Anyone else ever notice their speedometer tend to run about 5mph over actual speed at highway speeds? (e.g. gps showing 75mph, but speedo showing 80)

My Jetta was the same way. I always assumed a conspiracy to end warranties quicker and devalue used cars.

A long while back I asked my shop about it and they said they can't recalibrate it...

 

Ever think your GPS might be off? Are you running stock wheel/tire size from the OEM brand? At the recommended PSI? How old are your tires?

There are about 4-5 things I can think of that would affect the rolling diameter of your wheel/tire combo, each of which will have an affect on your speedo, good/bad... Park rhe conspiracy theory and put it up on bricks in the corner of the shop, nevr to be driven in public, again.

 

heh heh...oh the conspiracy theory was a bit tongue in cheek, but I haven't really done the math to see how substantial a differece of 5mph at highway speed would make your odo miles miss actual...

(wait I just did, double check me, I was an art major
...40,000 miles at 75 mph = 533 hours, x 5 mph against = 2,666 additional miles on the odo versus actual travelled)

Certainly this math is simple and I've not gone 75mph for every life of the car, some calculus should be able to figure it more exact....but 2,600 miles is not an insignificant figure.

Everything is stock the way it came, ...got brand new Michelin on it now, factory recommended sized, proper inflation.

In the case of the Jetta, I actually went 10mm wider (with the same profile) with the tires (on factory wheels), which should have made the speedo run slower...but really the difference was negligible. I can't imagine any reasonable amount of over-inflation going to have much of an impact either.

Can GPS be off??? I would think that would be the standard by which any independent mechanical devise would be tested against...? I really have no idea... I'll set both my Garmin and my phone up next time I'm riding and see if there's any discrepancy.

So back to conspiracy... Why would a dealer shop refuse to re-calibrate this?

 

I was pretty sure (about 105%) your consp. theory was punintended.

As far as the GPS goes, it's got "a little off" built into it. GPS is a Military invention. Satellites encircle the Earth and your device "polls" up to 4 of them, getting a two dimensional (longitude & latitude) "fix" on your location. Accurate to within about 100 meters (approx 1 football field). The approximation error is mathematically built into your device because it is NOT a Military GPS receiver. Your device takes those 4 approximations and spits out a location accurate to within 20 - 50 meters.

Military GPS accesses up to 12 GPS satellites, and can provide accuracy to within inches, in 3 dimensions, depending on how many satellites the device can "see" at any given moment.

As far as your dealership not wanting to re-calibrate? I have three possible theories:

1) They're too stupit to pull it off.
2) They don't want your money, since they already have more than enough to last them through the end of the year.
3) The Government has given them SPECIFIC instructions NOT to re-calibrate YOUR speedometer, because of something your Mother said, last year, that was picked up by one of our urban camouflaged surveillance monitoring vehicles (looks JUST LIKE an ordinary ice-cream truck... why do you think they keep coming around 15-20 times/day?) that happened to be driving through your neighborhood at that precise moment and was passed on to authorities who have been tracking you EVERY SECOND (after all GPS IS a two-way satellite transceiver, you know) since then.


Perfect paranoia is perfect awareness!!!

 

lol @ the dealer reasons. and thanks for the info on the GPS, I was unaware

Interestingly enough, I suspect a combination of reasons 1 and 2

All in all, a minor thing, just wondering if anyone else had similar experience

 

Prolly teh biggest reasons they wont hafta do with aftermarket wheels. You dint say if you were rollin OEMs, or not. Going +1 or +2 on your wheels, and/or rollin non-OEM rubber can throw off your speedo (goto: http://www.miata.net/garage/tirecalcold.html and play with teh numbers and you'll see what I'm referring to). Anything they do'd prolly be undone @ your next tire change/upgrade. still, I'd think they'd WANT to solve for X, and encourage you to come back with each l'il problem. Maybe even offer to rotate teh air in your tires, for good measure (you DO know the stock wheel/tire package is scented, right? R/F: Cherry, L/F: Lemon, R/R: Chocolate, L/R: Skankberry).

 

they all smell the same to me

yeh, stock package, optional 18's 245/40 what came on them, I assume stock. Thanks for the calculator link, I might fool around with that - way back in high school, me and this other car junkie used to calculate all kinds of wheel/tire sizes and the effect on speedo and gearing ratios instead of listening to our calc teacher. nerds

 

In the u.s. auto manufactures are allowed to have speedometers that show lower mph but not higher then actual speed. VW/Audi is err-ing on the side of caution, they don't want to put new speedos in 5 million cars. just think of all the speeding tickets you didn't get because of it. I stepped up from 215/55/16's to 215/60/16's and my speedo is dead-on @ 60 mph