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?

 

Yes. The tac is off as well. Also in reality the amount the Speedo is off is completely dependant on speed. The faster you are going, the more it will be off. The slower you go the more accurate it is.
Edit: I'm an idiot. Lol. I was thinking he was talking about the odometer. So, no, the tach will still be accurate.

 

http://www.jeepforum.com/forum/f11/l...t-tach-292397/

Like these guys said, the tach reads off the motor, not the drivetrain. The only things affected by larger tires are the speedo and the odometer

 

Unless I'm mistaken your tach will still measure your engine speed.

You will just have less engine speed to turn the wheel speed.

 

I'm going from 235/75/r15 to 32"s in about a month I'm assuming I can have my speedometer calibrated? And how much does it really effect the odometer?

 

No calibration, just buy a new speedometer gear. You can find a chart on google pretty easily to find out how many teeth you need.

 

The fix is changing the speedometer gear located on rear of transfer case.
The engine/drivetrain reads off this information also so it's a good idea to replace it. There is a chart for which gear to get.
http://www.seriousoffroadproducts.com/

edit: beat to the answer again.

 

It affects the odometer the same amount as the speedometer.

 

the tach will read correctly but the bigger your tires are on the same gears te lower your rpms will be

i.e. stock tires will have an rpm of 2300 ish at 65

33s will be about 1900 or so

 

so the trip odometer is wrong, so lets say i drive 100 miles on the trip odometer how many miles did i actually travel more or less? is there a formula for this?

 

With 3.55 gears and 33s your speedo gear is off by 18%, so you're actually going 18% faster then what the speedo says and you're traveling 18% farther then the odometer says. So if your odometer reads 100 trip miles you actually went 118

 

that makes sense

 

Having the trip odometer off is a pain in the *** if you're trying to follow a trail guide with mileage notes. I went through that with my old Jimmy on a trail that was like a spiderweb of ATV trails too, and I was immediately lost compared to the guide and had no clue where to turn.

Changing the speedo gear is seriously a piece of cake. Probably the easiest thing I've done to my XJ, aside from changing the blinker fluid.

 

find the gear for your tires. I have a hard time believing going to 33's would put you 18% off but the math seems to support it.

 

Should be a 30 tooth for 33s and 3.55s