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tire size and speedometer

 
 
ryoung
 
Reply Wed 29 Jun, 2011 08:49 pm
if you inssall a taller tire on your vehicle will you be going faster than your speedometer reads or slower.
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Type: Question • Score: 2 • Views: 4,885 • Replies: 8
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Krumple
 
  0  
Reply Wed 29 Jun, 2011 09:02 pm
@ryoung,
no because the speedometer is not hooked up to the tires. It is hooked up to the transmission.
parados
 
  1  
Reply Wed 29 Jun, 2011 09:20 pm
@ryoung,
In most cars the speed is calculated based on number of rotations and expected tire circumference.

Changing the tire size can cause the speedometer to give the wrong speed. It may be possible to reset the speedometer to correlate with the new tire size.
Rockhead
 
  2  
Reply Wed 29 Jun, 2011 09:37 pm
@ryoung,
smaller tires will make your speedo read fast.

it is calibrated from the factory for a certain diameter tire.

on older cars you can change the driven speedo gear (in the transmission) to a different tooth count gear.

newer cars run a magnetic pickup style electronic speedometer. there are different ways to make them read right, depending on what make it is...
ryoung
 
  1  
Reply Wed 29 Jun, 2011 09:40 pm
@parados,
I know I've heard that before but was not sure. The speedometer does connect to the transmission but as the trans turns it turns the wheels on the ground, I just wasn't sure if greater radius increased speed or slowed speed in relation to the speedometer.
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ryoung
 
  1  
Reply Wed 29 Jun, 2011 09:41 pm
@Rockhead,
Thanks, got it.
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roger
 
  1  
Reply Wed 29 Jun, 2011 09:41 pm
@Krumple,
Some have been connected to the front wheels on rear wheel drive vehicles. It's kind of interesting to hear the engine rev and the rear wheels spin on ice and still get an accurate speed indication.

To the original question, larger than standard tire diameters will give a greater speed and gas mileage indication unless the speedometer can be recalibrated as parados suggests.
Rockhead
 
  1  
Reply Wed 29 Jun, 2011 09:47 pm
@roger,
old air cooled VW's read from the left front wheel...

to make them read right with smaller tires, you gotta send the speedo off and get it rebuilt with different gearing.

itsa pain in the arse...
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gungasnake
 
  1  
Reply Thu 30 Jun, 2011 03:33 am
@ryoung,
Faster. You shouldn't need to think about that one more than a little bit.

Your odometer will also be off and the car won't accelerate quite as well although the difference might or might not be noticeable.
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