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Tire Wheel Backspacing Offset Calculators

Wheel Offset and Backspacing

So you finally saved up the cash to get the perfect set of wheels for your ride, now what. There’s diameter, that’s the easy part, your favorite song says you need some 24s! Now you just google your vehicle to find out the number of lugs and distance between them, 6 lugs 5.5″ spacing 6×5.5. Okay all set, go on eBay and order some wheels… Not so Fast! Did you know many wheels are sold in 3-10 different wheel offsets or ET (some German abbreviation for offset). The wheel offset is the distance in millimeters (mm) that the hub of the wheel (where you bolt the wheel to the car) is from the True center of the wheel. Basically a positive offset of +25mm means the center of the wheel is 25mm closer to the outside (curb/fender side) of the wheel, thus pushing the wheel in toward the caliper/brake. This basically sinks the tire and wheel into the fender. If the same wheel were to have a wheel offset of -25mm, that means the center (lug nut or mounting part) is closer to the inside of the wheel. This is what often creates a larger lip on the wheel or a concave style wheel, Thus pushing the tire out toward the fender making it flush or on your favorite monster truck way outside the fender creating that super aggressive 4×4 stance.

How to Measure our Backspacing (Back of the wheel)

Measuring Backspacing

Or hopefully you can just look on the back side of the wheel/rim and find this stamping!


(This would be a 18″ diameter with 8″ width and +55 offset.)


Let me start by just throwing all the math at you, then you will see some diagrams and explanation below.

Using 205/60-15 as an example:

Tire width = 205 mm

Aspect Ratio = 60 (sidewall height is 60% of tire width) or 0.6 * 205 = 123 mm side wall

Wheel rim dia. = 15 inches

So the INCHES diameter of a 205/60-15 tire is = (2*205*0.6/25.4) + 15 = 24.69 inches diameter is same as “height”


formula from mm and ratio to inches:

((2*tire width mm*tire ratio*)/25.4)+rim diameter inches = tire height in inches

For width tire width mm/25.4 = width inches


To convert from inches to mm

Tire width = 8″

Tire height = 25″

wheel dia. 15″


(tire width in * 25.4) = width mm then round to 5mm or 8*25.4 = 203.2 or 205mm

((Tire height – rim diameter) / 2) / (tire width in * 25.4)


formula from inches to ratio in %

((Tire height inches- rim diameter inches) / 2) / (tire width inches)= % again round to nearest 5 (60 or 65 using our previous numbers)

For width tire width inches * 25.4 = width mm

top view wheel offset w logo

As the offset is moved toward the Curb Side (positive + offset) – toward the fender – it has the opposite effect on the wheel stance, moving it in toward the caliper which creates a larger gap between the outter edge of the tire and the outer edge of the fender. As the offset moves toward the caliper (negative – offset) the wheel and tire are moved out toward the fender and with a large enough negative offset the tire will stick out past the fender.


The following diagrams assume the wheel is the same width in both examples. Remember if the wheel width changes this will affect how much the offset will move the wheel. Example: a 7.5″ wide wheel with a +25 offset will be “tucked” into the fender while a 10″ wide wheel with +25 wheel offset could be flush. How? You have to account for the 2.5″ of extra wheel width. That mean 1.25″ on each half of the wheel from the center line. Since every inch is worth 25.4 mm that means you have added 31.75mm of wheel toward the fender and another 31.75mm toward the caliper, even though the offset was the same.


Assuming you are keeping the same width wheel, this is what offset would do as you change from positive offset such as +25 to less positive such as +12 or past the center line of the wheel to negative wheel offset such as -12.

First an example looking from the rear of a vehicle with a stock or positive offset. The suckin or tucked, factory look.

rear view positive wheel offset w logo

This second diagram looking from the rear of a vehicle with a negative offset (same width wheel). This setup will give the flush or with a larger negative offset one can achieve the aggressive look as you move outside the fender.

rear view negative wheel offset w logo


So now that you know exactly what wheel offset is we will move on to the other word used by the custom wheel world. Backspacing. Not all that different however to keep you on your toes it changes where we measure from. We are still looking for the distance from the mounting surface of the wheel, however rather than measuring to the ‘center line’ of the wheel, backspacing is measured from the back edge of the wheel to the mounting surface. Like below.

What is Offset?
by: Custom Offsets

Confusing? Not a big deal that’s why CustomOffsets.com exists. You don’t have to worry about all this because we have created a place to SEARCH. BROWSE. and SHARE pictures of your favorite rides with the specifications of the wheels, including wheel offset to make finding the perfect wheel for your ride sooooo much easier!

Tire Wheel Backspacing Offset Calculators