bigger tire why isn't he recalibrating for tire change? Sorry for the stupid question I just can't seem to figure this out. Hammer can you help me? Thanks

 

You DO NOT have to recallibrate for the gears
You DO have to recallibrate for larger diameter tires because of the revolutions per mile.

Reason being is all due to how the computer reads how fast your going. some time ago probes measured the gear speed, in term figuring you MPH, now they go off the revolutions of your tires.

 

yep, so it's all based on tire revolutions per mile (or KM for those who use that wacky system), so the PCM needs to know the diameter of the tire. Obviously a 29" tire will rotate many more times than a 35" tire to travel a mile. That's why if you do not recalibrate when going to a larger tire, you are actually traveling faster than your speedometer indicates and you are racking up more miles on you odometer than you are actually traveling...

 

Thanks, I didn't know that about gears. I thought they where the same as tires as far as recalibrating, good to know. As for the tires I guess I didn't read the thread well enough, I thought he upgraded tire size too. Thanks again.

 

Exactly the respn why i havent Recalibrated mine with the bigger tires! lol! i can drive 100 miles and only rack up like 82 on my odometer haha. i have other sources of telling me my MPH

 

Actually I worded that wrong, when I re-read my post in your quote, I meant to say just the opposite, you're odometer is showing LESS miles than you are actually travelling.

Yeah, I know you are not the only one did not recalibrate their speedo for that reason, thank goodness for GPS's...

 

yes sir!

 

The computer reads the speed off of a sensor that uses a toothed wheel bolted to the ring gear in the rear differential. Since the speed is measured after the pinion gear, ratio means nothing. They used to use a cable driven from the tailshaft of the transmission or transfer case. With a different gear ratio, the driveshaft spins different thus making the speedo off. Only tire diameter will change the reading on the Ram.