How to use it
- Enter the current tire size and wheel: width in inches and offset (ET) in millimeters, as stamped on the back of the wheel.
- Enter the new tire and wheel the same way, and the spacer thickness if you plan to run one.
- Measure the three gaps on the car as it sits today, as described below, and enter them.
- Read the new gap at each point and its verdict, then check the drawing to see where the tire gets closest.
How to measure the three gaps
Park on level ground with the car at its normal ride height, the wheels pointing straight ahead and the usual load on board. A tape measure or a ruler is enough. Measure the front axle, where steering makes room tightest, and the rear if you also change the rear wheels.
- Inner gap: from the inner sidewall of the tire, or the wheel lip if it is closer, to the nearest suspension part, usually the strut, spring or upper control arm. Take the smallest distance you can find.
- Outer gap: hold a ruler straight down against the fender lip and measure horizontally from the ruler to the outer sidewall. If the tire already sticks out past the lip, enter the gap as a negative number.
- Top gap: from the top of the tread straight up to the wheel-well liner or the lowest part of the fender above it.
How the gaps change
- Lower offset or a spacer moves the whole wheel outward: every millimeter takes 1 mm from the outer gap and adds 1 mm to the inner gap.
- A wider tire grows by about half the extra width on each side, so it eats into both the inner and the outer gap.
- A taller tire lifts the vehicle by half the extra diameter and grows by the same half above the hub, so the top gap shrinks by half the diameter increase.
- Track width grows by twice the outward move, because both sides of the axle move out.
| Gap | Measured today | With the new setup | Verdict |
|---|---|---|---|
| Inner | 55 mm (2.17 in) | 89 mm (3.50 in) | Clears |
| Outer | 40 mm (1.57 in) | 6 mm (0.24 in) | Tight |
| Top | 65 mm (2.56 in) | about 51 mm (2.00 in) | Clears |
The tire is 28.5 mm taller, so the top gap loses about 14 mm. The 34 mm drop in offset pushes the tire 34 mm (1.34 in) farther out on each side and widens the track by 68 mm (2.68 in). Add a 15 mm spacer to that setup and the outer gap goes to −9 mm: the tire would stick out past the fender lip.
Reading the verdicts
| Verdict | New gap | What it means |
|---|---|---|
| Clears | 10 mm (0.39 in) or more | Room to spare with the wheels straight and the car at rest. |
| Tight | 0 to 10 mm | It fits standing still, but there is little margin left for steering, bumps and tire variation. |
| Rubs | Below 0 | The tire or wheel would touch that part, or stick out past the fender lip. |
This is a static check. It does not model steering lock, suspension compression, body roll or the tire growing at speed. Turning the wheel swings the front of the tire toward the bumper and the rear toward the inner fender and frame, and a big bump pushes the tire up into the liner. That is why a tight gap often rubs at full lock or over bumps even when it looks fine in the driveway.
Sizes on the sidewall are nominal. The same size from two brands can differ by several millimeters in width and height, and mud-terrain tires with big shoulder lugs often run wider and taller. When the result is tight, check the published dimensions of the exact tire, test-fit one wheel and turn the steering from lock to lock before buying a full set.
Wheel spacers
- A spacer moves the wheel out by its full thickness: it buys inner room and costs the same amount of outer room.
- Use hub-centric spacers made for your hub and bolt pattern, so the wheel is centered on the hub and not only on the lug nuts.
- Slip-on spacers need longer studs so each lug nut keeps full thread engagement. Thicker bolt-on spacers bolt to the hub and carry their own studs.
- Never stack thin spacers. Use one spacer of the right thickness, torque to spec and check the torque again after a short drive.
When it does not fit
If the outer or top gap comes out tight or negative, the usual options in the US and Mexico are a wheel with more offset, a narrower or shorter tire, rolling or pulling the fender lip, trimming the plastic liner and bumper corners, fender flares that cover a tire that sticks out, or a leveling kit or lift to raise the body. A lift helps the top gap, not the inner or outer one. If the inner gap is the problem, a wheel with less offset or a spacer helps, at the cost of the outer gap.
Tires that stick out past the fender can break state rules on fender coverage or mud flaps, and every change in wheel position also changes the load on bearings and the steering feel. Check your local rules and have a shop confirm the fit before you drive on it.
Questions
How do I know if a bigger tire will fit without a lift?
Measure the top, outer and inner gaps on your truck today, enter them with the new size, and look for verdicts that clear with room to spare. A taller tire uses up half its extra diameter at the top, so a tire 50 mm taller needs more than 25 mm of top gap before you even count steering and bumps.
What does a tight result mean?
The new gap is between 0 and 10 mm with the wheels straight and the car at rest. Turning, braking and bumps use up more room than that, so a tight setup often rubs at full lock or when the suspension compresses.
Do wheel spacers help a bigger tire fit?
Only on the inside. A spacer moves the tire away from the strut by its thickness and closer to the fender by the same amount, and it does nothing for the top gap.
Where should I measure fender clearance?
At the front wheels, with the car on level ground and the wheels straight. Measure horizontally from the fender lip to the outer sidewall and vertically from the top of the tread to the liner.