2001 Speedometer vs. GPS

[XChuck]

Like many guys this Christmas I was fortunate enough to buy a GPS to give to my wife to give to me for Xmas. Since then, I've noticed that the GPS consistently shows me going 10km/h slower than my speedometer does.

Who would you trust? My gut says to trust the GPS for a couple of reasons: 1) I think it's more advanced technology than whatever drives that needle on the speedometer 2) I've always felt, since getting this car, that doing 130 km/h on the highway didn't feel like 130 km/h but I had a truck before so I chalked that up to better ride. 3) perhaps this is some sort of secret safety feature from our safety conscious friends in Sweden to slow us all down?

I need to take this in anther car as well to get a second opinion.

Does anyone else have this issue? Thanks!

[uscgc70]

I would trust the GPS more than your cars speedometer. it could be off from slightly different size tires than stock.

[colin]

I know for a fact that the speedometer on my motorcycle used to deliberately read faster than I was actually going. A good explanation for this was discussed ad infinitum on a webforum dedicated to that bike. In a nutshell it had to do with legal liability for the manufacturer and them protecting themselves from the speedometer reading slower than you were actually going. At highway speeds it was about 10 Km slower than indicated as Xchuck suggests. It actually works on a percentage so the difference varies over the whole range.
I have since replaced the stock speedometer faceplate with an after market one that gives me a much more accurate reading. Things like tire size, pressure and wear can still affect a truly accurate reading however. I still use my GPS to check from time to time to check.

[Omega5]

I used to work at a Go-Kart track when i was in highschool and as government regulation, we had to ensure that the karts NEVER exceed 35MPH. What this meant is that during the regular maintenance (once every few days) every car's governor was adjusted and hte car was clocked at 30MPH exactly. (if i had it my way, i would have taken the damn thing out and let it do 60MPH as it should have)

Point is, every time the car came in the speed was always a good 3 -5MPH slower then previously adjusted. WHY? Because the tires were actually wearing out. In out case, the wheels were only about 14" diameter overall when new (not rim diameter). When completely bald (the fibers and wire showing) they were a good 2" smaller in diameter. This resulted in the kart slowing down.

Circumference new was 2(Pi)7" when old it was 2(Pi)6" difference was ~6.3" so you lost half foot at every rotation. Meaning the car traveled ~14% slower. See the numbers? 3-5MPH less then 30MPH?? Kind of makes sense don't it?

PS. this is the first time i do the math, and I'm personally surprised it worked out this well LOL

[n00bkiller944]

my GPS shows 3 mph over my speedo

[JRL]

No argument, go by the the GPS.
My 2000 R's speedo and the odometer are 10% optimistic (fast) and there's nothing Volvo can do about it.
They asked me if I wanted to just start putting parts and sensors on as there's no other way to test it.
My other 2000 R is about 7% over.
My 06 is a little over
Most car's speedometers read over, but usually not by that much.
Usually at 60 mph they may read 62-63

[PJ810]

At 110km/h on my speedo my GPS reads 112-113km/h. I thought they deliberately overstated by about 10%. Anyway, apparantly my speedo is very very accurate.

[Jake529]

JRL,

What is the governer on these cars? I remember racing a punk a couple of years ago in a Mustang on 128 in Bedford and I think it shut off at 150, of course I was going kinda fast at the time and it was dark and I was wearing SUN GLASSES :>..... Ah racing a stank in a Soccor Moms car, memories........

Jake

[JRL]

I don't think so.
Most Volvos are 130, XC's are (I believe) 118
(New 04-07) R's are 150
Mine is unlimited
I've done an (indicated) 148 in my R, I would be scared to death if a tall XC could go that quick, wallowing around

[Omega5]

JRL: In a straight line on the free way. Why would you be scared? Assuming tight suspensions and steering you got nothing to worry about until you get to the offramp. I have not taken mine over 200 (125MPH) yet because i keep running into traffic and in Ontario they introduced a new law where if they clock you 50km/h over they impound your car and fine you UP TO 10,000$!! If you ask me that it a hell of an incentive to see if you can outrun the cops.