Hot Rod Forum banner
1 - 6 of 6 Posts

·
Registered
Joined
·
3,925 Posts
Discussion Starter · #1 ·
When determining rim width for a tire application which do you use "rim width" or "overall rim width"

As noted here:

http://www.usacomp.com/terms.htm

"rim width" is measured from the inside shoulder to the inside shoulder of the rim lip (where the tread seats) while "overall rim width" is the distance from the very outer edge to the very outer edge of the wheel lip. Which of these two do tire makers use when they provide their "approved rim range" for their tires.

If it's "overall rim width" it seems to make sense for the rims/tires that were original on my 81 F-150. If mfrs. use "rim width" then everything goes out the window. The actual "rim width" (inside shoulder) on my F-150 stockers is about 5-5 1/4" , yet the Dunlop tire site says the recommended tire for this vehicle is approved for rims from 6" to 8" wide.

On the other hand the actual "overall rim width" on the F-150 wheels is just about 7". This puts the tire Dunlop "approved rim range" right in line with my 7" wheel width.

Any tire/rim experts out there?
 

·
Registered
Joined
·
2,677 Posts
"INSIDE to INSIDE" of the rim is the number used in all tire and rim width call outs. That is the surface the tire bead seats on.
You can generally go up and down an inch either way when fitting new tires without hurting to much.
As far as "OVERALL WHEEL WIDTH" is concerned steel wheels, with a thinner material thickness then aluminum, will not be as wide, overall. A 7" wide steel wheel may be about 7 1/4 inches
"OVERALL" while an aluminum wheel will be around 8".
Mark
 

·
Registered
Joined
·
3,925 Posts
Discussion Starter · #4 · (Edited)
astroracer,

Well that's what I thought too until I actually started measuring my F-150 rims. As I said in my original post, the stock rims measure 5" to 5 1/4" from the inside shoulder to the inside shoulder (I'm measuring the inside shoulder as shown in the link I provided in my initial post - if this is in error let me know.) This would make the STOCK rims too narrow for the range allowed for the STOCK tires which were P235/75 R15 XL. This, of course, makes no sense.

But sure enough, according to the Michelin web site, this tire can only go on a rim 6.5" to 8" wide. And the Dunlop site says their P235/75 R15 XL can only go on a rim 6" to 8" wide. So my 5 1/4 " rim is well outside the allowable limits.

I am further confused when you say,

" As far as "OVERALL WHEEL WIDTH" is concerned steel wheels, with a thinner material thickness then aluminum, will not be as wide, overall. A 7" wide steel wheel may be about 7 1/4 inches"

Either I am measuring from the wrong spot or my rims are very very unique. When I measure inside shoulder to inside shoulder (using the inside of the shoulder as shown in the link in my initial post) I get 5 to 5 1/4" - the variance due to the fact that this shoulder is slightly tapered as it moves away from the rim center. When I measure from the outside to the outside of the rim I get 6 3/4" - at least 1 1/2 inch difference rather than the 1/4" that you suggest.

This leads me to believe I am not measuring something in the right place but if the diagram in the link is correct, then ALL steel tires are 1" to 1 1/2" wider at the the "Overall width" than at the "tire width".

Just to make sure we are talking about the same thing, allow me to "over describe" my confusion. If one were to look at a cross section of a steel rim starting from the very outside top edge, one would have approximately 1/8" thickness of material. Then the edge tappers inward 1/2 - 5/8 inch and downward approx 5/8"- 3/4". This forms the surface the tire "beads" against.
.
And this is the surface I am using to make my measurement. As you can tell from just my simple description, the difference between the "overall width" and the "tire width" would be right around 1 1/2". (1/8" material on each side plus 5/8" tapper to the inside on each side for a total of 1 1/2").

Hope you can see why I am confused here...and mostly I hope you can explain what I am doing wrong.

MI2600 said:
I measure the width from outside rim to outside rim and subtract an inch.
If I used this rule of thumb and I sort of "stretched" the tape a little bit, I could come up with a "rim width" of 6" - which would put the stock rim right at the lower end of Dunlops allowable range. However, it would still put me outside of Michelin's range for their P235/75/R15 - and if I'm not mistaken, the truck came equipped with Michelins.
 

·
Member# 3287
Joined
·
2,625 Posts
I kind of got lost in all this, so I went to the "Parts Department" (basement) and measured two of the old Corvette style ralley rims. One is a 15X6 and it measured 7". The other is a 15X7 and it measured 8".

If the above holds true, your 6 3/4" measurement would make your rims 5 3/4". I don't know if a 1/4" would make a big difference.

There must be a tire expert here somewhere on the board.
 

·
Registered
Joined
·
3,925 Posts
Discussion Starter · #6 ·
MI2600 said:

If the above holds true, your 6 3/4" measurement would make your rims 5 3/4". I don't know if a 1/4" would make a big difference.
I think you are right. My guess is these are rated as 6" rims even though they are not exactly that. Could be different for aluminum wheels but I'll bet your 1" rule of thumb holds true for almost all steel rims.

Anyhow, I feel pretty safe now that these are the proper width rims for the tires I intend to put on them.
 
1 - 6 of 6 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top