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Tire sizing

46K views 49 replies 36 participants last post by  Slammer67 
#1 ·
As a new member, I've noticed that a quick tutorial about tires might help in answering some questions. Not necessarily about what size will fit what year, but some basic info that could help answer questions before you ask. Basically what the numbers mean, and more important, making sure that AWD owners get what they need. It has been pointed out that AWD owners already know that different size tires don't do well, but do they know why, or know when they are being led astray by a tire dealer?

The sizing is a 3 part "sentence". The last part of the tire size is the easiest. It's the size of the rim. I believe our vehicles(stock) take 15". The first part is the width, in millimeters. Here's the tricky part. The middle part is a percentage of the first part, and represents tire height. So this means that the height of a 235/70/15 is different from a 245/70/15. Height has everything to do with circumference, and that has a dramatic affect on AWD vehicles.

How I learned this lesson: I bought a set of 4 tires. They sold me(on the receipt) 70 series size tires, but the tire they installed was 75 series. Not a problem in itself, but 2 years later, when I went to another branch of the same company to get 2 of them replaced, I flashed the original receipt and they sold me the 70 series. The difference between 70s on the front, and 75s on the rear ate up my transfer case in about 3 weeks. It was my transmission guy that figured out why, and that info forced the tire company to pay for the transfer case repair, about $800.00, about 7 or 8 years ago.

This experience has taught me that I should never trust that I actually get what I bought.
 
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#7 ·
Good info StanVan. I have lost count of the discussions that I have had with guys trying to get them to understand aspect ratio. The conversation usually starts with them saying "I want wide tires, maybe 60's".
 
#13 ·
i'd like to add to this great post:

For AWD owners:

-not all tire brands / models are made to equal diameters.
-it is important that you ROTATE your tires frequently, front-to-back only ALWAYS keeping your tires on the same side of the vehicle.
-keep the same air pressure, all around.
-if you replace a tire, unless they are within 5000mi or km, you will be running a larger diameter on the new tire. best to find a used one with similar tread depth or youll have to buy a whole set of 4
 
#14 ·
StanVan said:
..... The last part of the tire size is the easiest. It's the size of the rim. I believe our vehicles(stock) take 15". .....
Is that part true?
My 95 Astro quit, so I bought a 2004 to replace it.
When I needed a new tire this past weekend, the stock size was listed as 215/70/16 so that's what I had installed.

BTW, I felt silly driving on the donut spare, so I thought I'd get a used tire for a leftover rim from the 95 and make that my spare. Then I realized it WAS a 15 inch rim/ tire from the 95 and 5 lugs, not six (as on the 2004).
 
#17 ·
The stock tire size for a 2001 2wd should be 215/75/15. This creates an overall diameter (including the tire) of 27.70" when changing the width, aspect ratio, or rim diameter you should always strive to maintain, as close as possible, the overall diameter. The biggest affect of altering this to much will be a change in the accuracy of your speedometer because the revolutions per mile will change enough to through it off. With newer cars that have traction control, stability control and those type of computer aided features, altering the overall diamter to much will cause these systems to have a fit and constantly make adjustments becuase the revolutions per mile will make it think something is wrong and it will try to compensate for it. A 235/75/15 tire will have an overall diameter of 28.8" and be 1" taller than stock. That will also make your van sit 1" higher off the ground and is enough to most likely make your speedometer inaccurate.

If you want to put a 235mm wide tire on your van, you should go with a 70 series tire. A 235/70/15 tire will have an overall diameter of 27.95" and only be 1/4" taller than stock. The other thing you must always keep in mind when putting wider tires on a vehicle is the rim width. There is a point where it is unsafe to have a much wider tire on a narrower rim. If you look at the specs for a tire, it will tell you the rim width range for it. A 235mm tire should have a rim width range between ~6" to 8".

I know a 235mm tire will fit on the Astro since that is what came stock on my 98. I purchased it with the sport option that included 235/65/15 white oulined letter tires. I maintained those for a while but they were only made by Goodyear and that is a very odd size that nobody made. I don't think you can even get them anymore from any manufacturer and they were very expensive.

For those that want to learn how I arrived at most of my numbers, it is just a matter of doing some math. Let's use the example of the 215/75/15 tire.

1. Take the width of the tire in milimeters, 215 and multiply it by the aspect ratio, 75. (215 x .75 = 161.25mm) This tells you the height of the sidewall.
2. Since there is a sidewall above and below the rim, we need to multiply step 1 by 2. (161.25 x 2 = 322.5mm)
3. The rims are measured in inches, not mm, so we need to convert the sidewall height to inches. there is 25.4mm to 1" so, (322.5/25.4 = 12.70")
4. Now we just need to add the sidewall height to the rim diameter and we will have the overall diameter. (15" + 12.7" = 27.70")

Just follow these same calculations for your new tire/rim size and you can see how close it matches the original height.

Hope that helps.
 
#19 ·
brickyard said:
......... A 235/75/15 tire will have an overall diameter of 28.8" and be 1" taller than stock. That will also make your van sit 1" higher off the ground and is enough to most likely make your speedometer inaccurate.
actually it will make your van sit 1/2" higher. measurements are from center of hub.
30" tires make it go up 1 1/2"
 
#21 ·
Does anyone have experience as to the various tire Load ratings when selecting a larger off- road tires for our lifted Vans? Given our Vans have a GVWR of 6100 lbs, and the stock tires were 215/75/15, alot of the larger tire choices are either LT or SUV rated. 6100/4 = 1525# per tire stock. I am looking to go to a 235/70R16 or 245/70R16 tire size and wonder about ride quality. The LT tires are 10 ply and rated up to 2460# load, whereas the P series SUV are rated to more like 2065#, still well above the stock load ratings. I guess want the ride quality of a 6 ply P rated tire, but do no want to sacrifice sidewall flex in cornering. Seems to me with the wider tire footprint and higher load rating, that an SUV tire should be fine? Any thoughs ?
 
#22 ·
you can't have your eggs over easy and scrambled.
It's always going to be a trade off. Stability is either an LT or a low profile tire, smooth ride is P rated marshmallow tires.
Todays cushy ride P/U's are more in the suspension, steering isolation etc.

A couple of weeks ago I went from BFG radial TA's to BFG AT KO's. At 35 psi it was a notable difference in ride comfort but by no means bone jarring. I had to go up to 40 PSI for my heavy load but it's still not to bad, plus I don't have to nearly stop at speed bumps any more. It corners much much better, better breaking control and is just all around more stable and feels safer on the road.
 
#24 ·
Here is a list of sizes that fit
Plus Zero
225/70-15
225/75-15
235/70-15
Plus One
215/70-16
225/65-16
225/70-16
235/65-16

Plus Two
225/65-17
235/60-17
245/55-17
Plus Three
245/50-18
Plus Four
245/45-19
Plus Five
235/45-20

Found this in an other post, Thanks to Phanthom april 3rd 2011.
 
#25 ·
On a 15 inch rim the biggest you can go on the front is a 255 60 15 without rubbing on the fender wheels, you can go as wide as 265 on a 50 aspect ratio on the rears you can go as large as 275 60 15 or 255 70 15.

I used a alloy wheel off a 95 Chevy truck 5x5 bolt pattern 15 x 8 255 70 r15 rear 255 60 r 15 front, note I have a rwd van.
 
#26 ·
mmadden55 said:
On a 15 inch rim the biggest you can go on the front is a 255 60 15 without rubbing on the fender wheels, you can go as wide as 265 on a 50 aspect ratio on the rears you can go as large as 275 60 15 or 255 70 15.

I used a alloy wheel off a 95 Chevy truck 5x5 bolt pattern 15 x 8 255 70 r15 rear 255 60 r 15 front, note I have a rwd van.
I am halfway through my second set of 295/50/15's all the way around.
 
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