TR7/8 Tyre Pressure Low Profile Tyres

TR7/8 Low profile tyres

Many people, myself included, choose to fit larger 15″ wheels so that newer and better modern tyres can be used. Tyres for the original 13″ wheels are becoming increasingly difficult to source and often lack the performance, safety, and technology found in modern tyre designs.

The question is: what tyre pressures should be used with these larger wheels to achieve the best balance of grip, handling, and safety?

The general advice seems to be to increase pressures by around 1–3 psi when using low-profile tyres. On my cars, running 15″ wheels with 50-profile tyres, I used 26 psi at the front and 30 psi at the rear. For normal road use this felt fine, but when driving at higher speeds the car began to feel lighter and less planted than it should.

Interestingly, many owners running low-profile tyres appear to use even higher pressures, often 32 psi or more all round.

However, increasing tyre pressure too much can make the ride noticeably harsher and may actually reduce ultimate grip. In general, lower tyre pressures (within reason) tend to provide more grip because they increase the tyre’s contact patch with the road, although they can make the steering feel slightly less responsive and slower to react.

For reference, the factory workshop manual and owner’s handbook for both the TR7 and TR8 recommend the following tyre pressures:

  • Front: 24 psi
  • Rear: 28 psi

These figures were, of course, specified for the original tyre and wheel combinations, so the question remains whether and how much they should be adjusted when running modern 15″ wheels and low-profile tyres.

The 4 PSI Rule

A useful method for fine-tuning tyre pressures is the “4 PSI Rule”.

How it works:

  1. Start with your chosen cold tyre pressures
    Set the tyres to your desired cold pressures before driving.
  2. Drive for 20–30 minutes
    This allows the tyres to reach normal operating temperature and build pressure as they warm up.
  3. Check the pressures again
    Measure the hot tyre pressures using a reliable pressure gauge.
  4. Compare the increase
    • If the pressure has risen by approximately 4 psi, your cold pressure is likely close to optimum.
    • If the pressure has increased by more than 4 psi, the starting pressure was probably too low and should be increased slightly.
    • If the pressure has increased by less than 4 psi, the starting pressure was probably too high and could be reduced slightly.

Important: The 4 PSI Rule is primarily intended for normal road use on tarmac surfaces and should be used as a guide rather than an absolute rule.

Using this method on my TR8 fitted with 15″ wheels and 195/50 VR tyres, I have settled on the following cold pressures:

  • Front: 25.5 psi
  • Rear: 28.5 psi

These pressures provide a good balance of grip, stability, steering response, and ride quality while remaining close to the original factory recommendations.

My gold 15″ wheels in the picture above came from these guys  https://www.tyresave.co.uk/classic-cars/ who I highly recommend, and they will supply wheels Balanced with tyres fitted!