If my speedo is 10% off is my odo also 10% off?

[alf]

I have about a 10% variance on my speedo (faster than actual), curious if my odo is also off by that amount.

alf

[Vipergrün]

If you drive in reverse, does that subtract miles? (Thinking Ferris Bueller)

[alf]

But would it be 10% more or less to the odo in reverse if they are linked?

Seriously...

[Bob's Flat-Six]

I would think it would be.

[87coupe]

I have seen a 911 that the speedo was not working but the odometer still worked, this leads me to believe they are independent of each other. Maybe someone can confirm this.

[Fishcop]

My guess would be "Yes".

Odo is driven by either a cable or signal from the g/b. If you're running smaller diameter wheels/lower profile tyres, then the signal being sent to the odo is going to "sped up". I can't see the signal/cable rpm not effectng both mechanisms...

[jaimekop]

My answer is maybe, and probably not. At least mechanical speedos (with a cable) have a direct connection to the odometer (fixed gearing) and a magnetic drive for the needle.

I've never been in a electronic speedo, but I'd guess that the needle is driven a similar way and can have variations over time (electromagnet with a spring?). The odometer, on the other hand, is most likely still direct drive.

Best way to figure it out is just compare highway markers... or take it apart and see how it works! 10% is a lot, you should be able to figure it out.

Jaime
'88 Targa
and some mercedes, with old speedos.

[RoninLB]

Originally posted by alf

I have about a 10% variance on my speedo (faster than actual), curious if my odo is also off by that amount.

Not in my car after 100k mi.

And not in my car after I adjusted the speedo pot to correct speed and the odo was off same amount as before, 3%.

[Ray 911s]

You didn't mention what year - is your speedo mechanical or electric?

I only have half of an answer, since I know nothing of the electric jobbies. On the mechanical type that have a flex cable running from the gearbox to the gage - absolutely your speed and distance measurements are directly linked.

There used to be a device called a "half-miler" which was a gear reduction coupler that could be installed between the gearbox and flex shaft. It reduced the odometer readings by 50% - but you had to remember that your speedo was also reduced by 50% so a 25mph reading meant you were travelling at 50mph.

Again, I know nothing of the newer devices that do not utilize a flex shaft so I would not infer anything about the link between mph and miles on those devices.

[Rot 911]

The speedo on my '86 also was reading off by 10% as tested with my GPS. However the odometer read correctly. I had the speedo recalibrated by North Hollywood Speedometer and now it is accurate.

[rick-l]

Originally posted by alf
I have about a 10% variance on my speedo (faster than actual), curious if my odo is also off by that amount.

alf

If x is the distance traveled the indicator on your dash tells you (outputs) x (odometer) and dx/dt (speedometer). The input is x in the form of pulses (or rotation) from the transmission.

I would suspect that since x is easily calculated and recorded there would be no adjustment to the odometer. Computing the derivative however is not so straightforward. I would suspect this (SPEEDO) could have a calibration adjustment.

If the input is wrong both outputs will be afffected but the speedometer could be calibrated correctly. Likewise if the odometer was correct the speedometer could be calibrated incorrectly

[safe]

Most speedometers shows 5-10% more than the actual speed, they are calibrated that way.
I don't think that will affect your odo, but if you speedo is off because you have got smaller or bigger wheels, that MUST affect the odo.

[Mysterytrain]

From the pictures I've seen it looks like the speedo and the odie have their own stepper motors..so in theory one could put a little switch on the odie motor and still have the speedo working.

[juanbenae]

Originally posted by Fishcop
My guess would be "Yes".

Odo is driven by either a cable or signal from the g/b. If you're running smaller diameter wheels/lower profile tyres, then the signal being sent to the odo is going to "sped up". I can't see the signal/cable rpm not effectng both mechanisms...

i had a truck do the same thing, even the 10% number when i installed bigger tires. mine with the bigger tires was 10% lower. did your car come stock with 15" rims and you switch to 16"?

[alf]

88, stock cir on the wheels and tires.

I could live with the calibration offset if the odo is not off as well, seems that my tach is also about 10% off. Wonder if my clock is

[juanbenae]

here is a tire/rim height deal that may give you an idea if you have upsided the tires.

[rick-l]

Originally posted by alf

I could live with the calibration offset if the odo is not off as well, seems that my tach is also about 10% off. Wonder if my clock is

You could measure it against the mile markers on the interstate.
Anyone know how accurate they are?

[Zef]

No...for sure.They work independantly.

[safe]

Originally posted by zefsuper911
No...for sure.They work independently.

Not if you have altered the wheel diameter...