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MECP Certified
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Discussion Starter · #1 ·
if and when i do put some Xtreme wheels on my 94 sonoma, how far will the speedo be off??

i currently have the base ugly 15x7" steel wheels with some 255/70/15

and am wanting to put on:

Xtreme 16x8" wheels with somewhere in the area of 235/55/16 tires or 235/50/16's


somewhere in that range...any idea if it will be off enough to notice.
 

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Umm... are you sure it is 255??? I think that is a little wide for a 7" wheel? are you sure it isn't 205? Mine is a 205/75/15. If it is a 205, the 235/55/16's will be really close, the 50's will be be about 2 mph off at 60. If they are 255, you will be about 10 mph off with those. I am getting this info from http://www.miata.net/garage/tirecalc.html . That page has a really nice menu and graphic of the tire/wheel and tells a lot of info. I would go there and mess with the tire sizes and see what suits you best.

DogP
 

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Deadbeat
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1,331 Posts
the formula for that is

(New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH

to calculate tire diameter the formula is

(Section Width x Aspect Ratio x 2 / 25.4) + Rim Diameter = Tire Diameter

Width in inches = section width / 25.4

Section Height in inches = Width in inches X Aspect Ratio (%)


so yours is (29.1 / 26.2) * 50 = 45

so for every 50 mph your going on the new tires your speedometer will read 5 less ex. your speedo says your going 100, your actually doing 90
 

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Discussion Starter · #5 ·

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Deadbeat
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ok if your speedo said you were going to 100 youd actaully be doing about 97 mph....

i didnt think the calc was that hard considering i used my fingers to calculate that but i've seen calculators on the web...i think s-series.org has one...i think
 

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gumby green
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i got to do that soon, i went from a 195/50/15 to a 245/50/16
 

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Deadbeat
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wow thats a cool tire calc...definately have to bookmark that one....
 
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