Dear John: Tyre sizes

Dear John: Tyre sizes

No, not the breakup letters. Our friendly neighbourhood agony uncle – of the automotive kind – answers your most burning questions…


"What’s the best size track tyre for my Astra VXR? I’ve heard 18s are better than 19s?"

There’s quite a lot more to tyres than just wheel size! Given that’s the basis of the question though, I’ll assume that you’re looking to get a set of wheels specifically for track. Here are some considerations (other than good looks!) to take in to account when selecting your wheel/tyre combination:

a) Rolling circumference. The overall circumference of the tyre can affect acceleration and top speed (and ground clearance). Your best bet is to keep within the range the manufacturer recommends, which on an Astra H varies from 1985mm for 16s, to 2032mm for 19s. It would be safe to assume that anything between or around those will be fine. It's marginal, but smaller circumference setups will give better acceleration but lower top speed, whereas larger circumference setups will give a higher top speed and less acceleration.

b) Tyre width. Wider tyres will offer more grip, naturally, but will also make the setup heavier. Narrower setups will feel more lively because of the lower weight and grip. How much tyre you want on the road will be determined by how much you can fit under the arches, how much power you have, and your driving style and personal preference. How a tyre feels can also be altered by the width of the wheel itself, as you'll see below...

c) Tyre profile. This is the height of the tyre, measured as a percentage of the width of the tyre. Sidewall stiffness varies from tyre to tyre, so do some research when selecting them, but all else being equal a taller sidewall will result in more tyre roll and vice versa. A bit of tyre roll can make the car feel a bit more predictable, the increased flexibility allowing for a larger contact patch outside of optimal camber. Depending on how you prefer your car to feel, to achieve your desired rolling circumference you may want a shorter tyre on a bigger rim or a taller tyre on a smaller rim. I won’t consider comfort here as that’s not usually a big concern on track!

d) Wheel width. How wide a wheel you want will depend on the width of the tyre desired, the cross-sectional shape of the tyre desired, how much you can fit in your arches, and weight considerations. Having a tyre that is narrower than the wheel introduces ‘stretch’ – too much stretch and your tyres just won’t work as intended, but a little stretch can artificially stiffen up the sidewalls. Or, for slightly more tyre roll for a given profile and potentially more progressive feel, go for a tyre and wheel of the same width.

e) Weight. The overall weight of your wheel and tyre combo is an important consideration – it can really affect how your car feels. Lighter wheels feel more lively and accelerate better, whereas a heavy combo can make the car feel more sluggish. Do your research to find out whether a larger wheel and smaller tyre works out lighter than a smaller wheel and larger tyre. It’s not always the way round you think it will be!

f) Tyre availability. The above is all well and good - unless you can’t get a tyre in the theoretical size you need. Not all brands make every conceivable variation of tyre size, so do a little research and see what you can get your hands on. Sometimes your 'ideal' tyre size might not be available, so decide what you're willing to compromise on.

g) Budget. As always, it comes down to what you can afford. Larger diameter wheels often mean more expensive tyres, and lightweight setups can also be pricey. Don't get impatient and compromise on performance - do your homework and save up for your ideal setup for just a little longer – it’ll be worth, it I promise!

There’s a lot to take in to account, and to make things easier I use to calculate the numbers and make comparisons between two setups. This will also help you visualise any changes in offset and how that might affect scrub radius. You should also consider whether your selection will impact future upgrades; there would be little point in getting shiny new wheels if the mega 8-pot big brake kit you have your eye on for next winter’s upgrade won’t fit underneath them!

Top tip: beg, borrow or steal your mates’ setups to try before committing to anything if you can!

Good luck and have fun!
John x