went to 33s speedo and tach question

i went to 33s from 30s and now my speedo is of by 8mph, with 30s my speedo was of by 3mph ( i used my navigation to check speed )

my question is is my factory tachometer also off due to tire change or just the speedo?

Nope, just the speedo.

91to99 said:

Nope, just the speedo.

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odometer, too.

Tach runs off the crankshaft position sensor.
Odometer / speedometer run off of the vehicle speed sensor / VSS (or whatever lingo Jeep uses), a sensor mounted in the transmission tail cone on 2wd and in the transfer case rear output in 4wd Jeeps.
There is a gear attached the VSS that meshes with the output shaft, the gear drives the shaft of a proximity sensor that makes a pulse for each revolution. Computer counts the pulses per second or minute or whatever & calculates the vehicle speed & distance traveled based on an assumed gear ratio / tire size.
Look up "Jeep speedometer gear chart" with google and find the part numbers that match your gears / tire size, if you want. The gear on the VSS can be changed to correct your speedometer.
Of course... if you just know that on the freeway, you're going almost 10 mph over indicated... that's good enough for most of us.

also another weird thing i noticed after going from 30s to 33s my rpm is actually lower at 65mph, i was spinning at 2250 or so with 30s and now i am at 2000 rpms with 33s

does this sound right?

Yep, that's correct. Larger wheel diameter = larger circumference = fewer revolutions required to go the same distance = decreased RPMs of all components in the drivetrain at the same speed.

That decrease in engine RPM is what makes OD unusable for me on anything but flat ground, there's not enough power there to maintain speed. Which is why people regear their axles.

donthelegend said:

Yep, that's correct. Larger wheel diameter = larger circumference = fewer revolutions required to go the same distance = decreased RPMs of all components in the drivetrain at the same speed.

That decrease in engine RPM is what makes OD unusable for me on anything but flat ground, there's not enough power there to maintain speed. Which is why people regear their axles.

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understood, the good thing is my XJ is an occasional Off roader and i havent had any problem on the hwy with overdrive yet.. actually i cant tell the diff from 30s that i had prior.. it feels like it drives the same

If you'd climbed the Grapevine on 30s then on 33s in the same day... you'd be able to tell

MUKAK714 said:

i went to 33s from 30s and now my speedo is of by 8mph, with 30s my speedo was of by 3mph ( i used my navigation to check speed )

my question is is my factory tachometer also off due to tire change or just the speedo?

Click to expand...

Did you use a navigation on phone or a standalone? If on phone which app?

Most phones have built in GPS. As long as that is the reference and you are doing it at constant speed it "should" be ok. If it drives the same and it has 3.55 gears in it still, he's in for a wakeup when he swaps to 4.56's.