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Discussion Starter · #1 ·
Right now I have P235/75/R15 tires on my truck now, my friend has some tires that are 30x9.50r15. How close are they in size? Will it throw off my spedo alot? Please dont laugh cause I dont know anything about tire sizes.
 

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The 30x9.5x15 are about 1.12"'s bigger than your current tires resulting in about a 2.5mph diffrence in your speedo. So if your doing 60 your speedo is only reading 57.5. Hope this helps
 

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Discussion Starter · #3 ·
The 30x9.5x15 are about 1.12"'s bigger than your current tires resulting in about a 2.5mph diffrence in your speedo. So if your doing 60 your speedo is only reading 57.5. Hope this helps
that answer was PERFECT, the rim width is the same right?
 

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The 30x9.5x15 are about 1.12"'s bigger than your current tires resulting in about a 2.5mph diffrence in your speedo. So if your doing 60 your speedo is only reading 57.5. Hope this helps
Officer I was only doing 57.5 in a 55 zone! :lol:
 

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Right now I have P235/75/R15 tires on my truck now, my friend has some tires that are 30x9.50r15. How close are they in size? Will it throw off my spedo alot? Please dont laugh cause I dont know anything about tire sizes.
If you use the tire size calculator at redrock4x4 you get this:
Result:
235/75/15 = 28.9x9.3/15


If you use the online calculator at onlineconversion.com
You will see that if you speedo shows 60mph you will actually be traveling at 62.3mph
 

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One thing most do not consider is that bigger tires require more power at all speeds, present a larger profile to the wind at speed. They cost you in lost fuel mileage and more wear and tear on suspension parts but yes they do look cool!!
 

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Discussion Starter · #9 ·
im just adding them so i have traction, i plan on buying a cheap set of rims from the junk yard to use for them so if i wanna go offroading or when it snows.
 

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One thing most do not consider is that bigger tires require more power at all speeds, present a larger profile to the wind at speed. They cost you in lost fuel mileage and more wear and tear on suspension parts but yes they do look cool!!
When I dropped from 305's to 265's I noticed much better pulling power and it is much easier to turn at low speeds. Of course, you can change the gearing to improve pulling, but that is expensive with 4x4 since both diffs have to be the same ratio. When increasing tire sizes more than 3% the increased loading on the brakes is dangerous.
 

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I went over and inch and half bigger on my last truck with no decrease in mpg. Maybe .2 or .3 which isn't enough to matter. Now upgrading from say a 29" tire to 35's. Yeah that will hurt you.
 

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You could also spring for a programmer. A good one will usually correct for over/under sized tires. I had stock wheels and when I put on my aftermarkets I could definitely tell a difference, but I put a programmer in and recalibrated the computer and it seems like everything is back to normal. As far as gas mileage is concerned, its always sucked so it makes no difference to me.
 
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