Rocambolesque
Rocambolesque Reader
4/27/20 4:41 p.m.

I am currently shopping for new tires for my project car.  I have a question regarding the tire's load ratings. 

The car is a 190E 2.3. According to Wikipedia, it weights about 2700 lbs. The stock tire size is 185/65/15. I looked on various tire sites and this size usually comes with a load index of 88, which is 1235 lbs.

Five years ago, I installed 195/50/15 Kumho PA31's with a load index of 82 (1047 lbs) on it. I didn't even look at the load index at first, drove the car for a few years and parked it to do major work on it. Never had a problem and the tires worn evenly.

Now that the car is almost ready, it needs new tires as the old Kumhos are pretty much dry and cracked from sitting in the sun for all that time. I am either going with the same Kumho PA31 in 195/50/15 again or with Kumho PS31 in 205/50/15. The 205's have a load index of 86 (1168 lbs).

When considering a tire's load rating, is it with the static load only? Or do I have to consider the dynamic weight transfer when cornering and braking? Since I will probably never load the car to GVWR, I could surely get away with a lower load index, right? Assuming a driver and passenger weight of 150 lbs each and a 60% front weight bias, the static weight on each front tire would be 900 lbs. Therefore, I should be good with the 195's. Is my reasoning good? Or is this totally unsafe?

The look of the 195's is very cool, and since this car will see mostly street driving I don't think I'll see the handling difference between the 195's and the 205's. I plan on attending at least one autocross or track day this year (first time for both). If I like it, then I would buy a dedicated set of tires/wheels. 

What do you think?

Toyman01 (Forum Supporter)
Toyman01 (Forum Supporter) MegaDork
4/27/20 4:48 p.m.

The weight of the vehicle and contents only. 

collinskl1
collinskl1 Reader
4/27/20 5:11 p.m.

The load ratings are static, not dynamic. The door sticker should tell you the mfg's load rating, which will be based on gross axle weight ratings. If you've significantly changed the cars weight or won't ever load it to the gawr loads, you could use a lighter rated tire, in theory. 

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
4/27/20 5:43 p.m.

I have used load ratings to adjust recommended tire pressure to suit a new size  with some success.

 

Used to have a book with a chart in it for those sort of shenanigans.  The load rating changes with pressure.  Lower rated tire will want more pressure for the same vehicle.

bobzilla
bobzilla MegaDork
4/27/20 5:48 p.m.

Our rule of thumb when mixing and matching tires and load ratings were as such: try to stay similar with the load rating and speed rating. If you do drop load rating do not also drop speed rating. 
 

I can tell you that a 2800lb fwd car set up for H rated tires and 88 load rating does not like 87T tires. 

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