Thinking about bigger tires for your truck, or a different size for your car? Use this free tire size calculator to compare any two sizes side by side. Enter your current tire and the new one, and you will instantly see the difference in overall diameter, width, sidewall height and revolutions per mile, plus exactly how far off your speedometer will be.
How to Use the Tire Size Calculator
- Find your current tire size on the sidewall of your tire or on the Tire and Loading Information placard inside the driver’s door jamb (for example, 265/70R17).
- Choose the width, aspect ratio and rim size from the drop-down menus, or type any size into the box, including light-truck sizes like LT285/70R17 and flotation sizes like 33x12.50R17.
- Enter the new size you are considering in the second box. The results update instantly.
- Check the colored verdict, the to-scale tire drawing, and the speedometer table before you buy.
- Use the Copy link button to save or share your exact comparison.
How to Read a Tire Size
Every number on your sidewall means something. Here is what each part of P265/70R17 115T tells you:
- P – tire type. P means P-metric (passenger). LT means light truck, built for heavier loads and towing. No letter usually means a Euro-metric size.
- 265 – section width in millimeters, measured from sidewall to sidewall.
- 70 – aspect ratio. The sidewall height is 70% of the width, so a higher number means a taller sidewall.
- R – radial construction, which nearly every modern tire uses.
- 17 – wheel (rim) diameter in inches.
- 115T – load index and speed rating. A load index of 115 means about 2,679 lbs per tire, and T means the tire is rated for up to 118 mph.
For more on the numbers that affect safety, see our guide to finding the right tire pressure and the NHTSA tire safety information.
How Tire Diameter Is Calculated
The calculator uses the same simple math tire shops use. First, find the sidewall height: width × aspect ratio. Then add two sidewalls to the rim diameter.
- Sidewall: 265 mm × 0.70 = 185.5 mm, or 7.30 inches
- Overall diameter: 17 + (2 × 7.30) = 31.61 inches
- Revolutions per mile: 63,360 ÷ (31.61 × 3.1416) = about 638
Flotation sizes are even easier: in 33x12.50R17, the tire is 33 inches tall, 12.50 inches wide and fits a 17-inch wheel.
The 3% Rule: How Much Bigger Can You Go?
A common rule of thumb among tire shops is to keep the new tire’s overall diameter within 3% of the original. Staying inside that range keeps your speedometer reasonably accurate and lets your ABS, traction control and stability control keep working the way the engineers intended. The calculator colors your result to make this easy:
- Green (within 3%) – generally a safe, simple swap.
- Amber (3% to 5%) – noticeable speedometer error; check clearance at full steering lock and over bumps.
- Red (more than 5%) – usually needs a leveling or lift kit and speedometer recalibration.
Going a little bigger on a truck is popular for looks and ground clearance. If you drive a pickup, our list of the best tires for the Ford F-150 covers the most common upgrade sizes.
What Changes When You Change Tire Size
Speedometer and odometer
Your speedometer counts wheel revolutions. A taller tire covers more ground per revolution, so your speedometer reads lower than your real speed and your odometer under-counts miles. A smaller tire does the opposite. The speedometer table in the calculator shows the exact numbers for your sizes.
Acceleration and fuel economy
Taller tires act like a taller gear, so acceleration and towing feel weaker. Bigger tires are also usually heavier and wider, which tends to lower fuel economy. Keep in mind that your trip computer will also miscalculate MPG until the speedometer is corrected. The U.S. Department of Energy shares more tips for keeping your fuel economy up.
Clearance and rubbing
Width matters as much as height. A wider tire can rub the fender liner, suspension parts or frame when you turn or hit a bump. Check the “width changes by” line in the results, and look at the backspacing of your wheels.
Load capacity and ride comfort
Never choose a tire with a lower load index than your original equipment. Lower-profile tires (smaller aspect ratio) sharpen steering but ride firmer and are easier to damage on potholes, while taller sidewalls ride softer.
Plus Sizing: Bigger Wheels, Same Overall Height
Plus sizing means fitting a larger wheel while lowering the aspect ratio so the overall diameter barely changes. Here is a real example for a compact SUV:
| Setup | Tire size | Overall diameter | Difference |
|---|---|---|---|
| Original | 225/65R17 | 28.52 in | – |
| Plus one (18-inch) | 235/55R18 | 28.18 in | −1.2% |
| Plus two (19-inch) | 235/50R19 | 28.25 in | −0.9% |
Both upgrades stay well within the 3% range, so the speedometer stays accurate. The trade-off is a firmer ride and usually more expensive tires.
Popular Tire Size Comparisons
Click any comparison to open it in the calculator above.
| Comparison | Diameter change | At 60 mph you are really going | Verdict |
|---|---|---|---|
| 265/70R17 vs 285/70R17 | +1.10 in (+3.5%) | 62.1 mph | Use caution |
| 275/65R18 vs 275/70R18 | +1.08 in (+3.4%) | 62.0 mph | Use caution |
| 245/75R16 vs 265/75R16 | +1.18 in (+3.9%) | 62.3 mph | Use caution |
| 265/70R17 vs 285/75R17 | +2.22 in (+7.0%) | 64.2 mph | Needs lift / recalibration |
| 33x12.50R17 vs 35x12.50R17 | +2.00 in (+6.1%) | 63.6 mph | Needs lift / recalibration |
| 285/75R16 vs 33x12.50R16 | +0.17 in (+0.5%) | 60.3 mph | Good swap |
| 215/55R17 vs 225/55R17 | +0.43 in (+1.6%) | 61.0 mph | Good swap |
Off-roaders often move to 33- or 35-inch tires. If that is you, our guide to the best tires for the Jeep Wrangler covers trail-ready options in these sizes.
Metric vs. Flotation Tire Sizes
Off-road and lifted-truck tires are often sold in flotation sizes, while most factory tires use metric sizes. To compare them, convert both to overall diameter, which is exactly what the calculator does. For example, a 285/75R16 measures 32.83 inches tall, so it is commonly called a “33-inch tire” and is within 0.5% of a true 33x12.50R16.
Tips Before You Switch Tire Sizes
- Start from the size on your door placard. That is the size your vehicle was tested and certified with.
- Match or exceed the original load index and speed rating.
- On AWD and 4WD vehicles, always replace tires as a matching set. Different diameters front to rear can strain the drivetrain.
- Plan for a speedometer recalibration if the difference is more than about 3%.
- Get an alignment after a size change, especially if you also install a lift or leveling kit. Learn more in our guide to tire balance and alignment.
- Buy from a reputable seller and check the manufacturer’s fitment notes. The U.S. Tire Manufacturers Association publishes helpful tire care resources.
Frequently Asked Questions
Can I put bigger tires on my car or truck?
Usually yes, as long as the new tire fits without rubbing, carries at least the same load index, and stays close to your original diameter. Within about 3% is generally a simple swap; bigger jumps often need a lift or leveling kit.
How much does a bigger tire change my speedometer?
It changes by the same percentage as the diameter. A tire that is 3.5% taller makes your speedometer read about 2 mph low at 60 mph. Use the speedometer table in the calculator for your exact sizes.
Do bigger tires lower gas mileage?
Most of the time, yes. Larger tires are heavier, wider and act like a taller gear, which usually reduces fuel economy. Your dashboard MPG reading will also be wrong until the speedometer is recalibrated.
Do I need new wheels to change tire size?
Not always. You can change the width and aspect ratio on the same rim diameter if the new width is approved for your wheel width. Changing the last number in the size (for example, 17 to 18) always requires new wheels.
Is 265/70R17 the same as a 32-inch tire?
Not quite. A 265/70R17 is about 31.6 inches tall. A 285/70R17 (32.7 inches) is the size most people mean by a “33-inch” upgrade on half-ton trucks.
Can I use different tire sizes on the front and rear?
Only if your vehicle was designed with a staggered setup from the factory. On AWD and 4WD vehicles, mismatched diameters can damage the transfer case or differentials.
Looking for your next set of tires? Browse all of our tire guides for reviews, buying advice and maintenance tips.