Truck Tire Question

MrNightly

Let's say I have 235/75/15's on my truck and I need to replace them cause they are shot... can I put on 225/70/15's? What is the second number there? Obviously, it would be slightly smaller tires from the 235 to the 225, but I have no idea what the 75 and the 70 stand for.

Fill me in you car guru's!

BaggedLancer

70 is the width of the tire. Think of it as the distance from the edge of the rim to the tire tread.

Curious, why would you want to downsize tires on your truck? 99.9% of people go the next size up.....

MrNightly

Cause they are cheaper and it's a beater truck! And I found some tires on CL for a great cheaper price... and trying to save money. That's why!

So, would they actually fit, or would I need to stay with a 75 for my rims?

treitz3

The second # is the height from the rim to the road. You'll be fine, it'll just look like a smaller tire.

strider

The tires you are looking at will be both shorter and narrower. You should be ok, if you want to know for certain if the new tires will fit on your rim, either check out the tire manufacturer's website, or tirerack.com. Both sites should have a suggested rim width for the given tire size, as well as a range of rim sizes the tires will work on.

The numbers in the tire size mean the following:

tread width/aspect ratio/rim size

So for a 225/70/15, the tread is 225 mm wide, the sidewall is 70% of 225 mm tall, and the opening in the tire is 15 inches in diameter.

tom t

i always thought the lower the second # the fatter the tire was. 60's were always fatter them 70's. 50's are real fat. i thought the first #was the hight .

strider

Here's what I was looking for: http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=46

More info then you'll ever need on tire sizes.

ben62670

The second number is the aspect ratio of the tire width. If your tires are 235 75 15's the the height of you sidewall would be 176.25mm. If you had 225 70 15's the hight of the sidewall would be 157.5mm. Thats a 18.75mm difference in sidewall times 2 equals 37.5mm, or about 1.5 inches.
So the 235 tire is 29 inches
and the 225 tire is 27.5 inches
so at 2500 rpm with the 235 tire you would be going 80mph
and the 225 tire 76mph
Final answer...
Yes they would fit witha slight decrease in mpg, and slight increase in power.

These calculations have been made assuming a rear end ratio of 3.08, over drive of .87, and 2500rpm engine speed.

MrNightly

ben62670 wrote: »

The second number is the aspect ratio of the tire width. If your tires are 235 75 15's the the height of you sidewall would be 176.25mm. If you had 225 70 15's the hight of the sidewall would be 157.5mm. Thats a 18.75mm difference in sidewall times 2 equals 37.5mm, or about 1.5 inches.
So the 235 tire is 29 inches
and the 225 tire is 27.5 inches
so at 2500 rpm with the 235 tire you would be going 80mph
and the 225 tire 76mph
Final answer...
Yes they would fit witha slight decrease in mpg, and slight increase in power.

These calculations have been made assuming a rear end ratio of 3.08, over drive of .87, and 2500rpm engine speed.

Wow... did you do all that in your head

ben62670

MrNightly wrote: »

Wow... did you do all that in your head

Not all of it. Some of the voices in my head helped me out:) I actually used to to do a lot of calculations for my 68 Bronco to get the Cam/intake/exhaust to match my gear ratios. Having a 3 speed auto you had to be careful not to get too wide with your tranny gears. Ideal would have been 4.56 rears, but even with the 4.11's I only get 8mpg with 12 gallon tank:eek:

MrNightly

ben62670 wrote: »

Not all of it. Some of the voices in my head helped me out:) I actually used to to do a lot of calculations for my 68 Bronco to get the Cam/intake/exhaust to match my gear ratios. Having a 3 speed auto you had to be careful not to get too wide with your tranny gears. Ideal would have been 4.56 rears, but even with the 4.11's I only get 8mpg with 12 gallon tank:eek:

I'm jealous... that the voices only talk to you!

Thanks for the info. I actually found 4 Goodyears with the same aspect ratio that I am currently running... now if I could only get a hold of the guy!!! If not, then back to the drawing board for me. Maybe I should go with 13"? Mean lean, ugly machine kinda style... hmmm...

ben62670

http://www.offroaders.com/info/tech-corner/reading/mph-rpm.htm

tom t

what would be the aspect ratio of a glazed doughnut from dunkin doughnut's?????

bobman1235

Don't forget if you change your tire size your speedo will also be off by a bit. Not that they're super accurate to begin with but it's still something to remember.

ben62670

tom t wrote: »

what would be the aspect ratio of a glazed doughnut from dunkin doughnut's?????

Seeing the height is usually equal to width it would be 100%. Now cruller style dough nuts are taller than wide I would have to say about 80%. Now jelly, and cream filled just have me completely baffled seeing there is no whole

MrNightly

bobman1235 wrote: »

Don't forget if you change your tire size your speedo will also be off by a bit. Not that they're super accurate to begin with but it's still something to remember.

Gotta love the Brotherhood of Blue and my trusty ID I keep handy in my wallet for moments like these!


















Even if I am no longer on the force... hehehe... gotta get perks from somewhere right?

tom t

good one ben, boy, are we bored or what!!

BAD ASP

another point....... if you are driving an all wheel drive truck the front and rear wheel diameters need to be the same + / - 1/2" of each other.
The transfer case can be damaged by the wheels having different circumferences therefor different rotation rates. Additionally Lower profile tires can provide a harsher ride than your stock higher profile tires. My jeep srt8 had extremely low profile tires stock and runflats at that. They were like driving a roller skate with metal wheels. I changed to 60's from 30's and the ride is now much more pleasurable.

audiobliss

I've never understood the ****/xx/xx naming scheme, so I'm VERY glad to have read this thread and read an understandable explanation of it. However, if I'm thinking about it right...wouldn't going from 60's to 30's be DECREASING the sidewall? I mean, from what I read in strider's first post, the second number is the height of the sidewall, in percent, in relation to the width of the tread. So a higher aspect ratio would give a taller sidewall while a lower aspect ration would give a shorter sidewall, no?

ben62670

audiobliss wrote: »

I've never understood the ****/xx/xx naming scheme, so I'm VERY glad to have read this thread and read an understandable explanation of it. However, if I'm thinking about it right...wouldn't going from 60's to 30's be DECREASING the sidewall? I mean, from what I read in strider's first post, the second number is the height of the sidewall, in percent, in relation to the width of the tread. So a higher aspect ratio would give a taller sidewall while a lower aspect ration would give a shorter sidewall, no?

You've got it!

disneyjoe7

ben62670 wrote: »

The second number is the aspect ratio of the tire width. If your tires are 235 75 15's the the height of you sidewall would be 176.25mm. If you had 225 70 15's the hight of the sidewall would be 157.5mm. Thats a 18.75mm difference in sidewall times 2 equals 37.5mm, or about 1.5 inches.
So the 235 tire is 29 inches
and the 225 tire is 27.5 inches
so at 2500 rpm with the 235 tire you would be going 80mph
and the 225 tire 76mph
Final answer...
Yes they would fit witha slight decrease in mpg, and slight increase in power.

These calculations have been made assuming a rear end ratio of 3.08, over drive of .87, and 2500rpm engine speed.

Beautiful Ben it was in line of what I was going to say, and I was afraid this smaller tire size well save $ now but cost him later in MPG. If you what to go smaller 235 to 225 you need to increase the second ratio number 75 to 80 may work. But then you could always buy the correct size but a cheaper tire brand.

bobman1235

The ****/xx/xx convention is the worst measuring convention in the world.

millimeters / percent / inches? REALLY?

The percent one is the most nonsensical, but just the fact that the standard involves having two sets of units in the same number is absurd.