A tire size like 285/75R17 breaks down into three parts: the section width in millimeters, the sidewall aspect ratio as a percent of that width, and the wheel rim diameter in inches. Once you know which number is which, that string of digits on your sidewall stops looking like a secret code and starts telling you exactly what tire you are dealing with.
Most of us have squinted at a tire and wondered what all those numbers mean. Whether you are matching a spare, ordering a new set, spec-ing a work truck, or just trying to talk straight with the tire shop, reading the size right saves you money and headaches. This guide walks through every part of the sidewall the way a good shop hand would explain it to you standing at the rack: the metric size, the flotation size, the hidden math for real diameter, the load and speed markings, and the mistakes that cost people a return trip.
Key Takeaways
- A metric size like 285/75R17 reads as section width in millimeters, then aspect ratio as a percent of that width, then rim diameter in inches.
- Aspect ratio is a percentage, not a fixed measurement, so a lower number means a shorter sidewall and a higher number means a taller one on the same width tire.
- To find real diameter, multiply width by aspect ratio, double it, divide by 25.4, and add the rim: a 285/75R17 lands around 33.8 inches.
- Flotation sizes like 35x12.50R17 hand you the height in inches up front, so no formula is needed to know how tall the tire stands.
- Changing overall diameter by even one to two inches shifts your speedometer, odometer, and effective gearing, so verify before you buy.
- Load range, ply rating, load index, and speed rating are stamped separately and matter as much as the size for any truck that hauls or tows.
The Metric Size, One Number at a Time
Take a common truck tire: 285/75R17. Here is what each piece is telling you.
| Part | Example | What it means |
|---|---|---|
| First number | 285 | Section width in millimeters (widest point, sidewall to sidewall) |
| Second number | 75 | Aspect ratio, the sidewall height as a percent of the width |
| Letter | R | Radial construction (almost all modern tires) |
| Last number | 17 | Wheel rim diameter in inches |
So 285/75R17 is a tire about 285 mm wide, with a sidewall that stands 75 percent of that width tall, built as a radial, mounted on a 17 inch wheel. That is the whole code. Everything else on the sidewall, and there is a lot of it, is supporting information layered around those four values.
One point that catches people off guard: the section width is not the tread width. Section width is measured at the widest bulge of the sidewall, which sits below the tread. The part that actually touches the road, the contact patch, is narrower. So a 285 mm tire does not lay down 285 mm of rubber on the pavement. Keep that in mind when you are eyeing fender clearance.
A Word on the Aspect Ratio
The aspect ratio trips up a lot of folks. It is not a fixed measurement. It is a percentage. A 75 series sidewall is taller than a 65 series sidewall on the same width tire. Lower number means a shorter, stiffer sidewall. Higher number means a taller sidewall with more give. That matters for ride comfort, load, and how the tire soaks up rough roads.
Here is the part that surprises people: because aspect ratio is a percentage of width, a wider tire with the same aspect ratio has a taller sidewall in absolute terms. A 315/70 sidewall is taller in millimeters than a 285/70 sidewall, even though both read 70. So you cannot compare sidewall height across two tires just by glancing at the middle number. You have to run it against the width. That is exactly why the diameter math below matters.
The Prefix Letter You Might Be Ignoring
Plenty of truck tires carry a letter or two in front of the size, like LT285/75R17. That prefix is not decoration. It tells you what service the tire was engineered for, and it changes the construction underneath.
| Prefix | Stands for | Typical use |
|---|---|---|
| LT | Light Truck | Hauling, towing, heavier half-ton to one-ton trucks; stiffer casing, higher load ranges |
| P | P-metric | Passenger cars and lighter duty; softer ride, lower load capacity |
| ST | Special Trailer | Trailer axles only, where the tire does not steer or drive |
| (none) | Euro-metric | Passenger rating common on SUVs and crossovers |
Two tires can wear the exact same 285/75R17 numbers and behave very differently because one is an LT and one is a P-metric. The LT version holds more air, carries more weight, and rides firmer. If you tow or haul, the prefix is not optional to understand.
Working Out the Real Diameter
Here is where the size string earns its keep. You can figure the overall height of a metric tire with a little arithmetic. The formula looks like this:
- Multiply the width by the aspect ratio to get sidewall height in millimeters. For 285/75, that is 285 times 0.75, which comes to about 214 mm.
- Double it for the top and bottom sidewalls: 214 times 2 equals roughly 428 mm.
- Convert to inches by dividing by 25.4: about 16.8 inches of sidewall stack.
- Add the rim diameter: 16.8 plus 17 equals about 33.8 inches overall.
So a 285/75R17 stands right around 33.8 inches tall. That is close enough to call it a 34 inch tire in everyday talk.
Let me run a second example so the pattern sticks. Take 265/70R17, a size you see on a lot of stock half-ton trucks. Section width is 265 mm. Sidewall height is 265 times 0.70, which is about 185.5 mm. Double it to roughly 371 mm, divide by 25.4 to get about 14.6 inches of sidewall stack, then add the 17 inch rim. That gives about 31.6 inches overall, call it a 32 inch tire. So swapping from a 265/70R17 up to a 285/75R17 takes you from roughly 31.6 inches to roughly 33.8 inches, a jump of a bit over two inches in diameter. That is a real change, and it is exactly the kind of jump that throws a speedometer off if you do not account for it.
Notice that the same width can give very different heights. A 285/75R17 stands near 33.8 inches, but a 285/65R17 with the shorter 65 series sidewall works out to about 31.6 inches. Same width, same rim, more than two inches of difference in height, all from the aspect ratio. This is the single most common thing people miss when they read a size.
If arithmetic in the shop is not your idea of fun, our Tire Size Calculator runs these numbers for you. Punch in the size and it hands back the height, width, and circumference without you doing a lick of math. Bear in mind that the calculated diameter is a nominal figure. The molded shape, the load on the tire, and the inflation pressure all shift the real rolling diameter by a fraction of an inch, so treat the number as a very good estimate rather than a caliper reading.
Flotation Sizes, the Ones Written in Inches
Now here is the part that confuses people the most. Some tires, especially on pickups and off-road trucks, use what is called a flotation size. Instead of millimeters and percentages, flotation sizes just spell it out in inches.
Look at 35x12.50R17:
| Part | Example | What it means |
|---|---|---|
| First number | 35 | Overall tire height in inches |
| Second number | 12.50 | Section width in inches |
| Letter | R | Radial construction |
| Last number | 17 | Wheel rim diameter in inches |
The nice thing about a flotation size is the height is right there in the first number. A 35x12.50R17 is about 35 inches tall and 12.5 inches wide on a 17 inch rim. No formula needed to know how tall it stands.
The name flotation comes from the idea of a wide, tall tire that floats over soft ground like sand or mud instead of digging in. That is why you see them on trucks that spend time off the pavement. There is one honest catch worth knowing: the height stamped on a flotation tire is a nominal marketing number, and real-world measured height often runs a little under the label once the tire is mounted and carrying weight. A tire sold as a 35 may measure closer to 34.3 inches on the truck. Different brands run slightly different, so if precise clearance matters, look up the specific tire’s measured specs rather than trusting the round number.
Metric and Flotation, Side by Side
A lot of tire sizes overlap between the two systems. That 285/75R17 we worked out earlier and the 35x12.50R17 are nearly the same size tire, just written two different ways. Here is a rough comparison to show how close they run.
| Size | Written as | Rough height | Rough width |
|---|---|---|---|
| 285/75R17 | Metric | About 33.8 in | About 11.2 in |
| 35x12.50R17 | Flotation | About 35 in | About 12.5 in |
They are not identical, but they land in the same neighborhood. This is worth knowing when you are shopping, because a shop may offer you either style to fit the same truck. Always compare the actual height and width, not just the label. The flotation tire in this pair is both taller and wider, so it is not a straight swap even though the two get talked about interchangeably. Check your fender clearance and your wheel well before you assume a flotation size drops in where a metric size lived.
The Rest of the Sidewall: Load and Speed
The size is only the headline. Right after it, most tires carry a load index and a speed rating, something like 121/118R on a light truck tire. The two numbers before the letter are load indexes: the higher one for single-tire fitment and the lower one for dual-rear-wheel setups, where each tire carries a bit less. Those indexes map to a specific weight capacity in a chart, not to the number itself, so a load index of 121 is not 121 pounds or 121 anything obvious. You look it up in a load index table.
The letter after the load index is the speed rating, which is the maximum sustained speed the tire was tested for. Truck and trailer tires often carry conservative speed symbols, and a trailer tire in particular may be rated for a lower top speed than you would expect. Then there is load range, shown as a letter like C, D, E, or F, tied to a ply rating and a maximum inflation pressure in psi molded on the sidewall. Higher load range letters hold more air and carry more weight.
None of this changes how you read the size, but it changes whether the tire is right for your rig. Load index, speed rating, load range, and max pressure all vary by tire and get revised over time, so confirm them against the vehicle’s door jamb placard and the tire maker’s published load and inflation tables. When a tire is carrying heavy or towing, the correct inflation pressure comes from those tables, not from the round number on the sidewall, which is only the maximum.
Common Mistakes
Reading a size wrong usually is not about the math. It is about skipping a step. Here are the ones that send people back to the shop.
- Grabbing only the width. Two tires can both be 285s and stand two inches apart in height because the aspect ratios differ. Read all four parts, every time.
- Confusing section width with tread width. The section width is the widest bulge of the sidewall, not the rubber on the road. Do not size your fender clearance off the tread alone.
- Trusting a flotation number as exact. A tire sold as a 35 may measure closer to 34 inches mounted and loaded. If clearance is tight, use the specific tire’s measured diameter.
- Ignoring the prefix. An LT, a P, and an ST version of the same numbers are built for different jobs and different loads. Match the prefix to how you actually use the vehicle.
- Mixing systems on the same axle. Do not put a metric on one side and a flotation on the other, or mismatch diameters across an axle. It stresses the differential and wears parts unevenly.
- Forgetting to recheck the speedometer. Any real change in diameter changes how fast the wheels turn for a given road speed. Skip this and your odometer quietly lies to you for the life of the tires.
Why Getting the Size Right Matters
Tire size is not just about clearance in the wheel well, though that counts too. Change your overall diameter and you change how fast your wheels turn for a given road speed. That throws off your speedometer and odometer, and it changes the effective gearing of the truck.
Go with a taller tire and your engine turns fewer times per mile, which can feel sluggish on hills and hurt your pulling power. Go shorter and it works the other way. If you are switching sizes by any real amount, it is worth checking how it lines up with your axle gears using our Gear Ratio Calculator. That way you know whether your new tires and your rear end still make a good match.
There is a speedometer angle too. If your old tires stood 31.6 inches and your new ones stand 33.8 inches, that is roughly a 7 percent increase in diameter. At an indicated 60 mph, your true speed would be a few miles per hour higher than the dash shows, and every mile logged would be short of the real distance. On a work truck where mileage feeds into cost per mile and maintenance intervals, that drift adds up over a year. Some trucks can be recalibrated to the new size, and it is worth doing when the change is more than a small fraction.
A Quick Checklist Before You Buy
- Read all four parts of the size, do not just grab the width.
- Confirm whether you are looking at a metric or flotation size.
- Check the prefix, LT, P, or ST, so the tire matches how you use the vehicle.
- Figure the real diameter, or let the Tire Size Calculator do it.
- Compare the new diameter to your old one so your speedometer stays honest.
- Check load range, load index, and speed rating for your weight, all stamped separately on the sidewall.
- Verify inflation pressure for your load against the door jamb placard and the tire maker’s tables.
The Bottom Line
Reading a tire size comes down to knowing what each number stands for. In metric, it is width, aspect ratio, and rim, and you do a little math for the height. In flotation, the height is handed to you in inches right up front. Once you have that straight, and once you have glanced at the prefix, the load range, and the speed rating, you can shop with confidence and talk to any tire shop without getting talked into the wrong set.
Load range, speed ratings, inflation tables, and legal requirements for commercial tires do change over time, so verify the specifics for your rig with the tire maker or a qualified tire professional before you commit. When in doubt, measure twice and buy once. Your wallet and your speedometer will both thank you.