ZTWsquared,Apr 5 2009, 10:10 PM Wrote:

Gabriel,Apr 5 2009, 03:26 PM Wrote:

ZTWsquared,Mar 24 2009, 01:53 PM Wrote:

NikiterZTS,Mar 24 2009, 12:05 PM Wrote:3000=115km/h in focus on winter tires
247k on the engine

i think it also depends on how you engine was tuned at the factory
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Nope - strictly a mechanical / mathematical thing based on transmission gearing, final drive ratio, and tire circumference.

Mechanical problems like slipping clutch or unlocked torque converter vs. locked t.c. could have an influence ... but otherwise it doesn't matter the size of the engine, the torque or the hp ... or its condition - the revs / kph will remain the same so long as those three factors remain the same.

A good analogy is the bicycle ... if the gearing and tires are the same, it doesn't matter who's riding it, the number of rpms per distance will remain the same.
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I agree with you , just with one exception : tire circumference . It doesn't matter whether you're on low or high profile tires , if your tach says 3000 and you drive let's say 120 km/h the gear ratio stays the same and the transmission turns at constant rpms . Now if you have tires with lower profile than the manufacturer installed , at 3ooo rpm the speedometer will show you 120 km/h but in reality you will have less than 120 . And for higher profile tires , for same readings you will actually move at over 120 km/h . That is for a manual transmission . As for automatic , I don't know , I am european , I don't play with that s#%t .
My 03 Focus SVT reads around 3200 rpm at 120 km/h in 6th . Slightly more than the 03 ZTS I had before read in 5th .
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I think we're saying basically the same thing ... which is why I mentioned tire circumference and not rim circumference ... as the profile gets lower the circumference of the tire will also get smaller (all other things being equal) ... bottom line being that when you reduce the circumference of the tire you cause the speedo to register higher.
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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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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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