If you have ever looked at the sidewall of your tire and seen a string of numbers like 285/70R17 and wondered what it all means, you are not alone. Understanding tire sizing is one of the most important things you can do before upgrading your truck, Jeep, or SUV. This tire size guide breaks down every number, letter, and rating so you can make a confident, informed decision the next time you shop for rubber.
Whether you are running stock tires on your daily driver or shopping for aggressive off-road rubber for a full build, knowing how to read tire sizes saves you money, prevents fitment headaches, and helps you get the exact performance characteristics you want.
Let us use a common truck tire size as our example: LT285/70R17 121/118Q. Every character in that sequence tells you something specific about the tire.
The first letters indicate the tire type:
For trucks and SUVs that see any kind of work duty, towing, or off-road use, LT-rated tires are almost always the better choice. They feature reinforced sidewalls and higher load capacities compared to P-metric equivalents.
The first number — 285 in our example — is the section width measured in millimeters. This is the distance from one sidewall to the other at the tire's widest point when mounted on the recommended rim width and properly inflated.
Common section widths for trucks and SUVs include:
A wider tire puts more rubber on the ground, which can improve traction and give your truck a more aggressive stance. However, going too wide without adjusting your wheel offset or adding a lift can cause rubbing on the fenders or suspension components.
The second number — 70 — is the aspect ratio, expressed as a percentage. It tells you the sidewall height relative to the section width. A 70-series tire has a sidewall height that is 70 percent of 285mm, which works out to about 199.5mm or roughly 7.85 inches.
Here is what different aspect ratios mean in practice:
For off-road use, a taller sidewall is your friend. That extra rubber acts as a cushion between rocks and your expensive wheels, and it allows you to air down further for better traction without risking the rim.
The letter R stands for Radial construction, which is the standard for virtually all modern tires. In a radial tire, the internal cord plies run perpendicular to the direction of travel, which provides a flexible sidewall and better road contact.
You may occasionally see:
Unless you are building a vintage vehicle or have a very specific application, you will be buying radial tires.
The final number in the primary size — 17 — is the wheel diameter in inches. This must match your wheel size exactly. Common diameters for trucks and SUVs include:
Smaller wheel diameters allow for more sidewall, which generally improves ride quality and off-road performance. Larger diameters give you a more aggressive street look but reduce available tire options for serious off-road use.
After the primary size code, you will see numbers and a letter like 121/118Q.
The load index tells you how much weight each tire can support at maximum inflation. The dual numbers (121/118) indicate the load rating in single and dual rear wheel configurations:
This matters enormously if you tow or haul. Underrated tires are a safety hazard. Always ensure your tires can handle the gross vehicle weight rating (GVWR) of your truck plus whatever you plan to carry or tow.
The speed rating indicates the maximum sustained speed the tire is designed to handle safely:
Most truck and off-road tires carry a Q or R speed rating. If you are not tracking your truck, this is perfectly adequate for highway driving.
One of the most important things to understand is how a tire size translates to the tire's overall diameter — the measurement from the ground to the top of the tire. This is what determines your ground clearance, speedometer accuracy, and gear ratio effectiveness.
To calculate overall diameter:
1. Convert section width to inches: 285mm / 25.4 = 11.22 inches
2. Calculate sidewall height: 11.22 x 0.70 = 7.85 inches
3. Double the sidewall (top and bottom): 7.85 x 2 = 15.7 inches
4. Add wheel diameter: 15.7 + 17 = 32.7 inches overall
So a 285/70R17 is approximately a 32.7-inch tire. This is useful when comparing metric sizes to the inch-based sizing used by many off-road brands (like 35x12.50R17).
You will encounter two different sizing conventions:
Metric sizing (285/70R17) is what we have been discussing. It is precise and used by most tire manufacturers.
Flotation or inch-based sizing (35x12.50R17) states the overall diameter first, then the section width, then the wheel diameter — all in inches.
Breaking down 35x12.50R17:
Both systems describe the same physical tire dimensions, just from different starting points. Many off-road and mud-terrain tires use the flotation system because it communicates overall diameter more intuitively.
You will also see a load range designation, which indicates the tire's maximum inflation pressure and load capacity:
More plies mean more durability but also more weight and a firmer ride. For most truck owners who want a balance of off-road capability and daily comfort, Load Range D or E is the sweet spot.
Changing your overall tire diameter throws off your speedometer. Larger tires make your speedometer read slower than you are actually going, while smaller tires make it read faster. A two-inch increase in diameter can result in a three to four mph discrepancy at highway speeds. Many modern trucks can be recalibrated through the dealer or with an aftermarket programmer.
Larger, heavier tires require more energy to rotate, which can reduce fuel economy by one to three mpg depending on the size increase. Aggressive tread patterns also create more rolling resistance than highway-oriented designs. This is a trade-off most truck owners accept willingly, but it is worth knowing.
Larger tires effectively change your gear ratio, making your truck feel sluggish on acceleration and potentially causing the transmission to hunt for gears. Many shops recommend re-gearing the axles when going to 35-inch or larger tires, especially on vehicles with automatic transmissions. Common regear ratios for 35s are 4.56 or 4.88, depending on the vehicle.
More rotating mass takes longer to stop. Upgrading to significantly larger tires without upgrading brakes can increase stopping distances. This is a safety factor that is often overlooked.
Selecting the right tire size depends on several factors:
1. What is the primary use? Daily driving, towing, off-roading, or a combination?
2. Do you have a lift kit? The amount of lift determines the maximum tire size without rubbing. here
3. What wheel size and offset are you running? Wider tires need more clearance, and aggressive offsets push the tire outward. here
4. Are you willing to regear? Budget for axle gears if going to 35 inches or larger.
5. What is your budget for tires? Larger sizes cost more, and specialty sizes can be significantly more expensive.
Here are some general guidelines:
Here is a quick reference for popular truck tire sizes and their approximate overall diameters:
| Metric Size | Flotation Equivalent | Overall Diameter |
|---|---|---|
| 265/70R17 | 31.6x10.4R17 | ~31.6" |
| 275/70R17 | 32.2x10.8R17 | ~32.2" |
| 285/70R17 | 32.7x11.2R17 | ~32.7" |
| 285/75R17 | 33.8x11.2R17 | ~33.8" |
| 305/70R17 | 33.8x12.0R17 | ~33.8" |
| 35x12.50R17 | — | ~35.0" |
| 315/70R17 | 34.4x12.4R17 | ~34.4" |
| 37x12.50R17 | — | ~37.0" |
| 37x13.50R17 | — | ~37.0" |
| 40x13.50R17 | — | ~40.0" |
On the sidewall you will also find the Uniform Tire Quality Grading (UTQG) ratings, which include:
These ratings are useful for comparing tires within the same brand, but comparisons across brands can be inconsistent since each manufacturer conducts its own testing.
Reading tire sizes is straightforward once you know the system, but choosing the right size for your specific truck, lift, and driving style involves more nuance. At Redline Auto Creations in Tampa, we have fitted tires on over 61 full custom builds and work with more than 100 brand partners to source the exact tire for your application.
Whether you need help figuring out what fits your lifted truck or you want a complete tire and wheel package, our team can walk you through every option. Stop by our shop at 11626 N Florida Ave in Tampa, call us at (813) 544-4009, or here to schedule a consultation.
At Redline Auto Creations, we specialize in crafting custom vehicles with precision and passion. From wheels and tires to suspension, performance upgrades, and custom designs, we bring your vision to life. Contact us today and transform your ride.
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