Speedometer Question

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meinmi
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Joined: Sat Dec 24, 2011 10:29 am

Speedometer Question

Post by meinmi »

I've had my Vibe a little over a month and really like it. I've put close to 3,000 miles on it and it runs and drives like a dream. I only have one question - as my speed increases the accuracy of the speedometer drops off. I use a Garmin GPS which shows mph; I've used this in several of my cars and numerous rentals. It is very accurate. When the GPS shows 60 mph the Vibe's is at 61 or 62 but with the GPS at 75 the car reads almost 80. This is nothing I can't live with, just wondered if it is common. The car has the correct wheels and tires.Thanks for the help.
djkeev
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Re: Speedometer Question (meinmi)

Post by djkeev »

There is probably not a production car with a 100% accurate speedometer on the highways. There is an accepted "slop" in speedo accuracy within the industry and the Government.You may have the correct size tires and wheels but are they NEW tires or OLD tires? Also there is also circumference variation by brand. A new Dunlop will be different than say a Yokohama and they may vary from a Bridgestone. Not much at all, but they vary.As tires wear, rubber is removed from the tire. When the rubber is removed, the circumference of the tire is reduced thus it will spin just a tiny bit faster than it previously did. Spinning faster = faster Speedometer. Under inflation or over inflation changes the circumference of a tire as well.A lot of variables, that's why as a rule Police allow a certain level of being able to exceed the posted limit before they pull you over.There is no ECONOMICAL way to be constantly recalibrating for this tire wear and brand variation. With today's modern electronics, sure it CAN be done, but let's go back to that word.....ECONOMICAL.Dave
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tpollauf
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Re: Speedometer Question (meinmi)

Post by tpollauf »

On the 2009-10 Vibes, at least the GT's, this is VERY common and you're pretty much right on the money with the numbers. When driving a true 40, speedo shows 42, 60 shows 63, 80 shows 84, etc ... Seeing how yours in a 2006 I can't comment on if it is common for that year OR any of the older 1st gen Vibes. My guess is that it IS ..........
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jkm311
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Re: Speedometer Question (tpollauf)

Post by jkm311 »

With 15" steel wheels and snow tires, I am experiencing what you have described. My 2006 came with 16" alloys, so right there a difference will exist. I will test this out over the next month or so. The reason being, there is a school zone that has a speed radar display on it that I pass every day. It's off by 3 MPH every time. If I'm cruising at 35, it reads 32, without fail. The interesting part is after school hours, the sign is programed to change the speed limit to 45 MPH in that zone (because of school being out, of course), but when I pass through going, say, 50 on my speedometer, it will read 47. Like clock-work. I'd be interested to know if I experience a bigger change on the highway, I bet I do. But what I will test and report back in a month or so is if my stock 16" rims and taller sidewall all-season Bridgestones yield the same 3 MPH discrepancy. I bet it regulates back closer to my speedometer.
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jake75
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Re: Speedometer Question (jkm311)

Post by jake75 »

My speedo over reads mph but under reads odometer miles. One would think if the car electronics thought it was going faster it would also think it was traveling more miles. Practical matter - when your odometer reads 35,999 you are still under warranty whereas the car actually has perhaps 36,700 miles on it. Also is you calculate that your mpg is 28.0 it is really maybe 28.7 mpg.
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skimask
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Re: Speedometer Question (jkm311)

Post by skimask »

Quote, originally posted by jkm311 »I will test this out over the next month or so. The reason being, there is a school zone that has a speed radar display on it that I pass every day. It's off by 3 MPH every time. If I'm cruising at 35, it reads 32, without fail. The interesting part is after school hours, the sign is programed to change the speed limit to 45 MPH in that zone (because of school being out, of course), but when I pass through going, say, 50 on my speedometer, it will read 47. Like clock-work. I'd be interested to know if I experience a bigger change on the highway, I bet I do.The problem with using road side speed radar displays like that is that the people that set them up generally (I say generally, not always) don't have a clue as to how to set them up. They drop them off, flip the switch, walk away, ????, profit.If the angle of the sensor isn't correct, or the sensor receives reflections, the result will be off. Period. A 5 degree offset from the direction of travel can account for roughly 8% of error, which in this case, just happens to account for most of the 3MPH difference.You can prove this to yourself...if you can get away with it Turn the unit sideways with respect to the road (90 degrees), put somebody across the street to watch the display and drive past it. It'll read ZERO MPH no matter what speed you're going (well, it might read something, but it'll be WAYYY out in the weeds).The one and ONLY time those displays are correct is if you are traveling DIRECTLY towards (or from) the sensor itself, on completely flat ground with no chance of reflections.
Old Tele man
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Post by Old Tele man »

...I took my 2009 VIbe down to local shop that calibrates/certifies our Highway Patrol speedometers, and they produced a print-out showing that my speeedometer was "consistently" high by 2 MPH between 30-80 MPH, and is something that GM is noted for doing, and this 'offset' only affects the speedo, NOT the odometer.
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jake75
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Re: (Old Tele man)

Post by jake75 »

Quote, originally posted by Old Tele man »...I took my 2009 VIbe down to local shop that calibrates/certifies our Highway Patrol speedometers, and they produced a print-out showing that my speeedometer was "consistently" high by 2 MPH between 30-80 MPH, and is something that GM is noted for doing, and this 'offset' only affects the speedo, NOT the odometer.Is it something that Toyota also is noted for doing? I assume that the Vibe speedo is a Toyota part.
2009 Vibe 1.8L Carbon Gray AT Power Pkg 1/12/092003 Vibe 1.8L Neptune AT Mono Power Pkg 1/27/03 [sold 2/2/09]2007 T&C SWB 7/31/07 "Broke people stay broke by living like they're rich. Rich people stay rich by living like they're broke."
lowincash
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Post by lowincash »

My 09 GT's speedometer used to be about 5mph faster than my actual speed when going 30-80mph. My GPS will say I'm going 70 when I'm actually going 65. I then changed to some slightly wider tires and then the difference went down to about 2-3mph off.
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meinmi
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Re: (lowincash)

Post by meinmi »

Thanks to everyone for their comments. The tires are about 80% tread so I think it is just as simple as the speedometer being a little off at higher speeds. Definitely not something to bother fixing.
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