E85 tyres, load index and speed ratings.

[Finisterre]

E85 load index and speed ratings. - I searched for load index recommendations on the forum before I went shopping for my new tyres but didn't find much so I thought I might add something for other people to find in future.

I have just changed my tyres (Goodyear F1 Asymmetric 2) from 94Y XL to 91Y - the choice offered by Camskil - XL being extra load.

91 is rated for 615 Kg at 2.5 bar
92 rated 630 Kg
93 rated 650 Kg
94 rated 670 Kg
95 rated 690 Kg
96 rated 710 Kg
As far as I can tell the 2.5 bar tyre pressure is a nominal figure for testing, manufacturing and rating purposes, it isn't the correct pressure to run the tyre at on the Zed. That is your choice. it seems the Xl tyres will take 42psi and the standard go to 36psi.

Black Circles registration lookup suggested 225/45R17 Y 91 is correct for my E85 Z4 on equal sized wheels. Anyway - I found this formula (not necessarily definitive (as it was something some bloke just like me stuck on the internet)) for working out the load index Li=(Weight of car-100 kg)/2, which worked out for the 2.0 (1295 - 100) '/2 = 597

The LI along with the tire speed symbol and air-pressure help define the tire stiffness (tire spring rate). To save people having to look up their car weight in kilos and then attempting very complex calculations I have made a little look up table.
2,2 1325 = 612
2,5 1335 = 617
3,0 1365 = 632
2,0 1295 = 597
facelift
2,0 1295 = 597
2,5 SE 1360 = 630
2,5 si SE 1360 = 630
3,0 si 1385 = 642
3,2M 1485 = 692
3,0 si coupe 1395 = 647

Speed wasn't an issue for me.
"W" Is for cars capable of maintaining 168 mph (270 kmph)
"Y" is for cars capable of maintaining 186 mph (300 kmph)

So... What are the lower load index tyres like in comparison? Kind of comfy, slightly less noisy, maybe a little less rigorous in corners. and a more relaxed drive, but not actually better.


I am happy to be corrected if I have something wrong and I will edit it out.
Cheers, Fin

 

[Stuart Truman]

Did you save much money dropping down on LI and speed rating?

The big thing for me where I live is reinforced sidewalls. I went through a bunch of "egged" tyres from hitting potholes. Since switching to reinforced sidewall tyres it's not been a problem.

 

[Finisterre]

Did you save much money dropping down on LI and speed rating?

The big thing for me where I live is reinforced sidewalls. I went through a bunch of "egged" tyres from hitting potholes. Since switching to reinforced sidewall tyres it's not been a problem.

There was a fiver in it per tyre. I wasn't bothered about the £20, I decided to try the standards because quite a few comments are dotted around the internet suggesting that tram-lining is less obtrusive with the standard load tyres. They certainly aren't any worse going by my experience so far.

 

[mmm-five]

You have to have tyres for the max weight of the car, plus a certain amount of headroom to take into account extra stresses on the tyre when cornering or driving on bumpy roads - not the kerb weight.

So add 2 x 100kg occupants, plus 50kg of luggage and your car weighs 250kg more (so right at the Z4 max load of 1800kg). Then add in the extra load (20% say) the tyres have to take when turning or hitting bumps/potholes and you need 250kg of available load.

Then take into account that the Z4 doesn't run at 2.5 bar, but 2.1/2.2 bar and you begin to see why the tyre load index looks very high.

You're also unlikely to feel any difference between a 91 & 99 load tyre. The XL nomenclature is not directly rated to the 91/99 index, but rather refers to the fact that the sidewalls are reinforced - so a 91XL may have a stiffer (harsher riding) sidewall than a 99 non-XL.

In your example the Li=(Weight of car-100 kg)/2 would work out for an ///M at LI=(1500 - 100)/2 = 700, and would need a 96 rated tyre when in fact it came with 91f/95r Continental SportContact M3s.

 

[Finisterre]

In your example the Li=(Weight of car-100 kg)/2 would work out for an ///M at LI=(1500 - 100)/2 = 700, and would need a 96 rated tyre when in fact it came with 91f/95r Continental SportContact M3s.

Yeah, I wondered about that. I was curious to see how the Li might alter through the range... And I find it easier to work through stuff on paper, preferably with another head involved. That calculation looked too simple to be anything more than a rough rule of thumb.

For our 2.0L - Four 91 tyres gives me 2460kg - less (1300 +300) say 1600kg for a very heavily loaded car = 860kg headroom. ~50%, which sounds a bit low when you think about it. We shall see, I will report back any problems that arise.



EDIT - whoops edited numbers for the two litre rather than the 3.2.