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Old 10-26-2007, 08:28 AM   #1
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Just changed my 275 70 22.5 tires to 275 80 22.5.
Now, what is the formula to calculate my speedometer change. I assume there will be about a 10% speed increase. But I've been wrong once before this year!!
Thanks for your help!
Gerard
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Old 10-26-2007, 08:28 AM   #2
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Just changed my 275 70 22.5 tires to 275 80 22.5.
Now, what is the formula to calculate my speedometer change. I assume there will be about a 10% speed increase. But I've been wrong once before this year!!
Thanks for your help!
Gerard
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Old 10-26-2007, 11:09 AM   #3
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by walkingplow:
Just changed my 275 70 22.5 tires to 275 80 22.5.
Now, what is the formula to calculate my speedometer change. I assume there will be about a 10% speed increase. But I've been wrong once before this year!!
Thanks for your help!
Gerard </div></BLOCKQUOTE>
Hi Gerard,
Not sure about your chassis but I changed tire sizes on my 2006 MADP and all Spartan had to do was connect the dash to a laptop and reprogram the coach computer. It took 10 mins tops.
Thanks, Daryl

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Old 10-26-2007, 11:37 AM   #4
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New Speed = Previous Speed x (New tire diameter / Old tire diameter)

IOW, if the speedometer indicated 60 MPH before and was accurate, and you went from a 32" diameter to a 34" diameter tire, the formula above would look like:

New Speed = 60 MPH x (34/32)
New Speed = 60 MPH x 1.0625
New Speed = 63.75 MPH

So, with the taller new tire, the actual speed would be 63.75 MPH when the speedometer is indicating 60 MPH.

Plug your actual numbers into the above to get your results.

Rusty
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Old 10-26-2007, 11:47 AM   #5
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The picky engineer in me forces me to offer a very slight correction to Rusty's note. To be accurate, you need to use the radius as it sits on the ground, not the unladen diameter. In actual practice, diameter is probably close enough.
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Old 10-26-2007, 11:58 AM   #6
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The engineer in me forces a correction as well - the term is "loaded radius". Yes, I started to use that, but unless the original poster has access to the tire specifications, the diameter would be easier to measure or calculate from the tire size designation.

Actually, if one has access to the tire specs, the formula should be:

New Speed = Previous speed x (Previous revs per mile / New revs per mile).

Hey, it's all good!

Rusty
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Old 10-26-2007, 12:23 PM   #7
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Actually, the EMC is programmed with the RPM (revolutions per mile) of the installed tire. Each tire has a RPM and the Michelin XZA2 275/70R22.5 has 545 RPM while the Michelin XZA3 275/80R22.5 has 518 RPM.

Doing the math the difference between these two tires is .495% or 2.97 mph at 60 mph.

A Spartan or Cummins shop can connect a computer and change your RPM from 545 to 518 and then your speedometer will be correct.
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Old 10-26-2007, 05:11 PM   #8
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<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">To the engineer, the world is a toy box full of sub-optimized
and feature-poor toys. </div></BLOCKQUOTE>

Ken,

To a true engineer the glass is neither half full or half empty....it is twice as large as it needed to be.

But if you used tire diameter for both parameters or loaded radius for both the very slight difference you be pretty close to "why bother"?

Ken
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Old 10-27-2007, 04:14 AM   #9
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You could purchase the cables from Cummins that will allow you connect to the ECM through the big round plug which is located in both under the dash and in the engine compartment. Then download the PowerSpec software off Cummins website and you are all set. This will not only give you the tools to correct the speedo but will allow you other tools and information.

Peter
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Old 10-27-2007, 12:36 PM   #10
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My speedometer is off 5% after switching from 275/70s to 275/80s. I have a GPS on the dash, it reads 63 when my speedometer reads 60.
Tom
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Old 10-28-2007, 07:23 AM   #11
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Well I never thought that the size of my glass would come into question!! Thanks to all of you for your help!!!
Enjoy
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