Speedometer Incorrect?

[Kevin]

I checked my speedometer today against my GPS and the MPH was off by anywhere from 2-3MPH. I don't know how accurate the GPS is but the difference seems sizable so I'm wondering if I can investigate further. I looked into the effect of running a non-stock tire and the difference should be 1.1%, which in the 60MPH range should be about 1MPH or so.

I guess it boils down to which one is wrong. In my last car, the GPS was off by 1MPH, so I kind of believe it over the Liberty...

 

[Corwyyn]

I'd trust the GPS over any speedometer - even though it only updates once per second (typically) it is more accurate. About the only time this may not be true is if you are in a deep urban environment with lots of tall buildings. You could always do the tried and true time vs. distance method to determine speedometer error if you have a good stretch of road to do it.

 

[06Liberty]

I'm guessing you have bigger tires? If so, then yeah it'll be off unless you get the speedometer re-calibrated. I know my JK is about 5 mph off if I'm going 60 (it says 60, i'm actually doing 65)....but my tires are 2 inches taller than stock.

 

[Kevin]

Yes, they are 235/70/16 compared to 225/75/16 (at least that's what the window sticker says). The speedometer should be 1.1% too fast buts greater than that according to the GPS...

 

[06Liberty]

That shouldn't be a big enough difference in height to really matter. In fact, the 235/70 is actually shorter than the 225/75's so if anything you should be going slower than it says you are....if it says you're going 60, you should actually be going about 59.3.

http://www.paspeedo.com/calculator.htm

 

[JeepJeepster]

My speed-o is off with the stock tires and right on with the 245/70/16 tires.

 

[06Liberty]

that's weird...why is that?

 

[JeepJeepster]

I guess they want them to read alittle fast from the factory. Gives you alittle lee way.

 

[Corwyyn]

I guess they want them to read alittle fast from the factory. Gives you alittle lee way.

I kind of doubt that - mine was right on with the stock 215/75s on it.

 

[Kevin]

That shouldn't be a big enough difference in height to really matter. In fact, the 235/70 is actually shorter than the 225/75's so if anything you should be going slower than it says you are....if it says you're going 60, you should actually be going about 59.3.

http://www.paspeedo.com/calculator.htm

Um, thats what I said, 1.1% difference. [60*(1-.011) = 59.34]

The GPS speed was always slower than the Speedometer speed and it seemed to differ anywhere from 2-3mph, which seems kind of high to me.

 

[Marlon_JB2]

Mine is off too... most likely, no speedometer is correct, no matter the vehicle. Great way to fight speeding tickets.

 

[06Liberty]

Um, thats what I said, 1.1% difference. [60*(1-.011) = 59.34]

The GPS speed was always slower than the Speedometer speed and it seemed to differ anywhere from 2-3mph, which seems kind of high to me.

You didn't specify whether you are going faster, or slower than it says you are. I assumed you were talking about going faster than the speedo says....because if it's only 1, 2, or 3 mph difference, why would anyone care about going slower than the speedo says.....if anything that would help you NOT to speed.

 

[Kevin]

I'd rather it say I'm going faster than I really am but what's the relationship between the speedometer and the odometer? It would suck to lose mileage with regards to the warranty...

 

[06Liberty]

Good point, maybe try going to the dealer to get it re-calibrated? Mine is the opposite, when it's all said and done I would have driven MORE miles than my odometer will read.

 

[musicaldoc]

I can tell you that all the KJ sold in Europe with 236/65/17 are apx 8% faster (on the speedometer) than what checked on any GPS!!!