Check your speedometer

[Mr. Left]

I have the 2009 Mazda 5 GT for 3 weeks now and today I compared the speed indicated on my GPS to the one shown by the speedometer. Looks like the speedometer indicates the car is going 105 km/h when the GPs shows only 100 km/h. 5% is a lot, I will rake the milleage on the odometer faster than normal, with all the issues that come from this: depreciation, out of warranty faster...

Please check your readings and let me know if you experience the same problem.

 

[x10dude]

I have the 2009 Mazda 5 GT for 3 weeks now and today I compared the speed indicated on my GPS to the one shown by the speedometer. Looks like the speedometer indicates the car is going 105 km/h when the GPs shows only 100 km/h. 5% is a lot, I will rake the milleage on the odometer faster than normal, with all the issues that come from this: depreciation, out of warranty faster...

Please check your readings and let me know if you experience the same problem.

Why are you so quick to assume that the speedometer is off and not the GPS? I don't know what type of GPS you have so it's hard to determine but many cheaper GPS units are well known to be "off" by as much as 5-10% depending on the number satellites and also atmospheric conditions.

I'm not saying that your speedometer is correct either, but just don't be so quick to assume that your GPS is more accurate.

 

[Heat]

Hmm... I'll have to check mine against the mile markers on the interstate (United States). I know that my '99 Honda Accord reads 3% to 4% high, I assumed my '09 Mazda5 was OK...

I'll check it out on my next road trip. And yes, those mile markers are accurate.

 

[DKaz]

What is your tire size?

 

[x10dude]

Hmm... I'll have to check mine against the mile markers on the interstate (United States). I know that my '99 Honda Accord reads 3% to 4% high, I assumed my '09 Mazda5 was OK...

I'll check it out on my next road trip. And yes, those mile markers are accurate.

Mile markers and a stopwatch is probably the cheapest (but more accurate) way to check your speedometer. I wouldn't rely on a GPS unit.

 

[Heat]

Mile markers and a stopwatch is probably the cheapest (but more accurate) way to check your speedometer. I wouldn't rely on a GPS unit.

I agree - I was talking about using a stopwatch with the mile markers.

As far as a GPS unit - heck, I don't even own one. Don't need one. Don't want one.

 

[PATMAN65]

My '08 sedan w/ factory 17's seems to register about 3 mph. faster than the 'radar guns' that Massachusetts finest sets up near school zones..pretty consistently.

 

[SGT_OKINAWA]

My '08 sedan w/ factory 17's seems to register about 3 mph. faster than the 'radar guns' that Massachusetts finest sets up near school zones..pretty consistently.

I was going to say that, find a speed trap and drive by it a few times in two kids of cars.


OR


Go to a dyno garage (You know that big drum in the floor they
use to test Horse Power, that is 100 accurate) and they give you
a neat little print out. But it cost $$



Hope that helps ! (headbang)

Peace!(rei)

 

[x10dude]

A stand-alone speed trap might not be 100% accurate because the don't always calibrate them precisely. They are there just to get people's attention and let them know how fast they are generally going.

But a police officer with a radar gun is fairly accurate because they have calibrate them all the time. The speeds they record have to stand up in court.

Again the cheapest are a mile markers and a stop watch...

 

[KBrian]

Last road trip, I took note of the mile markers and they arrived exactly as expected, over a 20km side road, the odometer matched the signs perfectly.

BTW, perhaps getting a speeding ticket just to confirm the accuracy, or lack thereof, of your speedo/odo seems an extreme solution to a minor irritation IMHO.

 

[AwaKeN]

I have the same reading with my GPS, my car show me 105 km/h but the GPS show me 100 km/h !

 

[Mr. Left]

Thanks for all your suggestions.

I checked again yesterday against 2 high end GPS units and got the same result. The speed difference is 4-5%.

I will check again today the odometer for a known distance. I am not that much concerned with the speedometer accuracy or lack therof, as much as the odometer accuracy.

BTW, I have the stock wheels and tires.

 

[Saluki]

As far as a GPS unit - heck, I don't even own one. Don't need one. Don't want one.

. . . as written by someone who has never used one.

 

[OldGreyMike]

Friend of mine is a cop, I will get him to check mine against his radar gun sometime soon (before I put on the winter wheels). Have the stock 17s on my 5T.

 

[Phratt]

Hi Everyone,

I have had this same concern with my last car.

Had a policeman (he was already doing radar duty) check out my speed. I stopped and asked, he accepted. I had a GPS, went 100Km/h on the GPS with cruise control on a flat / straight part of the highway, the speedometer said 105Km/h. I did this twice.

The only thing that the policeman recommended was NOT to use an emergency vehicle U-turn path to do a third run (uhm)

But seriously - he explained to me that the consumer type GPS's are not perfect instruments to measure speed/distance.

First, for national security purposes, they do intentionally have a 5 to 10% error.

He also added that 5 to 10% error is proper if we take into consideration the distance between space and earth, combined with the vehicle moving, the speed of light delay (300 000 Km per second if I remember correctly).

Hope this information will help you out as it has helped me out. Now I know that the difference in GPS units is the size of the screen and the gadgets. The governments will not allow more precision for these units (national security).