So I got this older (2006ish) GPS from my father, as he got a new one. Rolling down the highway last night, it appears to read roughly 5.5% higher than my speedo, consistently. The truck has stock 265's on it (and the door tire pressure indicates that is the size that came on the truck). Any ideas? Assuming this translates over to the odometer too, that means both that my fuel mileage is a little better than I thought it was, and that the odometer will be wrong. How awesome is that?

Any ideas? Is that about the difference I'd see if the truck had stock 245's, and maybe the computer is calibrated for the wrong tire size?

 

Are you kidding? You're worried about that MINUTE of a difference?
You can see that small of a deviation from different inflation pressures, different tires with different tread patterns, etc.

If you want to figure out the difference between tire sizes, give this calculator a try.
http://www.miata.net/garage/tirecalc.html

 

It's enough to get a ticket-- I was rolling along at 75 and the GPS said 80, then later at 60 and the GPS said 64. Good think I live in ND so the ticket would be 20 bucks or less (if a cop would even bother to pull me over).

I'll check out the calculator, thanks for the link.

It's just one of those sort of things annoys me. My motorcycle reads 10% high, which is even more annoying, but there's no solution to that.

 

My speedo is off by 10mph (I asked a local police officer to clock me once) and I live by that. Speedometers have rarely been real accurate, so don't get your panties in a bunch over 5%.

 

Get a superchip and reset your tires size.

 

LOL.... I only glance at my speedo on the bike when I see a cop to decide if I should stick around or not. Other then that, I pretty much never look at it.

 

ok! :d

 

If you are running larger tires it should help.

 

Car and Driver did a story a few years ago (read about 10 or so) stating that among newer cars that they were off as much as 5-15 MPH among them, and about as much from a radar gun. Wich is why that 98% of coppers will give you 5 to 10 before they will pull you over. Also you cant really rely on a GPS unit, Remeber they come from Space.

 

http://www.youtube.com/watch?v=dpgAq579cRA