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Discussion Starter #1
Maybe I'm having a brain fart- I don't get it. I have a manual transmission 6i, for those of you who haven't ever visited this board before :p

When it was new, I curiously wondered what the engine hummed along at on the highway. I found the tach crossed over to 3000rpms right around 77-78mph.

I also was surprised to find the speedometer reading perfectly- cruising at 60mph gave me a mile marker exactly every minute, and on a seperate occasion a static police radar sign informed me that I was going 30mph just as the tach said. It's nice to know it's accurate.

But now this-

As the tach goes past 3000rpm, the speedometer is only reading 73-74mph. Why the change? The tach and speedo are electical not mechanical- could cold weather somehow effect the magnetism or electric current?

I originally was thinking it was lower tire pressure due to falling temperatures, but that can't be it- that would cause the speedometer to read incorrectly in relation to my actual speed, not in relation to the tach.

Again, this is with a manual tranny. The engine seems to idle at 700rpms once warmed up, just as it always has. The tach sounds like its reading correctly. The clutch definately isn't slipping- it grabs nicely. What gives?

Can I get my speedo recalibrated under warranty?
 

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Reading Topic: Have

Well, just because the speedo and tach have drifted in relation to each other doesn't mean the speedo is out of spec. You need to find one of those signs again and check your speed against that. Remember that the tach is electronic, not mechanical. There is signal dampening that will make the tach not exactly accurate. It's a driver's tool that is more or less at the level of fidelity required to serve its purpose. You can plug a scanner (if you have access to one) into the port and see what the true engine speed is (as far as the ECU is concerned). This will show you how far off your tach lags.
 

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Replying to Topic 'Have fun explaining this one'

The speed sing you see on the highways and back roads that townships may have have a flaw that does not usely come up. I know prople that have road bikes that have used them for speed test and when you max the system out it make it post funny speeds. The one prson I know on abike maxe one out on RT. 74 and it read about 10 mph to slow untill the cops came out and reset it. Also jut to let you know some of the ones that the state of PA own can take a pic of your tag when you pass it at a high speed. I was on the news one day. They just send you the fine in the mail!!!!!!! That sucks:)
 

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Replying to Topic 'Have fun explaining this one'

QUOTE
Originally posted by DIRT RACER


            Also jut to let you know some of the ones that the state of PA own can take a pic of your tag when you pass it at a high speed.  I was on the news one day.  They just send you the fine in the mail!!!!!!!  That sucks:)[/b]
Crap! I dodn't know that. I usually speed up when I see them to see how high I can get it to read.:D
 

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Replying to Topic 'Have fun explaining this one'

Yeah they do that in Illinois too... I know there was an ex state trooper somewhere that used to sell semi reflective plate covers....if one of them tried to take a picture of your plate...the lights on the radar would cause the image of your plate number to be unreadable at a certain angle...
 

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Replying to Topic 'Have fun explaining this one'

Tell me about it - we had "Photo Radar" for a bunch of years and then we voted the communist's (figuratively speaking) out. The new provincial government removed the photo radar campaign much to the pleasure of the local citizenry. :)
 

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Reading Topic: Have fun explaining this one

there can be many factors.... tire wear, clutch slippage, slope of the road, battery voltage, tire pressure, anything which may cause more rolling resistance... head wind, lots of things.

Remember that the gauges in a production vehicle ONLY have to meet a certain degree of accuracy +/-

The best way would be to get yourself a gps and do a few runs on the same exact road with the same exact weather conditions. The other way is to make a bunch of test and come up with an average.

You'll never get 100% accuracy, there are to many variables.
 

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Reading Topic: Have fun explaining this one

I think your tach and spedometer is fine. I think it is the cold weather, which then causes the tire pressure to drop. Therefore, the engine has to work harder (higher revs) to reach the same speed.
 

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Reading Topic: Have fun explaining this one

No, no, no.

It can't be tire pressure. Tire pressure would cause the speedometer to be innacurate, but that's not the problem.

The problem is that the tach and speedo have changed in relation to each other, which has nothing to do with the car's actual speed. I have no idea how THAT could have changed!
 

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Reading Topic: Have fun explaining this one

I think its your flux capacitor, or your balistics inhibitor. Make sure your flux capacitor reads exactly 133ozsd at 3000 rpms
 

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Reading Topic: Have fun explaining this one

"Great Scott!" I'll need some plutonium or a bolt of lightning to fix this.
 

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Reading Topic: Have fun explaining this one

Personally, I think you are just being overly sensitive to the car's behavior. Obviously, the difference of 5 mph in relation to your tach, neither of which are dead on accurate instruments, shouldn't make too much of a difference. The only way to really figuire out if there was a difference would be to eliminate all of the variables, such as the temperature, weather, barometric pressure, wind etc. I don't think there is a problem with the car at all, and it is impressive that you actually noticed such a minute difference. So I'd say don't lose any sleep over it.
 

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Replying to Topic 'Have fun explaining this one'

Yes it could be something in the Flux Compassitor that is holding back the power from reaching 1.21 jiggawats.:D :sarc
 

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Reading Topic: Reading Topic: Have fun explaining this one

not to be to retentive... but, tire pressure can be a factor... if the diameter of the tire were to be reduced (from less tire pressure) , then the tire must rotate more to obtain the same speed. This requires the engines crank shaft to rotate faster to obtain the speed.
 

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Replying to Topic 'Have fun explaining this one'

QUOTE
Originally posted by Commuta_Busa

            not to be to retentive... but, tire pressure can be a factor... if the  diameter of the tire were to be reduced (from less tire pressure) , then the tire must rotate more to obtain the same speed. This requires the engines crank shaft to rotate faster to obtain the speed.[/b]
That would change the inaccuracy of the speedometer, but the speedometer should always read the same in relation to the engine RPMs. In my case it is not.

If my tires were completely flat and I revved the engine to 3000rpm in 5th gear (ouch); my car (regardless of how fast I actually am going) should think I'm going almost 80mph, but in actuality I'd probably only be going 60mph.

In my case, I rev to 3000rpms, so the car should think it's going 78mph, but it does not. It thinks its going 74mph. Regardless of what my actual speed is, the speed it thinks the car is going should always be directly related to my RPMs and gearing.
 
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