[ZTWsquared]

Gabriel,Apr 6 2009, 10:16 AM Wrote:Sorry , but no , no matter what circumference tires you have on , or if you had your car jacked up in the air with the drive wheels not touching the ground , if the car is in gear (5th in this case , talking about 5 speed Foci ), with the engine running it should still read 120 km/h @ 3000 rpm. I am talking here readings , not the vehicle's real speed . Real speed will depend on the tires circumference . The speedo should not register different with various tire profiles because the car speed is not a radar-type system , but is in tight connection between engine speed and transmission shaft speed .
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Okay, I think I get what you're saying ... that in the Focus the speedo isn't actually measuring the speed at the wheels but rather the RPMs of the transmission shaft (which, as you say wouldn't be affected by tire circumference) ... so in that case, reducing the cirumference of the tire would have the effect of reducing the actual speed vs. the indicated speed as the same transmission shaft RPMs would result in a shorter distance travelled over the same period of time.

LOL - thanks for your patience in clearing that up for me - I learned something today (assuming I got it right this time). Cheers!