Is speedo/odometer Is speedo/odometer based on the front wheel?

dekaliber · 05-13-2010, 08:23 AM

Probably a n00b question, but I imagine it would have to be, otherwise when you get wheelspin in the back the speedometer would be totally off. Can anyone confirm?

Just curious whether it's more important to match the diameter of the factory setup on the front or the back.

inTgr8r · 05-13-2010, 09:22 AM

IDK exactly with BMW but most cars pick it up somewhere on the drivetain.
So, it's the rears that matter.

jragan · 05-13-2010, 09:54 AM

Keep in mind that the Odometer is not actually tied to the spedometer.

The car's internal computer "knows" what the speed is, and uses this to determine MPG and Odometer functions, however what's displayed by the spedometer is different from this "actual" value and is programmed to read off by ~5% faster. The only way to see the "actual" value is to put the instrument cluster into "test" mode and convert Km/h on the fly.

dekaliber · 05-13-2010, 01:04 PM

Quote:

Originally Posted by jragan

The car's internal computer "knows" what the speed is, and uses this to determine MPG and Odometer functions, however what's displayed by the spedometer is different from this "actual" value and is programmed to read off by ~5% faster. The only way to see the "actual" value is to put the instrument cluster into "test" mode and convert Km/h on the fly.

But how does it calculate the speed? It either has to based off of a gyro or some sort of rotation (Wheels/driveshaft/flywheel) -- all of which are connected and influenced by the size of the tire/wheel setup. I guess I was just wondering which one, or both. If you suspended the car in midair and stepped on the accelerator, would the odometer or speedometer increase, assuming the front wheel was kept still? Conversely, if only the front wheels were rolling and the back wheels were still, would the odometer or speedometer increase?

05-13-2010, 08:23 AM	#1
dekaliber Major 75 Rep 1,143 Posts Drives: '07 Z4MC, '11 JCW, '18 Z06 Join Date: Mar 2008 Location: Los Angeles iTrader: (1)	Is speedo/odometer Is speedo/odometer based on the front wheel? Probably a n00b question, but I imagine it would have to be, otherwise when you get wheelspin in the back the speedometer would be totally off. Can anyone confirm? Just curious whether it's more important to match the diameter of the factory setup on the front or the back.
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05-13-2010, 09:22 AM	#2
inTgr8r Lieutenant General 2426 Rep 11,667 Posts Drives: M2 Comp Join Date: Mar 2006 Location: Toronto iTrader: (10)	IDK exactly with BMW but most cars pick it up somewhere on the drivetain. So, it's the rears that matter. __________________ '21 M2C Hockenheim Silver '18 718 Cayman S Lava Orange (sold) '13 E92 M3 Santorini Blue (sold) '07 Z4 M Coupe Alpine White (sold)
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05-13-2010, 09:54 AM	#3
jragan Imola Lover 26 Rep 749 Posts Drives: 2007 Imola Red M Coupe Join Date: Jul 2008 Location: Fort Worth, Texas iTrader: (0)	Keep in mind that the Odometer is not actually tied to the spedometer. The car's internal computer "knows" what the speed is, and uses this to determine MPG and Odometer functions, however what's displayed by the spedometer is different from this "actual" value and is programmed to read off by ~5% faster. The only way to see the "actual" value is to put the instrument cluster into "test" mode and convert Km/h on the fly. __________________ His: RIP-"Ruby" *E86 ///M Coupe: Pics Track Pics Short Vids Long Vids Hers: 997-1 C4S*: Pics
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