# Does your speedometer lie to you ?



## mike dockal (Dec 20, 2004)

Mine does. I know that most manufacturers calibrate the spedometers so they would show higher speed than you are going. But isn't over ten percent little too much ? Could it be because you view the instrument panel from an angle so they are trying to compensate for that ?
I did couple of testes and it showed between 10 and 15 percent error.
Does anyone else have the same experience ?


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## Terranismo (Aug 31, 2004)

Same here, but mine reads lower. It must be because of the increased tire diameter. I never noticed it much until a friend told me I passed him while he was doing 190km/h in his 325i. :fluffpol:


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## mike dockal (Dec 20, 2004)

Terranismo said:


> Same here, but mine reads lower. It must be because of the increased tire diameter. I never noticed it much until a friend told me I passed him while he was doing 190km/h in his 325i. :fluffpol:


So that means if I increase my tire size, there is a chance it will correct itself ?


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## Terranismo (Aug 31, 2004)

Not really. Spedos are generally made to have between a 10 and 15% discrepancy with real speed. It's a safety issue I guess.


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## thetimster (May 6, 2005)

That's my problem now... my tires are bigger than stock.. so my speedometer reads lower than actualy speed. I'm looking at different 20 rims with more proper offset (my wheels bulge a bit out of the fender line). But that would necessitate smaller tires. When that's installed, it'll look like I'm going faster than the actual speed. Another bad thing is that the odometer will spin faster and give rise to falsely higher mileage. Oh well... this car's a keeper for now anyway. So that's less of an issue. :thumbup:


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## X-Traction (Dec 21, 2004)

Probably the best way to check your speed is with a gps.


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## aussietrail (Sep 24, 2004)

X-Traction said:


> Probably the best way to check your speed is with a gps.


GPS reading and am sorry to say this, is the most unreliable way to check your speed. (I've tried it and was amzed at the difference)

The best way I found to check my speedo was using the highway distance markers, not sure if these are available everywhere, but in Australia when hit the highway, every hour or so you get these markers starting at 1km and goes all the way to 5kms.

Zero your trip meter when you get to the 1km mark and then check it again when you pass the 5km mark, if there is a difference in reading (i.e. greater or less than 5km's) on your trip meter, you would know how much your speedo is out by.


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## thetimster (May 6, 2005)

Best way is to use a radar detector. Just drive by a cop fast... you'll know very quickly if your reading is accurate.  I don't know if it's worth the ticket though. From what I gather from previous research when I got caught speeding and needed to go to traffic court, those devices are accurate to within a few kph. So assuming you're going at a steady speed and not accelerating or decelerating while you're scanned, it should give you a good idea whether your speedometer is fooling you. And no, after my research I didn't argue my case anymore. Seemed kinda fool proof to me. Just paid the fine and moved on.


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## AlexP (Feb 24, 2005)

aussietrail said:


> GPS reading and am sorry to say this, is the most unreliable way to check your speed. (I've tried it and was amzed at the difference)
> 
> The best way I found to check my speedo was using the highway distance markers, not sure if these are available everywhere, but in Australia when hit the highway, every hour or so you get these markers starting at 1km and goes all the way to 5kms.
> 
> Zero your trip meter when you get to the 1km mark and then check it again when you pass the 5km mark, if there is a difference in reading (i.e. greater or less than 5km's) on your trip meter, you would know how much your speedo is out by.


There are mileage (kilometrage?) markers on a good number of secondary roads in Ontario (and probably the rest of Canada). This is a good way to check your odometer, but I wouldn't assume that the odometer and speedometer are synchronized. Most motorcycle odometers, for example, are very accurate but the speedometers tend to read 5 to 10 percent high even though they're both driven off the same cable or sensor. I'd suggest having a passenger run a stopwatch while you keep a steady speed between markers to test the speedo...

Al


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## X-Traction (Dec 21, 2004)

I only mentioned the gps idea because a friend was using one in our other car one day, and commented the speedometer was wrong. Investigation revealed the gear that takes the speed off the transmission had the wrong number of teeth. Now it corresponds exactly. Gps is, after all, used for precision navigation for all sorts of things, but I know that a simple system won't be accurate if you're changing direction a lot.

Can't argue with the distance post method, though.


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