The difference between a wheel and a tyre is simple: the wheel is the rigid metal part (rim plus disc or spokes) that bolts to the hub, and the tyre is the flexible rubber casing fitted on the rim that holds air and touches the road. Together, wheels and tyres carry the vehicle’s weight, pass driving and braking torque to the road, provide grip for steering and cushion small road shocks. This page covers the wheel side (types, rim width, offset, PCD), the tyre side (construction, radial vs bias ply, tubed vs tubeless) and how to read a tyre size, with a worked example on diameter, revolutions per km and speedometer error.
Difference between wheel and tyre
| Point | Wheel | Tyre |
|---|---|---|
| What it is | Rigid rim joined to a centre disc or spokes | Flexible rubber and cord casing |
| Material | Pressed steel, aluminium alloy, magnesium alloy, steel wire | Natural and synthetic rubber, carbon black, textile and steel cords |
| Main job | Carries load from the hub, locates the tyre, transmits torque | Grips the road, cushions shocks, supports load through air pressure |
| Fixing | Bolted to the hub with studs or nuts on a pitch circle | Beads seat on the rim’s bead seats, held by air pressure |
| Wear | Lasts the life of the car unless bent, cracked or corroded | Wears out; replaced for tread depth, damage or age |
| Size marking | Rim width x diameter, offset, PCD, e.g. 5.5J x 15 ET40, 4×100 | Width/aspect ratio, construction, rim diameter, load and speed, e.g. 185/65 R15 88H |
In everyday talk “wheel” often means the whole assembly. In engineering drawings and parts lists the two are separate items with separate specifications, and a tyre only fits a wheel whose rim diameter and width fall in its approved range.
Functions of wheels and tyres
- Support the weight of the vehicle and its load, including load transfer during braking and cornering.
- Transmit tractive and braking torque from the axle to the road through the tyre contact patch.
- Produce the side force needed to steer (see cornering power for how the tyre does this).
- Absorb small road irregularities before they reach the suspension system.
- Keep the tyre accurately centred and balanced so the car runs without vibration.
Types of wheels
Pressed steel disc wheel
The most common wheel on budget cars, commercial vehicles and spare wheels. A rolled steel rim is welded or riveted to a pressed steel disc. The disc has holes for the wheel studs, and often extra holes for brake cooling and weight saving. A plastic or steel cover (wheel cap) is clipped on for looks. The drawing above shows the rim, the well in the middle of the rim and the valve hole.
Steel wheels are cheap, easy to repair and bend rather than crack in a pothole. Their drawback is weight, which adds to unsprung mass and rotating inertia.
Light alloy wheels: cast and forged
Alloy wheels are made of aluminium alloy (sometimes magnesium alloy for racing). They are lighter for the same strength, conduct brake heat away better and can be made in open spoke designs.
- Cast alloy wheels are made by pouring or low-pressure casting molten alloy into a mould. Most factory-fitted alloy wheels are cast. They are affordable but need thicker sections because cast metal has some porosity.
- Forged alloy wheels are pressed from a solid billet under very high force. The grain flow makes them stronger, so they can be lighter again. They cost much more and are used on sports cars and motorsport.
- Flow-formed wheels sit between the two: the centre is cast and the rim barrel is spun and stretched over a mandrel to improve its strength.
Wire-spoke wheel
A hub is laced to the rim with many tensioned steel spokes. Loads are carried by spokes in tension. Spoke wheels are light and flexible, and are still standard on many motorcycles and bicycles. On cars they survive mainly on classic and vintage models because they need periodic truing and usually need tubes.
Split rims and multi-piece rims for heavy vehicles
Trucks, buses, tractors and earthmovers often use rims made in two or more pieces: a rim base plus a removable side ring and a lock ring. The stiff bead of a large truck tyre cannot be stretched over a one-piece rim flange, so the rim comes apart instead. Multi-piece rims must only be inflated inside a safety cage, because a lock ring that is not seated can fly off with lethal force.
Wheel terms you need to know
| Term | Meaning |
|---|---|
| Rim diameter | Diameter at the bead seat, in inches, e.g. 15 in. It must match the tyre’s rim diameter exactly. |
| Rim width | Distance between the inside faces of the rim flanges, in inches, e.g. 5.5J. The letter (J, B, JJ) gives the flange shape. |
| Offset (ET) | Distance in mm from the wheel’s centre plane to its mounting face. Positive offset (ET40) puts the mounting face towards the outer side, pulling the wheel into the arch. Changing offset changes scrub radius and loads on the wheel bearings. |
| PCD (pitch circle diameter) | Diameter of the circle through the stud or bolt centres, written with the stud count, e.g. 4×100 means 4 studs on a 100 mm circle. |
| Centre bore | The hole in the middle that fits over the hub spigot. On a hub-centric wheel it centres the wheel; if it is larger, a spigot ring is used. |
| Bead seat | The slightly tapered ledge on each side of the rim where the tyre bead sits and seals. |
| Hump (safety hump) | A small raised ring just inside the bead seat. It stops the bead of a tubeless tyre slipping into the well if pressure drops or the car corners hard. |
| Well (drop centre) | The deeper middle section of the rim. The bead is pushed into the well when fitting so it can pass over the flange. |
Tyre construction: the parts of a tyre
- Tread: the thick patterned rubber that touches the road. Grooves carry water out of the contact patch; the compound decides grip and wear.
- Shoulder: the corner between tread and sidewall. It works hard in cornering and is thick to spread heat.
- Sidewall: the thin flexible wall that carries all the markings. It flexes on every revolution and gives most of the ride cushioning.
- Bead: a bundle of high-tensile steel wire wrapped in rubber at each inner edge. It locks the tyre onto the rim’s bead seat and makes the air seal.
- Carcass (body plies): layers of textile cord (polyester, nylon or rayon) in rubber, wrapped around the beads. The carcass holds the air pressure and shapes the tyre.
- Belts: steel cord layers under the tread in a radial tyre, sometimes with a nylon cap ply on top for high-speed tyres. They keep the tread flat and stiff.
- Inner liner: a layer of butyl or halobutyl rubber on the inside that keeps air from leaking through the casing. It replaces the inner tube in a tubeless tyre.
Bias ply vs radial vs belted bias tyres
| Feature | Bias ply (cross ply) | Belted bias | Radial |
|---|---|---|---|
| Carcass cord angle | About 30 to 40 degrees to the centreline, alternate plies crossing | Same as bias ply | About 90 degrees, running bead to bead |
| Belts under tread | None | Yes, glass fibre or steel | Yes, usually steel |
| Sidewall and tread | Act together, both stiff | Tread stiffened, sidewall still stiff | Work independently: flexible sidewall, stiff flat tread |
| Rolling resistance | Higher | Medium | Lowest |
| Tread life and grip at speed | Lower | Medium | Best |
| Where used today | Some trucks, tractors, two-wheelers, off-road and cheap trailer tyres | Rare now | Almost all cars, most new trucks and buses |
A radial tyre’s flexible sidewall lets the tread stay flat on the road while the tyre deflects, so the contact patch keeps its shape in corners. That is why radials give better grip, longer tread life and lower fuel use. Bias ply tyres have tougher sidewalls, which suits rough off-road and agricultural work.
Tubed vs tubeless tyres
| Point | Tubed tyre | Tubeless tyre |
|---|---|---|
| Air held by | A separate rubber inner tube | The inner liner and the bead seal on the rim |
| On a puncture | Tube bursts or deflates quickly | Usually loses air slowly because the nail plugs the hole |
| Rim needed | Any rim; spoke wheels need tubes | Airtight rim with safety humps and a sealed valve |
| Heat | Tube rubs against the casing and runs hotter | Runs cooler |
| Repair | Patch the tube | Plug or patch from inside, often without removing the wheel |
Tubeless is now standard on cars and most two-wheelers with alloy wheels. Tubes are still common on spoke-wheel motorcycles, older commercial vehicles and farm tyres.
How to read a tyre size: 185/65 R15 88H
| Marking | Meaning |
|---|---|
| 185 | Nominal section width in mm, sidewall to sidewall |
| 65 | Aspect ratio: sidewall height is 65% of the section width |
| R | Radial construction (a dash or “D” means bias ply) |
| 15 | Rim diameter in inches |
| 88 | Load index: 88 = 560 kg maximum per tyre at the specified pressure |
| H | Speed symbol: H = 210 km/h maximum (S = 180, T = 190, V = 240 km/h) |
The sidewall also shows “TUBELESS” where applicable, the maximum inflation pressure, the ply and cord materials, and a four-digit date code (week then year: 2324 means week 23 of 2024). Tyres sold in India must carry the ISI mark: automotive tyres are under compulsory BIS certification, and passenger car tyres are covered by IS 15633 (Automotive vehicles, pneumatic tyres for passenger car vehicles, diagonal and radial ply).
Worked example: tyre diameter, revolutions per km and speedometer error
Step 1: sidewall height. For 185/65 R15, sidewall height = 185 x 0.65 = 120.25 mm.
Step 2: overall diameter. Rim diameter = 15 x 25.4 = 381 mm. Overall diameter D = 381 + 2 x 120.25 = 621.5 mm.
Step 3: circumference. C = π x D = 3.1416 x 621.5 = 1,952.5 mm, about 1.95 m.
Step 4: revolutions per km. 1,000,000 mm ÷ 1,952.5 mm = about 512 revolutions per km.
These are nominal, unloaded figures. Under load the tyre squashes a little, and the effective rolling radius is a few per cent smaller, so the real number of revolutions per km is slightly higher. Manufacturers publish measured figures in their data books.
Step 5: speedometer error with a different size. A speedometer counts wheel (or gearbox output) revolutions and assumes the original tyre diameter. Suppose the owner fits 205/55 R16 instead:
- Sidewall = 205 x 0.55 = 112.75 mm
- Diameter = 16 x 25.4 + 2 x 112.75 = 406.4 + 225.5 = 631.9 mm
- Ratio = 631.9 ÷ 621.5 = 1.0167, so the new tyre is 1.67% larger
Each revolution now covers 1.67% more road. When the speedometer shows 100 km/h, the car is actually doing 100 x 1.0167 = about 101.7 km/h, and the odometer under-reads distance by the same proportion. Going the other way, a smaller 195/60 R15 (diameter 615.0 mm) would make the car travel at about 98.95 km/h when 100 is shown. A common workshop rule is to keep a replacement size within about 2 to 3% of the original diameter and to use a load index and speed symbol no lower than the original.
Wheel and tyre fitment mistakes to avoid
- Using wheels with the right PCD but the wrong centre bore and no spigot ring, which causes vibration.
- Fitting a much lower offset for a “wide” look, which increases scrub radius, bearing load and steering kickback. See steering geometry for why scrub radius matters.
- Mixing radial and bias ply tyres on the same axle.
- Choosing a tyre with a lower load index than the vehicle maker specifies.
How these tyre choices change fuel use, braking and handling is covered in our companion page on how tyres affect vehicle performance. Wheels and tyres are part of the automobile engineering syllabus in most B.Tech mechanical programmes, and NPTEL has free vehicle dynamics lectures on tyre behaviour.
FAQs
What is the difference between a wheel and a tyre?
The wheel is the rigid metal part made of rim and disc or spokes that bolts to the hub. The tyre is the rubber casing mounted on the rim that holds air and grips the road.
What does 185/65 R15 88H mean on a tyre?
185 mm section width, sidewall height 65% of the width, radial construction, 15 inch rim, load index 88 (560 kg per tyre) and speed symbol H (210 km/h maximum).
Why are radial tyres better than bias ply tyres for cars?
The radial carcass lets the sidewall flex while steel belts keep the tread flat and stiff. This gives lower rolling resistance, longer tread life and better grip at speed.
What is wheel offset?
Offset, marked ET, is the distance in mm from the wheel’s centre plane to its mounting face. Positive offset tucks the wheel into the arch; lower offset pushes it outwards.
How many revolutions does a 185/65 R15 tyre make per km?
Its nominal diameter is 621.5 mm and circumference about 1.95 m, so it makes about 512 revolutions per km unloaded, slightly more in real driving because the loaded radius is smaller.