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I was leaving work today, and when I passed a couple of our radar signs, I noticed that my speedometer was acting a little weird. Most of the time, the car said I was going faster than what the signs were telling me. Other times, it would be right on. Its not really an issue at all, but I was just wondering if anyone else noticed anything like this. Anybody drive by radar signs lately? Maybe it has something to do with how small the meter is. Its usually pretty close though.
 

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all auto manufacturers have the speedo show faster speeds than the actual speed. Some Jap cars are up to several % off. Some people were complaining about the situation b/c the odometer may also read more miles therefore anyone using the old fashion away of calculating MPG, would get higher MPG. The speed radar things on the side of the road, I wonder about those. Only if I'm cruising at exactly 1 speed does it seem to give a proper reading, maybe b/c there is a delay in displaying a speed I was going 1 second ago? Anyhow, I use my little GPS to see approximately how fast I'm going and how off the speedo is.
 

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I have a Toyota Matrix right now (I know, I know, an 09 WRX will be all mine in MARCH!) and it reads consistently 4 mph higher than actual (compared using a GPS). I was recently Florida and rented a car in Miami to drive to the Ft. Myers area...it was a Mazda 6. The speedo and GPS agreed 100%. Kinda irritating on a car as new as this, my Matrix is an 06, and from a manufacturer with the reputation that Toyota enjoys. I think maybe it is irritating b/c I don't really like the car...my old civic Si did the same thing, and I couldn't have cared less. I'll easily forgive a WRX :D

joneSi
 

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I noted this as well for my WRX and Honda. Now, I put my faith on the GPS speed to show the accurate speed, and not the speedometer.
 

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^^^^^^^^
I agree with that. I am a computer programmer who believes that the parameter of ground speed being read from satellite positioning is fairly precise. Though this just shows my fault of putting too much faith in computers. I'm sure that isn't perfect either. I suppose there IS a way to test (by timing how long it takes to travel a mile as indicated by road-side signs), though I'm thinking that an engineer can tell me why that doesn't work...

joneSi
 

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I suppose there IS a way to test (by timing how long it takes to travel a mile as indicated by road-side signs), though I'm thinking that an engineer can tell me why that doesn't work...

joneSi
That would just give you your average velocity over a 1-mile stretch, has nothing to do with your instantaneous velocity which will vary over the course of that mile. Good luck holding 1 speed exactly for a mile too....not gonna happen.

As for the Speedo's being off, that is REALLY odd because it should be the easiest thing on the car to measure. All it is is a 'gear' with iron teeth on it with a magnetic pick-up. Every tooth that runs by the pick-up generates a pulse. You get a frequency based on how fast the thing is pulsing (DIRECTLY related to how fast the shaft is spinning) which gets converted to MPH. Unless the resolution on the speedo-needle is god awful or your tires are extremely over/under inflated it should be correct. That's also assuming the person who programmed the constants into the frequency counter isn't a total retard.
 

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I suppose there IS a way to test (by timing how long it takes to travel a mile as indicated by road-side signs), though I'm thinking that an engineer can tell me why that doesn't work...

joneSi
That would just give you your average velocity over a 1-mile stretch, has nothing to do with your instantaneous velocity which will vary over the course of that mile. Good luck holding 1 speed exactly for a mile too....not gonna happen.

As for the Speedo's being off, that is REALLY odd because it should be the easiest thing on the car to measure. All it is is a 'gear' with iron teeth on it with a magnetic pick-up. Every tooth that runs by the pick-up generates a pulse. You get a frequency based on how fast the thing is pulsing (DIRECTLY related to how fast the shaft is spinning) which gets converted to MPH. Unless the resolution on the speedo-needle is god awful or your tires are extremely over/under inflated it should be correct. That's also assuming the person who programmed the constants into the frequency counter isn't a total retard.
it's done intentionally. plug in an obdii scanner and drive around, you'll see that the speed your ecu is reading (your actual speed) is lower than the speed your speedo is displaying. you can also test it by setting your cruise at 60 mph on the speedo but each minute that passes, you're odometer/tripometer won't increase by a full mile.
 

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My GPS and Speedo are within a mile or two at 75 mph.
 

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Doesn't that bug you that your mileage is a certain % higher than it really is? Meaning your car is depreciating faster than it really is? Maybe I'm just anal but this ticks me off a bit. :-\
 

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I suppose there IS a way to test (by timing how long it takes to travel a mile as indicated by road-side signs), though I'm thinking that an engineer can tell me why that doesn't work...

joneSi
That would just give you your average velocity over a 1-mile stretch, has nothing to do with your instantaneous velocity which will vary over the course of that mile. Good luck holding 1 speed exactly for a mile too....not gonna happen.

As for the Speedo's being off, that is REALLY odd because it should be the easiest thing on the car to measure. All it is is a 'gear' with iron teeth on it with a magnetic pick-up. Every tooth that runs by the pick-up generates a pulse. You get a frequency based on how fast the thing is pulsing (DIRECTLY related to how fast the shaft is spinning) which gets converted to MPH. Unless the resolution on the speedo-needle is god awful or your tires are extremely over/under inflated it should be correct. That's also assuming the person who programmed the constants into the frequency counter isn't a total retard.
OK, so I should have said I would try to set the cruise for 1 mile. I understand that is just the average, but it might be close ( I do this on a flat, I mean FLAT road). The tires are right, I check them every other gas fill up. I'm also not sure that the person who programmed the constants into the freq. counter wasn't a total idiot. Also, the speedo needle resolution is fine. It is just not accurate ???

At least I know I'm not alone. I told the toyota dealer, and they didn't really care. They let me know that the speedo and the reverse gear 'crunch' (that happens unless I clutch in and wait 5 seconds to go to reverse) are 'normal' and that 'they all do it'. Even though they had no other corolla/matrix 5spds on the lot. Yet another reason to punt this car. I owe no $ on it, and if I am going to 'put up' with things that just are (because the dealer says so), I'd rather have a car I thoroughly enjoy driving (ie NOT the matrix). Oh, right, and a WRX + Snow Tires = much fun in snow

joneSi
 

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Doesn't that bug you that your mileage is a certain % higher than it really is? Meaning your car is depreciating faster than it really is? Maybe I'm just anal but this ticks me off a bit. :-\
the odometer is based off of the ecu which has the correct speed, so your odometer isn't wrong. manufacturers put a slight increase in the speedometer. it's typically not off by more than a few miles per hour, but it depends on the car
 

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Discussion Starter #14
Well it's good to know that I'm not going crazy. I'm sure those radar signs aren't perfectly accurate, but like tougemonster said, it just bugs me. Plus, when I drive my truck to work, everything is right on. No big deal. When I get away from work and onto some winding roads, I'm happy again.
 
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