[Gabriel]

ZTWsquared,Apr 6 2009, 12:28 PM Wrote:

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!
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:D sorry if I tend to lose myself in too many words . I think you got it right.The car's speedo is actually in connection with the transmission . I am not sure how the new electronic gadgets work , but I know the old school principle : let's say the speedometer is activated by cable that is hooked up to the gear box and is turned by the tranny shaft via a small gear .The faster the shaft turns (considering the gear box in the highest gear), the more speed I read on the gauge. And at certain speedo reading (120km/h for example) the car's engine will turn with certain rpm (3000 in our case ) . Now for the car to really move with 120 km/h according to a radar it has to wear 205/50/16 IIRC for a ZTS. If the car has bigger or smaller wheels , the speed read by the radar will be different , but the gauges should read still 120km/ @ 3000rpm . The rpm gauge gets the information from the distributor (old school ) or from a sensor that counts the crankshaft/ flywheel turns (newer cars). So the speedo tells you the right thing ONLY if the car wears the tires with the size given originally by the manufacturer