Does the speedometer calculate speed off the front wheels or the rear?

LarThaL · 07-30-2011, 10:21 AM

Basically, I'm trying to figure out which tire diameter is crucial to keep speedometer readings accurate.

Also, how much can you change the relative tire diameters from stock before it messes up the stability functions, brake anti-lock, etc.

The stock rear is 668mm and the stock front is 654mm. 14mm or 2.5% difference.

TheRealOrosie · 07-30-2011, 11:37 AM

front

Rob@StrasseWheels · 07-30-2011, 11:46 AM

Well the speed that the speedometer is read is measured by revolutions the speed sensor in the transmission picks up coming from the output shaft... That is all i know haha but you can use this chart to measure overall diameter between tire sizes!

http://www.miata.net/garage/tirecalc.html

m3keash · 07-30-2011, 12:07 PM

Quote:

Originally Posted by Rob@StrasseForged

Well the speed that the speedometer is read is measured by revolutions the speed sensor in the transmission picks up coming from the output shaft... That is all i know haha but you can use this chart to measure overall diameter between tire sizes!

http://www.miata.net/garage/tirecalc.html

Great calculator.

gthal · 07-30-2011, 12:08 PM

I would think it HAS to be the rear because the speedometer moves on a dyno (if memory serves me) and it would do that unless the speed is measured from the rear.

FVM3 · 07-30-2011, 03:15 PM

should be from the rear, dyno explains it

m3keash · 07-30-2011, 05:51 PM

Now I've got to recalculate.

JCtx · 07-30-2011, 07:03 PM

Speed sensor is in the tranny indeed; therefore rear tires affect speedo. Just ordered 255/275-35-19 tires, so error should decrease a little after that

.

pmeloche · 07-30-2011, 07:29 PM

The wheels are not the same size.

ase93 · 07-30-2011, 07:58 PM

It has to be rear because when you burn out the speedo goes up pretty fast

Mace · 07-30-2011, 09:28 PM

As others have said it's measured at the transmission not at the wheels. When it comes to changes due to wheel diameters only the rear tire diameters matter because they're the drive wheels.

elm3 · 07-30-2011, 09:36 PM

[QUOTE=Mace;10131128]As others have said it's measured at the transmission not at the wheels. When it comes to changes due to wheel diameters only the rear tire diameters matter because they're the drive wheels.[/QUOTE

+1 there is a speed sensor on the tranny.

07-30-2011, 10:21 AM	#1
LarThaL Colonel 336 Rep 2,940 Posts Drives: 2011 M3 Join Date: Apr 2011 Location: West Springfield, MA iTrader: (0)	Does the speedometer calculate speed off the front wheels or the rear? Basically, I'm trying to figure out which tire diameter is crucial to keep speedometer readings accurate. Also, how much can you change the relative tire diameters from stock before it messes up the stability functions, brake anti-lock, etc. The stock rear is 668mm and the stock front is 654mm. 14mm or 2.5% difference. __________________ **Audi S6 Audi S3 * Porsche Cayman GTS* --Former BMW M3 owner**
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07-30-2011, 11:37 AM	#2
TheRealOrosie BMW Fan Boy 446 Rep 1,385 Posts Drives: //M Join Date: Apr 2010 Location: Chapel Hill, NC iTrader: (0)	front
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07-30-2011, 11:46 AM	#3
Rob@StrasseWheels 461 Rep 2,586 Posts Drives: 2023 BMW G80 M3 Comp Join Date: Jul 2011 Location: South Florida iTrader: (1)	Well the speed that the speedometer is read is measured by revolutions the speed sensor in the transmission picks up coming from the output shaft... That is all i know haha but you can use this chart to measure overall diameter between tire sizes! http://www.miata.net/garage/tirecalc.html __________________ @Strasse_Robert \| (786)701-3649 \| Robert@StrasseWheels.com \| www.StrasseWheels.com
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07-30-2011, 12:08 PM	#5
gthal Major General 1904 Rep 5,678 Posts Drives: 2018 340i xDrive Join Date: Jul 2007 Location: Canada iTrader: (2)	I would think it HAS to be the rear because the speedometer moves on a dyno (if memory serves me) and it would do that unless the speed is measured from the rear. __________________ 2020 X3 M40i \| Black \| Current DD 2020 C8 Corvette \| Z51 \| Torch Red ... built and waiting for delivery 2016 M2 \| Long Beach Blue \| 6MT 2015 M4 \| Austin Yellow \| DCT 2012 MB C63AMG \| 2011 E92 M3 \| 2010 E92 M3
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07-30-2011, 03:15 PM	#6
FVM3 Major General 273 Rep 5,751 Posts Drives: e92 m3, f30 328i Join Date: Sep 2010 Location: Los Angeles iTrader: (2)	should be from the rear, dyno explains it
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07-30-2011, 05:51 PM	#7
m3keash Second Lieutenant 7 Rep 243 Posts Drives: E93, JB/Bamboo, Adv.1 8.3 20s Join Date: Mar 2011 Location: NC iTrader: (0)	Now I've got to recalculate. __________________ JB, Bamboo, every option, 20" Adv.1
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07-30-2011, 07:03 PM	#8
JCtx Major General 258 Rep 5,012 Posts Drives: No BMW yet Join Date: May 2008 Location: El Paso TX iTrader: (0)	Speed sensor is in the tranny indeed; therefore rear tires affect speedo. Just ordered 255/275-35-19 tires, so error should decrease a little after that .
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07-30-2011, 07:29 PM	#9
pmeloche Lieutenant Colonel 24 Rep 1,604 Posts Drives: 2013 F25 X3 xDrive35i Join Date: Feb 2008 Location: Montréal, Canada iTrader: (0)	The wheels are not the same size. __________________ Patrice 2013 F25 X3 xDrive35i Vermillion Red
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07-30-2011, 07:58 PM	#10
ase93 ASE93 18 Rep 80 Posts Drives: E93 M3 Join Date: Dec 2010 Location: SW Florida iTrader: (0)	It has to be rear because when you burn out the speedo goes up pretty fast __________________ E93/ 6MT/ Fabspeed Catback/ Fabspeed X-Pipe/ Dinan Intake/ Active Autowerke Tune/ Hamann Rear Diffuser/ Chrome black Kidneys/ K40 with laser Jammer/ 15% tint
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07-30-2011, 09:28 PM	#11
Mace Second Lieutenant 2 Rep 239 Posts Drives: '11 E93 M3 Join Date: Mar 2007 Location: FL iTrader: (0)	As others have said it's measured at the transmission not at the wheels. When it comes to changes due to wheel diameters only the rear tire diameters matter because they're the drive wheels. __________________ GONE: '08 335i Coupe, Space Gray, Coral Red, Gray Poplar, 6-MT, ZSP, ZPP, ZCW, 19" Style 230 Wheels, Nav, CA, PDS, Sat, HD, IPod. ED 10 May '08. NEW ARRIVAL: '11 M3 Convertible, Mineral White, Fox Red, Extended Leather, Carbon Leather, 6-MT, 19" Style 220M Wheels, EDC, EPS, ZP2, ZCV, ZCW. ED 29 Apr '11
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07-30-2011, 09:36 PM	#12
elm3 ///M Fan 366 Rep 2,159 Posts Drives: '09 E92 M3 IB/'17 f85 X5M LBB Join Date: Sep 2008 Location: Canton, GA. iTrader: (1) Garage List 2017 BMW / X5M [9.00] 2009 E92 M3 [10.00] 2004 Yamaha R1 [10.00]	[QUOTE=Mace;10131128]As others have said it's measured at the transmission not at the wheels. When it comes to changes due to wheel diameters only the rear tire diameters matter because they're the drive wheels.[/QUOTE +1 there is a speed sensor on the tranny. __________________ '09 E92 ///M3 Interlagos Blue,6MT,Silver Novillo Leather,CF Trim,Premium Package,Technology Package,Cold Weather Package,Premium Sound,19"Wheels,Moonroof-Toy/'17 X5M, Long Beach Blue, Executive Package,Technology Package, Lighting Package, Driver Assist, Full Mughello Red Interior, 21" wheels-Toy/'14 535i with M Sport and 19" wheels-Daily
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