What does a tyre size change actually do?
Put in the size on the truck and the size you are pricing, and this shows the rolling diameter, revs per kilometre, what the speedo will be out by, and where the engine sits at 100km/h with your diff and top gear.
11R22.5
1074.4mm
overall diameter
- Rolling circumference 3274.1mm
- 305.4 revolutions per kilometre
Imperial size: no aspect ratio is stated in "11R22.5", so this assumes a nominal 90% profile. The maker's data sheet lists the exact diameter and outranks this figure.
295/80R22.5
1043.5mm
overall diameter
- Rolling circumference 3179.9mm
- 314.5 revolutions per kilometre
What the change does
- Overall diameter moves 1074.4mm to 1043.5mm, a change of -2.88%.
- The new tyre is shorter, so at an indicated 100km/h the truck actually travels about 97.1km/h. The speedometer and tacho over-read.
- At a true 100km/h in top gear, engine speed moves 1990rpm to 2049rpm. The shorter tyre is effectively shorter gearing: more revs at cruise and a bit more pull, at the cost of fuel.
Load rating check before anything else: a size change is only lawful and safe if the new tyre's load rating still covers the axle group mass at your operating weights. The rating is on the sidewall; the axle limits are in our axle mass calculator.
Rolling circumference uses a 0.97 deflection factor, the standard allowance for a loaded truck tyre flattening at the contact patch, so figures here are a close model rather than a measurement. Metric sizes compute exactly from the size; imperial sizes use a stated nominal profile.
Reading the answer
Metric sizes are exact, imperial sizes are stated approximations
A metric size carries its whole geometry: 295/80R22.5 is a 295mm section at 80% profile on a 22.5 inch rim, so the diameter is pure arithmetic, 1043.5mm. An imperial size like 11R22.5 states only width and rim; the industry treats the profile as roughly 90% for tubeless sizes and 100% for tube-type sizes like 12.00R20, and that convention is what this tool uses, flagged on every imperial result. Manufacturer data sheets list measured diameters, about 1054mm for a typical 11R22.5, roughly 2% under the nominal calculation, and they are the authoritative source for an exact figure.
Why a few per cent of diameter matters
Diameter feeds everything downstream: a 3% taller tyre drops engine RPM at 100km/h by 3%, under-reads the speedo by 3%, and stretches the distance in the work diary by the same margin. On a linehaul truck that is the difference between cruising in the economy band and labouring under it, and between a clean fatigue audit and explaining a discrepancy.
The check that is not about gearing
Whatever the diameter says, the new tyre's load rating has to cover the axle group mass, and the group limits themselves changed on 1 August 2026. Run the group numbers in the axle mass calculator before committing to a size.
Common questions
How is truck tyre diameter calculated?
For a metric size the arithmetic is exact: rim diameter in inches times 25.4, plus two sidewalls of width times aspect ratio. A 295/80R22.5 is 22.5 x 25.4 + 2 x 0.80 x 295 = 1043.5mm. Imperial sizes like 11R22.5 state no aspect ratio, so this tool assumes a nominal 90% profile and says so; the manufacturer's data sheet gives the exact figure and outranks the approximation.
Is 295/80R22.5 the same as 11R22.5?
Close but not the same. They share the 22.5 inch rim and similar overall diameter, which is why fleets swap between them, but the metric size is slightly shorter on most makers' data sheets, so expect the speedo to read slightly high and engine RPM at 100km/h to rise slightly. The tool shows the shift; the exact figure depends on the maker's stated diameters.
Does changing tyre size affect the speedometer and fatigue records?
Yes. The speedo counts wheel revolutions calibrated to one rolling circumference, so a taller tyre makes it under-read and a shorter one over-read, and the same error flows into the tacho and electronic work diary distances. A few per cent matters at audit. Recalibrate after any size change rather than absorbing the error.
Can I fit any size with the same rim diameter?
No. The new tyre's load rating must still cover the axle group mass at your operating weights, the width must suit the rim and clear the chassis, and dual spacing has its own limits. Load rating is the one that bites legally: an axle group is only allowed its regulated mass if the tyres are rated to carry it.
Pricing rubber? Someone here is selling it.
Truck steer, drive and trailer tyres are listed by size, so search the size this tool just told you fits.
Browse tyres and rims