How does changing to 15 inch wheels affect my speedometer?

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72_340_Demon

72_340_Demon

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I am putting 15 inch Crager SS rear wheels on my Demon. Any one know how that affects my speedometer? As an example if it says I'm going 60, will I actually going slower or faster than that?

Dan
72_340_Demon
 
Changing wheel size won't change your speedometer reading.

But changing rear tire height can.

What size tires will you be running on the rear?
 
for instance a 205/70-14 and a 205/65-15 are almost exactly the same circumference at 79.6" for the 14", and 80" for the 15", height is 25.4" for the 14" and 25.5" for the 15", so the 15" wheel will mean you are going .6 MPH faster at 60 MPH than the 14" wheel
 
google tire calculator,punch in factory size & size you are putting on,or find a freind with a garmin navigator ,i have use this to check speedo operation.
 
caferacerx said:
for instance a 205/70-14 and a 205/65-15 are almost exactly the same circumference at 79.6" for the 14", and 80" for the 15", height is 25.4" for the 14" and 25.5" for the 15", so the 15" wheel will mean you are going .6 MPH faster at 60 MPH than the 14" wheel
Click to expand...
cafercerx is correct to ..he can do the math faster than me.
 
72_340_Demon said:
255/60/15's replacing 245/70/14.
Click to expand...

on this the 14" 27.6" and the 15" is 27" so your speed will read 1.8% slower
or 58.9 MPH at 60 MPH than your old wheels, or 8.3% faster than stock size wheels and tires or you will be going 65 MPH when your speedometer is reading 60 MPH, here is a fun tire calculator to play with
http://www.rimsntires.com/specs.jsp
 
Wheel rim is 1 inch bigger but tyre sidewalls are 1.5 inches shorter so you may lose 0.5 inch which equates to about 1.7%. 60 mph speedo reading would be 59mph. However I've yet to come across a 100% accurate speedo so its with acceptable bounds for accuracy. As long as it wasn't way out in the first place.
AND the figures given on the sidewall will not necessarily reflect the true size of the tyre. As an example, my old 31x16.5R15 Hoosiers were WAAAAAAY wider than the MT Sportsman Pros of the same size. I got 27 inches for the 15s and 27.5 for the 14s.
 
very true even things like how much tread you have left effect your speedometer, best thing is to put the new wheels on then drive by a couple of those flashing "your speed" radar signs
 
Tony Fields said:
Here's a chart

Hope this helps
Click to expand...

That is the most easy to read chart that I have seen yet.
Thanks

Does it change with an OD trans since the gear is on the output shaft????????

I assume it is for a straight 1.1 high gear.
 
sireland67 said:
That is the most easy to read chart that I have seen yet.
Thanks

Does it change with an OD trans since the gear is on the output shaft????????

I assume it is for a straight 1.1 high gear.
Click to expand...

Doesn't change with an OD trans. All that matters is tire size & rear gear ratio.
 
My Sport has 255-60X15's on it. Going 100 miles on the interstate,and watching the mile markers,it was short 5 miles. Therefore 5% slow on the speedo. IE 60 mph=63. By the way,that is with the original gear for the speedo.
 
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