How to measure your car for proper wheel fitment

This is something I explain almost everyday on the phone so I thought it might be useful to some of you guys. I'll use my dad's 36 panel
For most wheels you really only need to take two measurements, we'll call them I (inside) and O (outside).

Here is the "I" measurement, Pretty simple, just lay a straight edge across the drum and measure inward. This one is about 6.5"


Here is the "O". Hang a plumb bob from the fender and measure inward to the mounting surface. This one is about 4"


And here is the equation to find your ideal backspacing(B). "W" is your wheel width measured at the tire bead. Our example is using a 15x8 wheel.

B=I-[(O+I-W-1)/2]

For our Example:
6.5-[(4+6.5-8-1)/2]=5.75" of backspacing is ideal.

Hope this is of some use.
Thanks,
Jimmy

 

Makes perfectly good since to me. HRP

 

Thanks man !!!

Wheels can be frustrating
I find it confusing when they mix metric units and inches and that freaking "offset"
When they do that, you really need to be on your toes and its a never ending math equation. 15x8 w offset +45mm. Way simpler with back space reference.

 

Yes, for fitment, "offset" is pretty much useless, it more of a suspension geometry measurement and has to be converted to backspacing and frontspacing before you can tell if the wheel will actually fit. Backspacing is much simpler.
Jimmy

 

One thing to keep in mind is if the car has a rear sway bar that projects beyond the inner body panel as it does with the CE rear end kit in the first measurement that was taken.

 

good tech. might add make sure weight of car is on the springs. also check for suspension travel. may still hit when driving over bumps.

 

Also, 15x8 wheel is actually 8.5"-9" wide, right?

 

9", yes. Thats what the "-1" in the equation is for.

 

I understand your measuring procedures and the equations, but, would you please explain
what the backspacing specification is used for? Thankyou

 

It's the measurement you use when ordering custom wheels, or a good measurement to use when going through a junkyard to see if a wheel will fit. Backspacing tells you where the back edge of a wheel is in relation to where the mounting surface is.
Thanks
Jimmy

 

Oh yea, simplest explanation I've seen. Thanks!

 

Simple but incorrect!
6.5 inches clearance taken up by 5.75 of backspace plus 1/2 to 3/4 of tire puts your tire hard into the sheet metal.

 

That's possible but most of the time 3/4" is enough, it just depends on the size of the tire.

 

Good stuff Jimmy!

 

Sure glad I'm not buying rims from you!!

 

Thanks for the reply. I'm building a 51 GMC I picked up in the 1970's. 350 sbc, 700r4 trans, and I'm not sure what rear end I'm going to use. I'm keeping the straight axle so
i'll want to keep the same measurement of 62" between the wheel mounting surfaces; I think. Any suggestions, from anyone" Thanks

 

The Internet, where people forget about etiquette. So if we have a 15x8 with a 5.75" of backspacing it will have a frontspacing of 3.25". In our example this makes the front clearance 3/4" which is also whats on the back. So, unless you just go to a smaller size, it seems to me that having an even clearance on both sides is the way to go. What would you suggest? Less Backspacing? Then you'd have a problem on the front.
Jimmy

 

Exactly.
You have described how to center the wheel in the space allowed. Then, I believe that the next step is the tire on that given wheel size. Obviously, tires with a substantially wider tread width or section width (in relation to the wheel width) must be considered. In my opinion, the wheel is only part of the equation. The fitment of a given tire on that wheel is the balance of the equation.

 

Good info as far as it goes, but the real bugger is getting the TIRE set right. Let's use the offset info and get into section width at different rim widths.......

 

With poor advice etiquette goes out the window!
With a proper width tire (at least a 235) on your 8" rim your cross section will be at least 10 inches. I have a 235 75 on my 15x8 rim as a spare. You'll be a 1/4 inch from sheet metal on the inside and 1/4 inch from the wheelwell lip on the outside. Way to close. Safe is at least an inch.

 

235mm=9.25"
5/8" is enough clearance

 

Great tech mate, thankyou.
Best to just ignore idiots like the guy whos trying to pick an argument with ya, after a while they tire and disappear.

 

You're sorta good at math. 235mm is 9.25".
On a 6 inch rim. On an 8 inch it's 10"
1/4 inch isn't enough!
And we do go away when we get tired of dealing with self proclaimed experts.

 

Ill just watch

 

What he said!
This is the exact reason why so many wheels have to be reordered. There are several steps to getting it right! We haven't even touched on how to make sure a tire/wheel will not rub when you turn the front wheels.
One way is to measure how much space a wheel and tire, that will bolt on, gives both inside and outside the well, when turned both ways. Take the wheel off and measure it's actual front and back space, including tire bulge. This can be done on the ground or any flat surface. Then, you can start the above processes to determine what will work, before you order wheels or buy a swap meet problem. This info may allow you to save money on wheels/tires that won't work on another car.
Another thing to remember is: backspace is figured from total wheel width, not advertised width. As stated by wheelkid and others: Add about 1" to advertised width for total width or roughly 1/2" lip on each side.
Don't forget to look at spacing around the entire diameter. Do the work, and the math, to save yourself the problems. There are some pretty trick tools available for someone that fits a lot of wheels, but anyone can do it manually, if they consider everything.

 

235mm isn't tread width. It's cross section.
Here's a much better way to figure out what wheel/tire combo fits without reordering at all. Do it once, do it right!
http://www.carcraft.com/techarticles/ccrp_0610_muscle_car_performance_wheels_fitment/viewall.html

 

Could not agree more!
I'm reading the link and a 17x8" wheel with 4" backspace is not centered. Backspace is measured from the inside lip. An 8" wheel has two, roughly, 1/2" lips for a total wheel width of 9" minus 4" backspace equals 5" front space.
Not to take away from the correct info in this link, this part is, as you say, incorrect. I'll keep reading.

 

Do yourself a favor and measure back and front space of any wheel you ordered or buy, before mounting tire. Most wheels can not be returned if tire was mounted on it, even if it does not match what you ordered. Sometimes a pair of wheels are not the same spacing, even brand new.
Next, measure again when one tire is mounted. If you catch a section width that is too wide on the wheel, you can usually change the tire size you are buying right then and avoid a problem with the car or the dealer.
For any of this to work, you have to have done your measuring of what will actually fit on the car. Sometimes, even then, there is a problem, and you own it. I had a set of Halibrands and tires that came with a car I was building which just rubbed on the outer fender lip. Luckily, the mounting pad was thick enough to machine a solution. Doing that, we had to consider strength and not creating a problem with lug nuts. The result of all this effort is wheels and tires that fit your car, even a factor on a fenderless car.

 

thanks for posting Wheelkid....

 

I'm confused as to your point:

In wheelkid's example, he has a total of 10-1/2" total to work with. Does not matter what the backspacing is, a 10" cross section tire has a 1/2" of available real estate to work with, period. His example centered the tire that was too wide to begin with.

That's what we want- a centered tire in a wheelwell, right? His tech was not about choosing tire width, it was about correct backspacing. If you have 10-1/2" to work with & want a 10" wide tire on an 8" wheel, how is his process wrong? Your point is an entirely different topic.