My truck came with 245/75/r-17 I just baught a set of 265/60/r-18. will this change my speedometer and mpg calculations ?

 

yes, your new tires are about an inch smaller in diameter. your speed will actually be about 3% slower than what the speedo reads.

 

How can they be smaller if I went from 17's to 18's ????

 

The first number is the width in mm. The second # is the height of the sidewall as a % of the width. 75% of 245 mm is taller than 60% of 265 mm

 

Wheel size doesn't matter, tire size is what changes the speedometer accuracy.

 

You went to a wider tire but the aspect ratio is lower thus making the sidewall almost 2.5" shorter. Tirerack.com has some pretty good information on tire sizes and what they mean.

 

technically, the wheel size does matter some... in order to get the overall diameter of the tire, you have to include the diameter of the wheel, not just the height of the sidewall. Of course, you also have to include the tread depth (since that's not part of the sidewall measurement). It's usually best to get the "revolutions per mile" of each tire and see how far off you are. This info can usually be gotten from the manufacturers' websites

what brand and model were the old ones? and the new ones?

 

old ones are Cooper "Grabber" new ones are Michelen. they came off of a 2011 f-250 FX4.

 

<TABLE cellSpacing=0 cellPadding=0 width=474 border=0><TBODY><TR><TD class=bodyText width=224>This might help.


What size is being run now?
<INPUT onkeyup="return autoTab(this, 3);" maxLength=3 size=4 value=245 name=origwidth>/ <INPUT onkeyup="return autoTab(this, 2);" maxLength=3 size=4 value=75 name=origratio>R- <INPUT onkeyup="return autoTab(this, 2);" maxLength=3 size=4 value=17 name=orim>

</TD><TD vAlign=bottom align=left width=25></TD><TD class=bodyText width=225>




What size would you like to run?
<INPUT onkeyup="return autoTab(this, 3);" maxLength=3 size=4 value=265 name=alterwidth>/ <INPUT onkeyup="return autoTab(this, 2);" maxLength=3 size=4 value=60 name=alterratio>R- <INPUT onkeyup="return autoTab(this, 2);" maxLength=3 size=4 value=18 name=arim>

</TD></TR><TR><TD width=474 colSpan=3 height=20></TD></TR><TR><TD width=474 colSpan=3><INPUT onclick=calculateSizes(this.form) type=button value=Specs/Compare name=boxChecker s_oidt="2" s_oid="functionanonymous(){calculateSizes(this.for m)}"></TD></TR></TBODY></TABLE><TABLE cellSpacing=0 cellPadding=0 width=474 border=0><TBODY><TR><TD width=474 colSpan=4 height=20></TD></TR><TR><TD class=bodyText width=474 colSpan=4>Results</TD></TR><TR><TD class=bodyText width=150>Totals are in inches</TD><TD class=bodyText width=80>Tire 1</TD><TD class=bodyText width=80>Tire 2</TD><TD class=bodyText width=164>Difference</TD></TR><TR><TD class=bodyText width=150>Sidewall Height:</TD><TD width=80><INPUT size=7 value=7.23 name=OINSideWall></TD><TD width=80><INPUT size=7 value=6.26 name=AINSideWall></TD><TD width=164><INPUT size=7 value=-0.97 name=DIFFside></TD></TR><TR><TD class=bodyText width=150>Section Width:</TD><TD width=80><INPUT size=7 value=9.65 name=OINTireWidth></TD><TD width=80><INPUT size=7 value=10.43 name=AINTireWidth></TD><TD width=164><INPUT size=7 value=0.78 name=DIFFsect></TD></TR><TR><TD class=bodyText width=150>Overall Diameter:</TD><TD width=80><INPUT size=7 value=31.47 name=OINTireDia></TD><TD width=80><INPUT size=7 value=30.52 name=AINTireDia></TD><TD width=164><INPUT size=7 value=-0.95 name=DIFFdia></TD></TR><TR><TD class=bodyText width=150>Circumference:</TD><TD width=80><INPUT size=7 value=98.86 name=OINTireCir></TD><TD width=80><INPUT size=7 value=95.88 name=AINTireCir></TD><TD width=164><INPUT size=7 value=-2.98 name=DIFFcir></TD></TR><TR><TD class=bodyText width=150>Revs per mile:</TD><TD width=80><INPUT size=7 value=640.9 name=OREVMile></TD><TD width=80><INPUT size=7 value=660.82 name=AREVMile></TD><TD width=164><INPUT size=7 value=19.92 name=DIFFrev></TD></TR><TR><TD width=474 colSpan=4 height=15></TD></TR><TR><TD width=474 colSpan=4></TD></TR><TR><TD width=474 colSpan=4 height=15></TD></TR></TBODY></TABLE><TABLE cellSpacing=0 cellPadding=0 width=474 border=0><TBODY><TR><TD class=bodyText width=237>If your speedometer reads: <SELECT onchange=calculateSizes(this.form) name=ospeed> <OPTION value=20>20</OPTION> <OPTION value=25>25</OPTION> <OPTION value=30>30</OPTION> <OPTION value=35>35</OPTION> <OPTION value=40>40</OPTION> <OPTION value=45>45</OPTION> <OPTION value=50>50</OPTION> <OPTION value=55>55</OPTION> <OPTION value=60>60</OPTION> <OPTION value=65 selected>65</OPTION> <OPTION value=70>70</OPTION> <OPTION value=75>75</OPTION> <OPTION value=80>80</OPTION> <OPTION value=85>85</OPTION> <OPTION value=90>90</OPTION> <OPTION value=95>95</OPTION> <OPTION value=100>100</OPTION></SELECT></TD><TD class=bodyText width=237>You're going approximately: <INPUT size=5 value=63.04 name=ASPERead></TD></TR></TBODY></TABLE>

 

Mich 265/60R18 - 682 rev/mile
General Grabber - 245/75/17 - 662 Rev/Mile

I knop OP said Cooper - but only found general Grapper

 

that's a great find for ballparking it. also keep in mind that really aggressive tires tend to have a much deeper tread than highway tires... so the revs per mile can vary significantly from model to model *even within the same size*. and of course, that changes with tire wear... your speedo isn't going to be 100% accurate no matter what you do. even straight from the factory

 

Thanks guys. I don't yhink i will worry about 2 mph. again thanks, this forum always helps with my questions.