Understanding tyre load ratings

azibux1 · 16 Apr 2019 at 16:08

Got a question here that I'm sure someone will know the answer to.

My car comes stock with:

F: 245/35 R19 Y 93
R: 265/35 R19 Y 98

The wheels are identical size to another model from the same manufacturer which comes with:

F: 255/35 R19 Y 92
R: 275/35 R19 Y 100

A lot of people fit these wider tyres instead for better grip.

PS4S are available (when they’re in stock) in:

255/35 R19 (92Y) TL – falls below the stock 93 rating on the front
275/35 R19 (96Y) TL – falls below the stock 98 rating on the rear
255/35 R19 (96Y) XL TL
275/35 R19 (100Y) XL TL

92 = 630kg load per tyre
93 = 650kg load per tyre
96 = 710kg load per tyre
98 = 750kg load per tyre

So I could just get the ones higher rated than stock but they are also XL (extra load) – any downside to that?

Interesting point is the car has 1550kg kerb weight so each tyre only needs to be rated to 387.5kg anyway surely? Then they all meet the requirement, albeit not meeting or exceeding the original specs. Or is my maths/understanding wrong somewhere?

Thanks!

GeX · 16 Apr 2019 at 16:55

Downsides from what I understand with XL tyres is possibly more noise, and increased weight leading to higher unsprung weight and rolling resistance.

Source; just fitted XL tyres on the back of my Civic.

NVP · 16 Apr 2019 at 17:50

Does the downforce at vmax need to be added into the weight calc?

azibux1 · 16 Apr 2019 at 19:05

GeX said:
Downsides from what I understand with XL tyres is possibly more noise, and increased weight leading to higher unsprung weight and rolling resistance.

Source; just fitted XL tyres on the back of my Civic.

Hmm fair enough..nothing too major then

NVP said:
Does the downforce at vmax need to be added into the weight calc?

Can't tell if you're being serious or mocking my seemingly in depth analysis lol

It's just something I've checked before and they're important of course so didn't want to mess it up

NVP · 16 Apr 2019 at 20:18

No mocking what so ever.

azibux1 · 16 Apr 2019 at 20:49

You're potentially right then. Otherwise even the stock tyres are massively overrated.

Amp34 · 16 Apr 2019 at 20:53

Curb weight is irrelevant, it’s MAM you’ll want to do your calculations for. There’s also not a perfect 25:25:25:25 weight distribution either. Depending where the engine is and how much weight is in the boot some of the wheels could have a lot more weight than others. That would need to be taken into account too.

XL tyres may also have a stiffer sidewalk, which could make for a slightly bumpier ride.

jpaul · 16 Apr 2019 at 20:57

Do you have to check with insurer at all ?
I was looking at fitting michelin cross-climates, exclusively in xl for my car

azibux1 · 16 Apr 2019 at 21:17

Amp34 said:
Curb weight is irrelevant, it’s MAM you’ll want to do your calculations for. There’s also not a perfect 25:25:25:25 weight distribution either. Depending where the engine is and how much weight is in the boot some of the wheels could have a lot more weight than others. That would need to be taken into account too.

XL tyres may also have a stiffer sidewalk, which could make for a slightly bumpier ride.

Ah as I suspected my calculations were well off then!

That's a shame.

Might end up going for the XLs. Will do some more thinking and reading.

Will also check if my current ones are XL as I have a strange feeling they might be. Will make the whole decision making easier lol

Edit: just checked a high res pic and sure enough they have "extra load" on the side wall.