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.

 

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

 

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.

 

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.