Speedometer Measurement with Plus-Sized Tire Setup
Thread Starter
Member
Joined: May 2005
Posts: 247
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From: Pasadena, CA
'12 C204 C250, '05 SLK350 (sold)
Speedometer Measurement with Plus-Sized Tire Setup
This may be a noob question, but I'm wondering if our cars' speedometers measure the speed from the front or rear tires? Reason I'm asking is that I'm currently running a 235/35R19 Front & 265/30R19 Rear setup.
Here's how the diameters compare:
Stock Front: 225/45R17 (25.0" diameter)
Plus-2 Front: 235/35R19 (25.5" diameter)
Stock Rear: 245/40R17 (24.7" diameter)
Plus-2 Rear: 265/30R19 (25.3" diameter)
I'm considering changing the rears to 275/30R19 for my next set of rears (25.6" diameter). Anyone see an issue with this? I want to figure out exactly how much my speedo is off (Plus-2 Original setup and Plus-2 Planned setup). Thanks in advance for your input.
Here's how the diameters compare:
Stock Front: 225/45R17 (25.0" diameter)
Plus-2 Front: 235/35R19 (25.5" diameter)
Stock Rear: 245/40R17 (24.7" diameter)
Plus-2 Rear: 265/30R19 (25.3" diameter)
I'm considering changing the rears to 275/30R19 for my next set of rears (25.6" diameter). Anyone see an issue with this? I want to figure out exactly how much my speedo is off (Plus-2 Original setup and Plus-2 Planned setup). Thanks in advance for your input.
Super Member
Joined: Jan 2003
Posts: 675
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From: Dallas, TX
2018 GLS 550
Try using this calculator, input stock size on first line then new size on second line. If I put in the right stock size, it will read 1MPH higher than actual with the 275 setup. input as 275/30/19
http://www.powerdog.com/tiresize.cgi
http://www.powerdog.com/tiresize.cgi
Super Moderator
Joined: Jun 2008
Posts: 19,942
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From: Llandudno, Cape Town, South Africa
late 2009 CLK 350 Coupe Elegance, '65 Jaguar S Type wires
The speedo picks up from the front LHS wheel. Processed via the ABS unit. Speedo can be recalibrated with Star for different rolling circumference tyres.
Keep front & rear rolling circumference within 2% of one another or the ESP will complain.
Keep front & rear rolling circumference within 2% of one another or the ESP will complain.
Thread Starter
Member
Joined: May 2005
Posts: 247
Likes: 0
From: Pasadena, CA
'12 C204 C250, '05 SLK350 (sold)
Thanks for your quick replies. So it seems 235/35R19 and 275/30R19 are better matches with essentially the same diameter, but both are 2% taller than stock. Besides the speedo being 2% off, is there anything else I need to worry about? Hopefully rubbing isn't an issue when I drop.
