How does speedo work?
Thread Starter
Burning Brakes
Joined: Oct 2002
Posts: 797
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From: Sunny Earthquake Country, USA On the Left Coast (which is becoming more Right!)
How does speedo work?
I have changed my rear axle ratio to 3.90 and my tires have been changed to front 275/35/18, rear 305/35/18.
Depending where the speedo picks up its data maybe my speedo is off.
Where does speedo get data?
Can a tech II change any of the parameters used to determine speed? i.e. correct for above changes.
I have a '97 mn6 coupe.
Thank you
Roy
Depending where the speedo picks up its data maybe my speedo is off.
Where does speedo get data?
Can a tech II change any of the parameters used to determine speed? i.e. correct for above changes.
I have a '97 mn6 coupe.
Thank you
Roy
Last edited by 7.0sc SuperVette; Sep 20, 2006 at 02:43 AM.
Le Mans Master
Joined: Mar 2002
Posts: 5,955
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From: PNW
Originally Posted by 7.0sc SuperVette
Depending where the speedo picks up its data maybe my speedo is off.
Melting Slicks
Joined: Sep 2005
Posts: 2,625
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From: Rhode Island
Not sure where it picks up it's signal, but you only need to have your gear ratio changed on the cars computer. I went from 2.73's to 3.42's and not only would the speedometer have been off, but being that my car is an automatic, my shift points would have been too high. Any tuner who works on Vettes can do the change for you, only takes about 10 minutes, then a quick road test.
Le Mans Master
Joined: Sep 2004
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From: Los Angeles
Originally Posted by cbx1047cc
Not sure where it picks up it's signal, but you only need to have your gear ratio changed on the cars computer. I went from 2.73's to 3.42's and not only would the speedometer have been off, but being that my car is an automatic, my shift points would have been too high. Any tuner who works on Vettes can do the change for you, only takes about 10 minutes, then a quick road test.
6000rpm is 6000rpm no matter what gear is in the car.
Team Owner
Joined: Apr 2001
Posts: 54,204
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From: S.W. Ohio. . . . . . NRA Life Member
Originally Posted by robvuk
Why would your shift points change???? 6000rpm is 6000rpm no matter what gear is in the car.
6000rpm is 6000rpm no matter what gear is in the car.Ths speedo gets pulses from a reluctor wheel attached to one of the output shafts inside the carrier (diff). Rear axle ratio changes will not alter the speedo accuracy on a C5... period - end of story.
But rear tire heigth changes will
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Le Mans Master
Joined: Sep 2004
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From: Los Angeles
Originally Posted by Mike Mercury
Because on the A4, WOT shifting must meet RPM and MPH thresholds.
Thread Starter
Burning Brakes
Joined: Oct 2002
Posts: 797
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From: Sunny Earthquake Country, USA On the Left Coast (which is becoming more Right!)
Thanks forum members...
It seems like the output shaft pick up is it.
So I can simply prorate my mileage by the ratio of new tire height by original tire height.
Looks like a wrap-thanks again guys!
Roy
So I can simply prorate my mileage by the ratio of new tire height by original tire height.
Looks like a wrap-thanks again guys!
Roy
Le Mans Master
Joined: Dec 2003
Posts: 5,561
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From: Atlanta metro Ga.
Originally Posted by 7.0sc SuperVette
It seems like the output shaft pick up is it.
So I can simply prorate my mileage by the ratio of new tire height by original tire height.
Looks like a wrap-thanks again guys!
Roy
So I can simply prorate my mileage by the ratio of new tire height by original tire height.
Looks like a wrap-thanks again guys!
Roy
An interesting point; when I swapped out my standard C5 tires for Z06 sizes (approx. 5% reduction in rolling diameter), I contemplated changing the multiplier to compensate. But when I looked at the numbers, backed up by tachymeter measurements with mile markers, the change from stock to Z06 sizes brought the accuracy of the speedo perfectly in-line, that is 60 MPH on the road registered as exactly 60 MPH on the display. The reason this occurs is by default the factories (all of them) bias the speedometer calibration on the high-side, so as to avoid liabilities / litigation. So ultimately I decided to leave the stock values in place, comfortable in the knowledge I have a highly accurate speedometer calibration.: cheers: