How to Adjust Your Tyre Pressure According to Your Speed

Did you know that having the right tyre pressure at high speeds is essential for your safety?

In Europe, speed limits generally restrict fast driving. However, on certain unrestricted roads or in specific circumstances, tyre pressure adjustments may be necessary.

When a tyre rolls at high speed, it is subjected to increased stresses, particularly deformation of the sidewalls, known as deflection, which can impact its performance and durability.

The Phenomenon of Tyre Deflection at High Speed

What is Tyre Deflection?

Engineers use the term deflection to describe the difference between the "unloaded" radius of a tyre (the distance between the top of the tyre and its centre, with no load applied) and the "loaded" radius (the distance between the centre of the tyre and the road when the tyre is in contact with the ground). This flattening depends primarily on the load, tyre pressure, and speed.

Impact of Speed on Deformation and Load Capacity

As speed increases, tyres are subjected to significant centrifugal and thermal forces. If the pressure is not adjusted correctly, this can lead to excessive deformation of the tyre, causing tread deterioration. Simultaneously, the tyre's maximum load capacity decreases as speed increases.

Why Adjust the Pressure?

Proper pressure reduces excessive deflection and helps maintain an optimal level of safety. Driving with tyres correctly inflated according to speed and load preserves performance, limits wear, and prevents overheating.

What Tyre Pressure is Required at High Speed?

Take, for example, a standard tyre with the size 215/50R17 91W, designed to make approximately 520 revolutions to cover a kilometre. At a speed of 50 km/h, this equates to about 7 revolutions per second. When the speed increases to 240 km/h, this number rises to 35 revolutions per second. This means deflection (tyre deformation) occurs every 28 milliseconds. This phenomenon results in gradual overheating and significant stress on the tyre’s internal structure.

etrto The ETRTO (European Tyre and Rim Technical Organisation) has recognised that excessive tyre deflection at high speeds can compromise driver safety. Indeed, significant deformation of the sidewalls and tread can lead to instability, overheating, and in extreme cases, tyre failure.

To minimise these risks, the ETRTO recommends adjusting tyre pressure according to speed. At high speeds (beyond 160 km/h), it is necessary to increase tyre pressure to compensate for the gradual reduction in their load capacity. This adjustment reduces deflection, improves vehicle stability, and limits tyre overheating.

Guidelines for Determining the Correct Pressure for a Given Speed

A tyre's maximum load capacity decreases as speed increases due to mechanical and thermal stresses. Here are the general rules according to the ETRTO:

1. Reference Thresholds for Speeds:

The reference speed depends on the tyre’s speed rating:

Q to V (160 to 240 km/h): Load reduction begins as speed exceeds 160 km/h.
W (270 km/h): Load reduction begins at 190 km/h.
Y (300 km/h): Load reduction begins at 220 km/h.

2. Reduction in Load Capacity:

Between 160 and 210 km/h (ratings Q to V), load capacity decreases by:

5% for every additional 10 km/h.

Beyond 210 km/h, reductions are specific to each speed rating:

Rating V (240 km/h): Total reduction of 15% at 240 km/h.
Rating W (270 km/h): Total reduction of 20% at 270 km/h.
Rating Y (300 km/h): Total reduction of 15% at 300 km/h.

3. Pressure Adjustment to Compensate:

The pressure must be increased to compensate for the loss of load capacity and limit excessive deflection (flattening) of the tyres.

Official ETRTO Recommendation Table

Speed Index R (max 106mph / 170km/h)
Car Speed		Increased Pressure		Load	Tyre Pressure
Km/h	mph	bar	psi	Load	bar	psi
160	100	0.00	0.00	615kg (maximum load)	2.5	36.3
170	106	+0.20	+2.9	584.25kg (-5%)	2.7	39.2

Speed Index S (max 112mph / 180km/h)
Car Speed		Increased Pressure		Load	Tyre Pressure
Km/h	mph	bar	psi	Load	bar	psi
160	100	0.00	0.00	615kg (maximum load)	2.5	36.3
180	112	+0.20	+2.9	584.25kg (-5%)	2.7	39.2

Speed Index T (max 118mph / 190km/h)
Car Speed		Increased Pressure		Load	Tyre Pressure
Km/h	mph	bar	psi	Load	bar	psi
160	100	0.00	0.00	615kg (maximum load)	2.5	36.3
190	118	+0.20	+2.9	584.25kg (-5%)	2.7	39.2

Speed Index H (max 130mph / 210km/h)
Car Speed		Increased Pressure		Load	Tyre Pressure
Km/h	mph	bar	psi	Load	bar	psi
160	100	0.00	0.00	615kg (maximum load)	2.5	36.3
190	118	+0.20	+2.9	584.25kg (-5%)	2.7	39.2
210	130	+0.30	+4.3	553.5kg (-10%)	2.8	40.6

Speed Index V (max 149mph / 240km/h)
Car Speed		Increased Pressure		Load	Tyre Pressure
Km/h	mph	bar	psi	Load	bar	psi
160	100	0.00	0.00	615kg (maximum load)	2.5	36.3
190	118	+0.20	+2.9	584.25kg (-5%)	2.7	39.2
210	124	+0.30	+4.3	553.5kg (-10%)	2.8	40.6
240	149	+0.40	+5.8	522.75kg (-15%)	2.9	42.1

Speed Index W (max 168mph / 270km/h)
Car Speed		Increased Pressure		Load	Tyre Pressure
Km/h	mph	bar	psi	Load	bar	psi
160	100	0.00	0.00	615kg (maximum load)	2.5	36.3
190	118	+0.20	+2.9	584.25kg (-5%)	2.7	39.2
210	124	+0.30	+4.3	553.5kg (-10%)	2.8	40.6
240	149	+0.40	+5.8	522.75kg (-15%)	2.9	42.1
270	168	+0.50	+7.2	492kg (-20%)	3	43.5

Speed Index Y (max 186mph / 300km/h)
Car Speed		Increased Pressure		Load	Tyre Pressure
Km/h	mph	bar	psi	Load	bar	psi
160-220	100-136	0.00	0.00	615kg (maximum load)	2.5	36.3
240	149	+0.20	+2.9	584.25kg (-5%)	2.7	39.2
270	168	+0.30	+4.3	553.5kg (-10%)	2,8	40.6
300	186	+0.60	+8,7	522.75kg (-15%)	3.1	45

The tables are based on a tyre with a load rating of 91, and all indicated pressures are measured cold.

Advice from Our Experts:
Adjusting your tyre pressure according to your speed is a simple yet essential action to ensure your safety and preserve your tyres. Before hitting the road, especially for fast or long journeys, take a few minutes to check your tyre pressure.