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Tire size and speedometer differences

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So guys I have a question for all of you who had 265/75R16 tires from the factory that have upsized to 285/75R16 or 285/70R17.

Who here knows or remembers how much the speedometer was off? How much was it off by?

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2-4mph. At lowers speeds its close, maybe 2 mph off. At 70 mph on the dash, my gps says 74

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17 hours ago, dodgedieselnewbie said:

My GPS shows 4 mph off at all speeds with 285/70/17

Shows 74 when your doing 70?

1 hour ago, Vais01 said:

Shows 74 when your doing 70?

It'll show 70when you're doing 74

Edited by BBHD

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3 hours ago, BBHD said:

It'll show 70when you're doing 74

Good to know. I'm trying to get within 2 percent of the speed on the speedometer. As of now I show 70 on the speedometer and I'm actually going 65.

You can calculate the difference in speed percentage based off the tire circumference as well.  Calculate the factor and you could make a table to look the speed up.  Or, you could just get a Smarty!  

Lets see...

C=Pi*diameter 

% factor = Coem tire / Cnew tire

% factor * indicated speed = real speed

Plugging in some fake numbers here... 

bg61TEC.png

Edited by CSM

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On Thursday, December 10, 2015 at 9:11 PM, CSM said:

You can calculate the difference in speed percentage based off the tire circumference as well.  Calculate the factor and you could make a table to look the speed up.  Or, you could just get a Smarty!  

Lets see...

C=Pi*diameter 

% factor = Coem tire / Cnew tire

% factor * indicated speed = real speed

Plugging in some fake numbers here... 

bg61TEC.png

Where did you find such a tire calculator? All the ones I've used give you some bogus revolution per mile. 265/70R17 lists about 660 revolution per mile and a 285/70R17 lists a 638 revolution per mile. 

 

Local dealership said they could program a 275/70R17 and they list a 650 revolutions per mile and the tire manufacturer lists a 646 so that's close to that size but not sure how far off that will put me.

1 hour ago, Vais01 said:

Where did you find such a tire calculator? All the ones I've used give you some bogus revolution per mile. 265/70R17 lists about 660 revolution per mile and a 285/70R17 lists a 638 revolution per mile. 

 

Local dealership said they could program a 275/70R17 and they list a 650 revolutions per mile and the tire manufacturer lists a 646 so that's close to that size but not sure how far off that will put me.

I just made it.  The math makes sense to me.  I did this years ago and just measured my own tires.  

Edited by CSM

  • Owner

I don't suggest using tire math because every brand is a bit different in tread depth and sizes so math doesn't work. Like I'm one size under 235 vs 265 but I'm 1.1 inches narrower and 0.04 inches TALLER. Makes for my 0.5 MPH offset. Over all odometer difference for my little 0.5 MPH could be roughly 30 miles difference between GPS and actual odometer.

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2 minutes ago, CSM said:

I just made it.  The math makes sense to me.  

:think: 

You mind running the numbers I put up above from zero to 100? 

265/70R17= Dia: 31.7= 99.58 Circumference = 660 revs per mile

275/75R17= Dia: 32.2 = 101.15 Circumference = 646 revs per mile

285/70R17= Dia: 32.8= 103.04 Circumference = 638 revs per mile 

 

Just concerned with the 265/70R17 vs 285/70R17

6 minutes ago, Mopar1973Man said:

I don't suggest using tire math because every brand is a bit different in tread depth and sizes so math doesn't work. Like I'm one size under 235 vs 265 but I'm 1.1 inches narrower and 0.04 inches TALLER. Makes for my 0.5 MPH offset. Over all odometer difference for my little 0.5 MPH could be roughly 30 miles difference between GPS and actual odometer.

I'm more concerned with the speedometer than just the odometer. Although fixing both would be great.

  • Owner

Odometer vs GPS will tell you if the how close you are to accuracy. As speedometer accuracy drifts so will the odometer. So if you correct one the other automatic will be corrected as well.

  • Author
7 minutes ago, Mopar1973Man said:

Odometer vs GPS will tell you if the how close you are to accuracy. As speedometer accuracy drifts so will the odometer. So if you correct one the other automatic will be corrected as well.

True. I just want to know what my speed actually is without some GPS APP on my cell phone.

  • Author
1 hour ago, Mopar1973Man said:

Best is to have the ABS module reflashed with the correct rev/per mile for your tire size. Dealer or Smarty is your choices.

That's the plan although I'd like to see how close it will be. Dealer can only flash what's on their Service Manual. Largest size listed is 275/75R70=32.2 or lowest revs per mile listed is 648 revs per mile.

285/70R17 is 638 revs per mile.

Edited by Vais01

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