Tire size and wheel offset comparison
Enter your current setup and one you are considering. See how diameter, speedometer reading and wheel position change, drawn to scale from the hub face.
Current setup
Height 31.61 inSidewall 7.30 inWidth 10.43 in
Backspacing 5.43 in
Proposed setup
Height 32.71 inSidewall 7.85 inWidth 11.22 in
Backspacing 4.75 in
Exact figures from the tire maker
Optional. The numbers above are calculated from each size, and real tires vary by model, tread depth and wear. If you have the maker’s spec sheet or a measurement, enter it here and it replaces the calculated value.
Current tire
Proposed tire
Add your own measurements
Optional. This tool cannot see your vehicle, so it never says a tire fits. If you measure the room on your own vehicle, it will compare each tire against your numbers, and draw your limits on the diagram.
Wheel off. From the flat hub face outward to the fender edge.
Wheel off. From the hub face inward to the nearest strut, spring, brake line or frame.
Wheel on, at normal ride height. From the top of the tire to the wheel arch.
Summary
Check before you buy
B is 1.10 in taller than A (+3.5%).
At an indicated 60 mph you would really be doing 62.1 mph, 3.5% faster than the speedometer shows.
The outer tire edge moves 1.67 in toward the fender and the inner edge moves 0.69 in away from the suspension.
Overall diameter
+3.5%
31.61 in to 32.71 in
True speed at 60 mph
62.1 mph
3.5% faster than shown
Outer tire edge
+1.67 in
toward the fender
Inner tire edge
−0.69 in
away from the suspension
Cross-section from the hub faceDrawn to scale. Drag a wheel by its offset handle to try a different offset.
Setup A: the wheel runs from 5.43 in inboard to 3.07 in outboard of the hub face, and the tire from 6.30 in inboard to 3.94 in outboard. Setup B: the wheel runs from 4.75 in inboard to 4.75 in outboard of the hub face, and the tire from 5.61 in inboard to 5.61 in outboard.
Wheel width, offset, tire width and diameter are drawn exactly from the numbers entered. The tire’s curve and the wheel’s inner profile are simplified, and tire width on the wheel is an estimate unless you entered a measured width.
Side viewBoth tires on the same axle, to scale.
Side view. Setup A is 31.61 in tall with a 7.30 in sidewall. Setup B is 32.71 in tall with a 7.85 in sidewall. The axle sits 0.55 in higher with B.
The numbersDistances from the hub face count inward toward the vehicle and outward toward the fender.
| Measure | A | B | Change |
|---|---|---|---|
| Tire | |||
| Size | 265/70R17 | 285/70R17 | — |
| Overall diameter | 31.61 in | 32.71 in | +1.10 in+3.5% |
| Sidewall height | 7.30 in | 7.85 in | +0.55 in+7.5% |
| Section width, published | 10.43 in | 11.22 in | +0.79 in+7.5% |
| Width on this wheelestimate | 10.23 in | 11.22 in | +0.99 in+9.6% |
| Circumference | 99.29 in | 102.76 in | +3.46 in+3.5% |
| Revolutions per mile, nominal | 638.1 | 616.6 | −21.5−3.4% |
| Wheel | |||
| Width | 7.5 in | 8.5 in | +1.0 in |
| Offset (ET) | +30 mm | 0 mm | −30 mm |
| Backspacing | 5.43 in | 4.75 in | −0.68 in−12.5% |
| Distance from the hub face | |||
| Wheel inner lip | 5.43 in | 4.75 in | −0.68 infarther from the suspension |
| Wheel outer lip | 3.07 in | 4.75 in | +1.68 incloser to the fender |
| Tire inner edgeestimate | 6.30 in | 5.61 in | −0.69 infarther from the suspension |
| Tire outer edgeestimate | 3.94 in | 5.61 in | +1.67 incloser to the fender |
| Track width, both sides | — | — | +2.36 inwider |
SpeedometerAssumes the speedometer was accurate on setup A.
| Speedometer reads | You are really doing | Difference |
|---|---|---|
| 30 mph | 31.0 mph | +1.0 |
| 45 mph | 46.6 mph | +1.6 |
| 60 mph | 62.1 mph | +2.1 |
| 70 mph | 72.4 mph | +2.4 |
| 80 mph | 82.8 mph | +2.8 |
| True speed | Speedometer shows | Difference |
|---|---|---|
| 35 mph | 33.8 mph | −1.2 |
| 45 mph | 43.5 mph | −1.5 |
| 55 mph | 53.1 mph | −1.9 |
| 65 mph | 62.8 mph | −2.2 |
| 75 mph | 72.5 mph | −2.5 |
The odometer is affected the same way: every 100 miles it shows is really 103.5 miles. Speedometers are calibrated to the original tire, and this assumes yours was accurate to begin with. A GPS speed app is an easy way to check.
What this means
Overall diameter +3.5%Check
Beyond the usual 3% guideline. Speedometer and odometer errors will be noticeable. Engine speed at the same road speed drops about 3.4%, so the gearing feels taller. Many vehicles let a shop or a tuner recalibrate the speedometer; check yours.
Tire is 0.99 in widerCheck
Estimated on the wheels you entered. Wider tires reach further toward both the fender and the suspension, and are more likely to touch the fender liner or suspension at full steering lock.
Outer tire edge 1.67 in closer to the fenderCheck
Whether the tire reaches the fender depends on your vehicle. Measure the room outboard of your hub face (see Add your own measurements) before you buy.
Axle height rises 0.55 inNote
That is half the change in diameter, from tire size alone. Real ride height also depends on your suspension and load. A taller tire also moves the top of the tire closer to the wheel arch.
Inner tire edge 0.69 in further from the suspensionNote
More room on the inside. The price is that the same amount moves outward, so check the outboard side.
Track width grows 2.36 inNote
Counted on both sides of the axle, from where the wheel centerline moves. A wider track usually steadies a vehicle in corners; changing it by a lot also changes steering feel and fender coverage.
Want a check against your own vehicle? Add your measurements in the left column (in), and clearance is compared against them.
What this tool cannot tell you
These are geometric estimates from the numbers you entered. They are not a fitment guarantee, and this tool never says a tire will clear your vehicle unless you supply your own measurements, and even then only for the points you measured.
It cannot see:
- The shape of your fender, the suspension travel, steering lock, camber and body roll.
- The inside of the wheel: spoke shape and brake caliper clearance differ by wheel model, so ask the wheel maker.
- Whether the tire is approved for the wheel width, or rated for your load and speed. Check the tire maker’s data.
- How your vehicle’s speedometer, transmission, ABS and stability systems react to a different size.
Real tires vary from their nominal size by model, tread depth, wear and pressure. Enter the maker’s figures under “Exact figures from the tire maker” when you have them.
Read the full accuracy and limits notice and the formulas.
The terms, in plain words
- Hub face
- The flat surface the wheel bolts against. Every distance on this site is measured from it, which is why drawings and numbers line up.
- Overall diameter
- Wheel diameter plus twice the sidewall height. It sets the speedometer reading and how tall the tire stands in the wheel arch.
- Sidewall height
- The width of the tire times its aspect ratio. A 265/70 tire has a sidewall 70% as tall as it is wide.
- Section width
- The widest point of the tire in millimetres. The published figure is measured on a specific wheel width, so on yours it can differ a little.
- Offset (ET)
- How far the wheel’s centerline sits from the hub face, in millimetres. Positive tucks the wheel inward, negative pushes it outward.
- Backspacing
- The distance from the hub face to the wheel’s inner edge. It carries the same information as offset and is common in the truck and off-road world.
- Track width
- The distance between the centers of the tires on one axle. Changing the offset on both sides changes it by twice as much.
Questions people ask
How much can I change tire diameter before it causes problems?
Many tire shops use about 3% as a rule of thumb. Within that, many vehicles cope with no changes. Past it, speedometer and odometer errors become noticeable and the gearing feels different.
Speedometer, transmission shift points, ABS and stability control are usually calibrated to the original tire size, so a large change can affect them. Some vehicles can be recalibrated by a shop. What your vehicle tolerates is a question for its maker or a good tire shop.
Will bigger tires make my speedometer read wrong?
Yes. A speedometer counts wheel rotations and assumes the original circumference. A taller tire covers more road per rotation, so you travel faster than the dial shows. A tire 3% taller turns an indicated 60 mph into about 61.8 mph. A shorter tire does the opposite.
The speedometer table above shows both directions for your two setups. A GPS speed app is an easy way to confirm the real figure.
What is wheel offset?
Offset, also written ET, is the distance in millimetres from the wheel’s centerline to the face that bolts against the hub. Positive offset puts the mounting face toward the outside, which sits the wheel further in toward the suspension. Negative offset sits it further out. Lowering the offset number moves the wheel outward.
How are backspacing and offset related?
Backspacing is the distance from the hub mounting face to the back (inner) edge of the wheel. They describe the same geometry. With the wheel width in inches and the offset in millimetres, backspacing in inches is (width + 1) ÷ 2 + offset ÷ 25.4. The extra inch allows for the rim flanges.
Will this tire and wheel fit my vehicle?
This tool cannot say, and it will not pretend to. It shows how far the tire and wheel move relative to the hub face, compared with the setup you have now. Whether that clears your fender, suspension and brakes depends on parts it cannot see.
If you measure the room outboard and inboard of the hub face and the gap above the tire, you can enter those numbers and each tire is compared against them. For the final answer, ask a shop or owners running the same combination on the same vehicle.
Why does the width on this wheel differ from the printed width?
A tire’s published section width is measured on a specific measuring rim. Mounted on a wider wheel, the tire spreads: about 0.2 in (5 mm) for every half inch of extra wheel width. On a narrower wheel it pinches by the same amount.
Tire makers do not publish the measuring rim as a formula, so the tool estimates it and labels the result as an estimate. If you have the maker’s measured width, enter it under Exact figures from the tire maker.
Does this site keep my numbers?
The page does not send what you type anywhere: the calculations run in your browser. The numbers appear in the page address so you can copy a link to the same comparison, which means anyone who opens that link sends those numbers to the host as part of the address. Your choice of units and colour theme is remembered on your own device, only after you make the choice. See the privacy policy for the details.