What are Offset and Backspacing, which do I need to worry about and how wide a wheel can I run? I get these questions (and combination/tangents thereof) often enough, I wanted to create a thread to which I could refer those with wheel and tire fitment questions. Here at Carli, we do 2.5” and 4.5” lifts for the Radius Arm Equipped 2005-17 Super Duties, this write-up will be from this perspective. Most guys running 6”+ of lift are more lenient with wheel specs as they’re not quite as important.
Width: This, Ladies & Gentlemen, is how wide your wheel is. Contrary to popular belief, this normally refers the distance between the outer and inner tire-bead seat. The overall measurement will likely exceed the stated width by 1” (up to 2” if Beadlock).
Backspacing: The backspacing of a wheel refers to the measurement from the hub mounting surface to the rear-most part of the wheel. Some measure from the hub mounting surface to the inner tire bead seat but, most commonly, you’ll see people place a wheel, face-down against the ground, place a straight-edge on the back of the wheel and run a tape from the mounting flange to the straight edge to determine backspacing.
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Offset: A wheel’s offset is the measurement, in MM, of a wheel’s hub mounting surface from the centerline of the wheel.
![Image]()
So… All that said, with what does a customer need to be concerned? We recommend focusing on Width and Backspace. With these two numbers, one can easily determine the wheel’s offset but it’s superfluous information as the former two (width and backspace) are the only factors with which one needs to be concerned when determining potential fitment. If Width and Offset are the only two specifications available, calculators exist to use those two characteristics to determine Backspacing.
This is the one I use: https://www.customwheeloffset.com/tire-wheel-backspacing-offset-calculators
Specifically:
Width: Up to a 4.5” lift, we recommend customers stick to a 8.5”-9” wide wheel. 2.5”-4.5” lifts balance fitment between the Radius Arm and Body. 6”+, most wheels/tires sit under the body and wider wheels can be run with less trimming/discretion for width assuming a reasonable offset/backspacing.
Offset: When considering a lower lift (2.5”-4.5”), we recommend keeping offsets in the 0 to +12mm area. In the 2.5”, 0 to +18mm (assuming 9” wide wheel) is recommended with a 35” tire, 0 to +12mm is recommended for a 37”x12.5” tire fitment. In 4.5”, that range shifts to -12mm to +12mm depending on Tire width and aftermarket radius arm option.
The issue with offset. Because it’s relational, it varies with rim width and can get confusing where backspacing is a constant regardless of width. If you know you’re after a 4.5”-5” Backspace and you’re within the recommended wheel width range (8”, 8.5” & 9” wide), you can be confident the wheel will fit. That said, the confusion comes in when customers focus on the offset. Even with the constant 5” backspacing among the above offerings, the offset varies from +12mm on the 8”, +6mm on the 8.5”, and 0 on the 9”. Even though the differentiation in fitment characteristics is inconsequential, the number shift in the offset can set off an unnecessary alarm.
Until you truly understand all three (Width, Backspace and Offset), we recommend ignoring the offset.
Backspacing: On a 2.5” lift, we recommend a 5”-5.5” Backspace for 37x12.5” tires or up to 6” for 35” tires. On a 4.5” Lift, 4.5”-5.0” will work with up to a 37x13.5” tire. The aforementioned are all guidelines and shift based on aftermarket radius arm design & wheel width.
Why this is important: If a customer purchases a 4.5” lift and wants to run a 37x13.5” Toyo MT (Actual width 14.5”), a 4.5”-5.0” Backspace is required to clear the factory radius arm. If the customer chooses a 9” wide, this equates to a 0 offset and won’t likely rub. If the customer elects to run a 10” wide with the same 5” backspace, the spec is the same short of the width. This means the wheel position is the same but 1” is added to the outside of the rim slightly shifting the tire to the outside potentially causing rub on the body. This can be offset (depending on the amount of room in the application) by slightly increasing the backspace to compensate for the outward shift of the tire on the wider rim width. Or, the backspacing can remain and the customer can trim the body to clear the tire to keep the deeper rim “lip”.
All the tech info out of the way:
I get questions all the time on 9" vs. 10", sometimes 12"-14". I haven't received a call on 16" wides yet but it won't likely be long. Rule of thumb, 10"+ wides are designed for 6"+ of lift and won't always tuck a tire.
I run a 4.5" Dominator Kit on my 2016 F250 with an 18x9 American Force Wheel with a 4.5" backspace on a 37x13.5" Falken Wild Peak AT. They clear lock to lock. When I get into the gnarly stuff, it'll stuff my passenger fender if I’m in a turn and compressed (due to the axle shift toward the passenger side on upward suspension cycling). Regardless of my recommendations, I choose to run a larger, wider rim. Why don’t I recommend this? Larger wheels + smaller tires = Stiffer ride. More on that here if you’re interested: Carli Blog: Tires and Wheels: How Do They Affect My Ride??
So the cool question: What do I need to do to fit a wider than recommended wheel on a 4.5” lift wheel with a 37” Aggressive tire?
Answer: How comfortable are you cutting and shaping metal? Some guys grab a body sway and hit the fender as they don’t care. This is what I did on my 2006 with my 4.5” kit and 18x9 with 4.5” Backspace (this was required on the older truck as the wheel-wells are smaller). For 10” wides on newer trucks, the same holds true. I recommend cutting off the bottom fender liner retaining bolt off on both sides (highlighted it in red).
![Image]()
My new wheels are more aggressive. They're 20x10 and 4" Backspace. If you’re wanting to go more aggressive in width or offset like I did, my recommendation is to get extremely comfortable with a body saw or take it to a good paint and Body Guy. I started by measuring the width of my new American Force 20x10 Beadlocks. Actual width is about 12”, measured the Backspacing, Actual 3-13/16” (claimed 4”). I threw them on the truck while it was on jack stands and cycled the steering carefully. These hit the bumper and fender enough that I knew the trimming should be done by a pro as I wasn’t hacking this truck up.
![Image]()
The Guys at Autobahn Collision in Torrance are known for doing this type of work. If you’re in the area, pay these guys a visit. If you’re somewhere else, find yourself a good body guy and be prepared to pay for a good result.
The Beginning:
![Image]()
The Process:
![Image]()
![Image]()
The Result: Coming this weekend
Given the offset, I would also recommend finding a vinyl guy to wrap the damage-prone areas of your truck (fenders and rockers) with a quality, clear film as aggressive, offset tires will throw debris at the side of the truck and tear up your paint if you don't.
Width: This, Ladies & Gentlemen, is how wide your wheel is. Contrary to popular belief, this normally refers the distance between the outer and inner tire-bead seat. The overall measurement will likely exceed the stated width by 1” (up to 2” if Beadlock).
Backspacing: The backspacing of a wheel refers to the measurement from the hub mounting surface to the rear-most part of the wheel. Some measure from the hub mounting surface to the inner tire bead seat but, most commonly, you’ll see people place a wheel, face-down against the ground, place a straight-edge on the back of the wheel and run a tape from the mounting flange to the straight edge to determine backspacing.
Offset: A wheel’s offset is the measurement, in MM, of a wheel’s hub mounting surface from the centerline of the wheel.
So… All that said, with what does a customer need to be concerned? We recommend focusing on Width and Backspace. With these two numbers, one can easily determine the wheel’s offset but it’s superfluous information as the former two (width and backspace) are the only factors with which one needs to be concerned when determining potential fitment. If Width and Offset are the only two specifications available, calculators exist to use those two characteristics to determine Backspacing.
This is the one I use: https://www.customwheeloffset.com/tire-wheel-backspacing-offset-calculators
Specifically:
Width: Up to a 4.5” lift, we recommend customers stick to a 8.5”-9” wide wheel. 2.5”-4.5” lifts balance fitment between the Radius Arm and Body. 6”+, most wheels/tires sit under the body and wider wheels can be run with less trimming/discretion for width assuming a reasonable offset/backspacing.
Offset: When considering a lower lift (2.5”-4.5”), we recommend keeping offsets in the 0 to +12mm area. In the 2.5”, 0 to +18mm (assuming 9” wide wheel) is recommended with a 35” tire, 0 to +12mm is recommended for a 37”x12.5” tire fitment. In 4.5”, that range shifts to -12mm to +12mm depending on Tire width and aftermarket radius arm option.
The issue with offset. Because it’s relational, it varies with rim width and can get confusing where backspacing is a constant regardless of width. If you know you’re after a 4.5”-5” Backspace and you’re within the recommended wheel width range (8”, 8.5” & 9” wide), you can be confident the wheel will fit. That said, the confusion comes in when customers focus on the offset. Even with the constant 5” backspacing among the above offerings, the offset varies from +12mm on the 8”, +6mm on the 8.5”, and 0 on the 9”. Even though the differentiation in fitment characteristics is inconsequential, the number shift in the offset can set off an unnecessary alarm.
Until you truly understand all three (Width, Backspace and Offset), we recommend ignoring the offset.
Backspacing: On a 2.5” lift, we recommend a 5”-5.5” Backspace for 37x12.5” tires or up to 6” for 35” tires. On a 4.5” Lift, 4.5”-5.0” will work with up to a 37x13.5” tire. The aforementioned are all guidelines and shift based on aftermarket radius arm design & wheel width.
Why this is important: If a customer purchases a 4.5” lift and wants to run a 37x13.5” Toyo MT (Actual width 14.5”), a 4.5”-5.0” Backspace is required to clear the factory radius arm. If the customer chooses a 9” wide, this equates to a 0 offset and won’t likely rub. If the customer elects to run a 10” wide with the same 5” backspace, the spec is the same short of the width. This means the wheel position is the same but 1” is added to the outside of the rim slightly shifting the tire to the outside potentially causing rub on the body. This can be offset (depending on the amount of room in the application) by slightly increasing the backspace to compensate for the outward shift of the tire on the wider rim width. Or, the backspacing can remain and the customer can trim the body to clear the tire to keep the deeper rim “lip”.
All the tech info out of the way:
I get questions all the time on 9" vs. 10", sometimes 12"-14". I haven't received a call on 16" wides yet but it won't likely be long. Rule of thumb, 10"+ wides are designed for 6"+ of lift and won't always tuck a tire.
I run a 4.5" Dominator Kit on my 2016 F250 with an 18x9 American Force Wheel with a 4.5" backspace on a 37x13.5" Falken Wild Peak AT. They clear lock to lock. When I get into the gnarly stuff, it'll stuff my passenger fender if I’m in a turn and compressed (due to the axle shift toward the passenger side on upward suspension cycling). Regardless of my recommendations, I choose to run a larger, wider rim. Why don’t I recommend this? Larger wheels + smaller tires = Stiffer ride. More on that here if you’re interested: Carli Blog: Tires and Wheels: How Do They Affect My Ride??
So the cool question: What do I need to do to fit a wider than recommended wheel on a 4.5” lift wheel with a 37” Aggressive tire?
Answer: How comfortable are you cutting and shaping metal? Some guys grab a body sway and hit the fender as they don’t care. This is what I did on my 2006 with my 4.5” kit and 18x9 with 4.5” Backspace (this was required on the older truck as the wheel-wells are smaller). For 10” wides on newer trucks, the same holds true. I recommend cutting off the bottom fender liner retaining bolt off on both sides (highlighted it in red).
My new wheels are more aggressive. They're 20x10 and 4" Backspace. If you’re wanting to go more aggressive in width or offset like I did, my recommendation is to get extremely comfortable with a body saw or take it to a good paint and Body Guy. I started by measuring the width of my new American Force 20x10 Beadlocks. Actual width is about 12”, measured the Backspacing, Actual 3-13/16” (claimed 4”). I threw them on the truck while it was on jack stands and cycled the steering carefully. These hit the bumper and fender enough that I knew the trimming should be done by a pro as I wasn’t hacking this truck up.
The Guys at Autobahn Collision in Torrance are known for doing this type of work. If you’re in the area, pay these guys a visit. If you’re somewhere else, find yourself a good body guy and be prepared to pay for a good result.
The Beginning:
The Process:
The Result: Coming this weekend
Given the offset, I would also recommend finding a vinyl guy to wrap the damage-prone areas of your truck (fenders and rockers) with a quality, clear film as aggressive, offset tires will throw debris at the side of the truck and tear up your paint if you don't.
