Piper645

My speedometer is off, but its not like you would think. I just bought a 98 cherokee classic 4.0 liter.

My speedometer reads 3 mph faster than what it should. When it says 30, im actually going 27. Also, regardless of speed, it is always 3 mph off, not an inclined speed difference. The inside of the drivers door says it came with 215/75R tires and it now has 225/75R on it from the previous owner. My question is why does it read fast even with supposedly larger tires than stock? Could the drivers door sticker be wrong? Any ideas would be welcome.

freegdr

this is normal behavior with larger tires than stock

freegdr

http://http://www.jeep-xj.info/HowtoSpeedoGears.htm

Piper645

Your link didn't work, but from what I have read, this is backwards. With bigger tires shouldn't i being going faster than it says? As in it says 30 but gps says 33. My speedo says 30 but i'm really going 27.

96_xj

it should read slower not faster.

It is all gear ratio's if you increase tire size you are changing the ratio and making the speedo read slower (typically)

However in your case I am unsure. I would measure your tire's and see what they really read. Maybe your new tires are not really the size they read.

However if you know it is 3mph off i would leave it.

freegdr

did you scroll down to highlighted speedo section just worked for me from post maybe my pc fup

Piper645

Lol I know its not a real big deal, and I am fine with leaving it, if I can't figure it out. But i'm curious what the problem is and if its a 20 minute fix, its 1 less problem and something to do thanks for your help though

bmp120

I have the same setup ... right down to the tire change size.

I have just lived with it but it is a good question.

I disagree with the previous post - Bigger tires than stock, meaning a larger circumference of tire, would yield a higher actual speed and a lower indicated speed.

Piper645

I'm glad i'm not the only one haha

96_xj

wrong.

Think about it.

The speedo gear is picking up revolutions of the transfer case output shaft.
that is directly corresponding to your driveshaft which is corresponding to you gear ration which is corresponding to how many revolutions it takes to your tire to travel in a mile


So if you do not change your speedo gear, and you do not change your axle ratio but you increase tire size it now takes less revolutions of the tire to travel a mile which in turn makes your speedo slow, your engine rpm will also be lower when compared to the same speed with the smaller tires. because it takes less revolutions of the output shaft to tun your tires in a mile.

Engine RPM and transmission gear ratio as well as transfer case ratio all lay a role. This is why your speed is taken at the transfer case output shaft.

That is why when you increase tire sizes dramatically. you need a numerically higher axle gear ratio. So when you do that you now compensating for the tires taking less revolutions per mile but making your axle have more revolutions per mile equaling out to a close to stock specification

shaneo417

mines off as well, but i have swapped out the gears and put butter tires on er, so all i need to to now is change the spedo gear in the transfer case

bmp120

Ummmm, I have thought about it. You stated "wrong" but you went on to basically go into the details of how the speedometer works. Good info but probably clouding the discussion.

And, I think you misunderstood the issueThe speedo is showing faster ... and that is the issue.


The poster is asking the question of why is this inverted - indicated (speedometer) is actually higher than actual (GPS) AND why is it always off 3 mph. It should be a greater deviation at higher speeds ... but it does not. Or, said another way, at 50mph the tires are going to be doing 2 times the revolutions then at 25mph so why is it still only 3 mph off versus 6 mph?

Anyone have any ideas on why it is inverted and why it is a constant offset?

Mike H.

96 XJ's info is correct.
Possibly, factory tolerances have allowed the cable/needle to be installed slightly canted allowing the needle to always be off a touch.
Possibly the factory does this on purpose to keep folks from speeding.
I know my WJ is the same way. I always set the cruise 5mph over the limit, and every radar sign I go by says I'm only a couple over.
I also noticed the company Honda Pilot and CryCo Pacifica are the same.
If you read many car rags, you will notice darn near every car they test in high speed events suffers the same dilemma... nearly all have a speedo that reads optimistic.

Also note that most XJ's came with 225/75-15's. You had to pretty much search for a stripper to find a set of 215/75-15's from the factory... so essentially, Piper is back to the tire size that was likely accounted for by the engineers... as the 215's would only make the speedo even more optimistic.