A truck tire size chart is only useful if you can read the size and check that an equivalent tire really fits. Two sizes that look different can be almost the same height, but height is not the only thing to verify.
As a tire specialist, I am asked about size equivalents every week. The question sounds simple, but a wrong swap can affect safety, speedometer accuracy and tire life. This guide explains how I read a truck tire size chart and what I check before I approve a change.
How to read a metric truck tire size
Take 295/75R22.5:
- 295 is the section width in millimeters.
- 75 is the aspect ratio: sidewall height as a percentage of the width.
- R means radial construction.
- 22.5 is the rim diameter in inches.
Older inch-based sizes such as 11R22.5 or 11.00R20 give the width in inches instead and use a different series, which is why they need a lookup table to compare.
Calculate the overall diameter
diameter (mm) = rim (in) × 25.4 + 2 × width × aspect ÷ 100
| Size | Sidewall (mm) | Calculated diameter (mm) |
|---|---|---|
| 295/75R22.5 | 221.3 | 1,014 |
| 315/70R22.5 | 220.5 | 1,013 |
| 295/80R22.5 | 236.0 | 1,044 |
| 315/80R22.5 | 252.0 | 1,076 |
By this calculation 295/75R22.5 and 315/70R22.5 differ by about 0.15%, so they are close in height. 315/80R22.5 is about 6% taller than 295/75R22.5. Real overall diameters vary by brand and pattern, so use the manufacturer’s data sheet for the final answer.
Check these before swapping sizes
Equal height does not mean interchangeable. Confirm the load capacity and load index for your axle, the tire inflation pressure for that load, clearance in the wheel well, rim width and offset, dual spacing, and the vehicle manufacturer’s requirements. Ask a qualified tire professional to approve the change.
Handy conversions
- Tread depth: 1/32 in = 0.79375 mm
- Pressure: 1 psi = 6.89476 kPa, so 100 psi is about 690 kPa or 6.9 bar
Look up equivalents instantly
Type your size into the Truck Tire Size Conversion Tool to see equivalent sizes across the 100, 90 and low profile series, and use its converters for pressure and tread depth. Once you pick a size, find the right inflation in our truck tire pressure guides such as the 245/70R19.5 tire pressure guide, and compare prices fairly with the Tire CPK Equalizer.
Why Size Matters More Than It Looks
A tire size is not just a label. It sets the diameter, the load capacity, the clearance in the wheel well and the way the vehicle tracks and brakes. Two sizes that appear close on paper can still behave differently once fitted, especially on a fully loaded truck. For that reason, I never treat a size chart as a final answer. I treat it as a shortlist to verify.
Load Index and Speed Symbol
After the size you will usually find a pair of numbers followed by a letter, such as 152/148L. The first number is the load index for a single tire, and the second is the load index when the tire is used in a dual assembly. Each index corresponds to a maximum load in kilograms, so a higher number means a higher capacity. The letter is the speed symbol, which shows the maximum speed the tire is rated for at its load. Common symbols are J (100 km/h), K (110 km/h), L (120 km/h) and M (130 km/h).
When you look for an equivalent, the replacement should carry at least the load index the axle needs, and a speed symbol that suits your operation.
Other Markings on the Sidewall
- DOT code. The last four digits show the week and year of manufacture, which helps you judge the age of a tire.
- TL or TT. TL means tubeless, and TT means the tire requires a tube.
- Ply rating or load range. This describes the strength of the tire construction, and it must match the pressure and load you plan to use.
- M+S or the three-peak mountain snowflake symbol. These show mud and snow capability, and the snowflake symbol shows that the tire passed a specific winter test.
Metric Sizes Versus Inch Sizes
Metric sizes such as 295/80R22.5 and inch-based sizes such as 11R22.5 describe similar tires in different systems. Tables often list them as equivalents, but equivalent does not mean identical. Section width, overall diameter and load rating can differ slightly between brands. I always confirm the data in the manufacturer’s catalogue before recommending a switch.
Why Diameter Changes the Speedometer
Using the example from the table above, 315/80R22.5 is about 6% taller than 295/75R22.5. A taller tire covers more distance with each revolution, so the speedometer and odometer will read lower than the true speed and distance. At a displayed 80 km/h, the real speed would be about 85 km/h. That affects speed limits, tachograph records and fuel calculations, so the difference should be checked and corrected where the vehicle allows.
Matching Tires on the Same Axle
On a dual assembly, both tires share the load. If one is taller or more worn than the other, the larger tire carries more weight and wears faster, and the smaller one can scrub. I recommend fitting tires of the same size, brand, pattern and similar wear level on each axle, and following the manufacturer’s tolerance for the difference in diameter between duals.
Mistakes I See When Fleets Change Sizes
- Relying on a chart without checking the load index for the axle.
- Ignoring rim width, which must suit the tire section width.
- Mixing sizes between axles without checking the effect on drivetrain and braking systems.
- Forgetting to adjust the inflation pressure for the new tire, which is why you should consult the correct inflation chart, such as our 245/70R19.5 tire pressure guide for that size.
- Not recalibrating the speedometer or tachograph after a significant change in diameter.
A Simple Process I Recommend
- Write down the current size, load index and speed symbol from the sidewall.
- Find candidate sizes in a conversion table, then calculate the overall diameter of each.
- Check load capacity and rim compatibility in the manufacturer’s data.
- Confirm clearance and dual spacing on the vehicle.
- Ask a qualified tire professional or the vehicle manufacturer to approve the change.
- Set the correct pressure for the new size and load.
Three Practical Examples
These examples use the diameters from the table above and show how I think through a change.
- 295/75R22.5 and 315/70R22.5. The calculated diameters are almost identical, so the speedometer will barely change. The 315 is wider, though, so I check that the rim width suits it and that there is enough clearance and dual spacing.
- 295/80R22.5 and 315/80R22.5. The second tire is about 3% taller, which is enough to affect speedometer readings and gearing. I would only approve it after checking clearance, load capacity and the vehicle manufacturer’s requirements.
- 11R22.5 and a metric equivalent. Charts commonly list them as equivalents, but I still compare the exact overall diameter and load index of the two specific tires, not just the size names.
Questions to Ask Your Tire Supplier
- What is the exact overall diameter and section width of this tire on the recommended rim?
- What are the load index and speed symbol for single and dual use?
- Which inflation pressure do you recommend for my axle load?
- Is the tire approved for my axle position and type of operation?
- What warranty applies if the size change causes fitment problems?
A good supplier answers these questions with data, not guesses. If you get vague answers, I would look for another source. When in doubt, a few minutes of checking costs far less than a wrong size on a loaded truck.
Keep Records of Every Size Change
If you change size on any vehicle, I recommend recording the old and new sizes, the date, the odometer reading and the reason for the change. Note any adjustments made to the speedometer, the tachograph and the inflation pressure. These records protect you if there is a later dispute with a customer, an insurer or an inspector, and they help you compare results before and after the change.
- Record the new overall diameter and load index.
- Note the approved pressure for the new size.
- Check the result again after the first few thousand kilometers, including wear pattern and clearance.
FAQ
What does R22.5 mean on a truck tire?
R means radial construction and 22.5 is the rim diameter in inches.
Can I replace 11R22.5 with 295/75R22.5?
They are commonly listed as equivalents, but confirm load capacity, pressure, clearance and rim fitment first.
Do I need to change the speedometer after changing size?
A different overall diameter changes the speedometer reading and distance, so compare diameters and ask the vehicle maker or a workshop.
