Tires: Low Profile Tire Inflation Pressure, stock tires, directional arrows

Question
QUESTION: I have two questions because I have had to drive my daughter's car for a few days while she uses my truck to move.

My first question: She has low profile (215/35) tires on a honda civic and says that she runs them at 50psi. This is of course not Honda's recommendation for the stock tires and is 6psi above the max on the tires. I am trying to find some good guielines to use. Since they are so different from the stock tires I do not want to just count on the factory recommendation. Any suggestions?

The second question: While checking the tires for specs to try get a good inflation pressure I found that the two tires on the right side of the car are from one manufacturer and the left side has a different manufacturer. I am not sure exactly how many miles are on the current configuration. In my own experience I have had to put two new tires on a vehicle (couldn't afford all four) but, I have always put matching tires on the same end rather than the same side. I know it is not good to have different tires on opposite sides for many reasons. The question is, would it be better to leave them where they are now that they are broken in or should I suggest that she move them so that the like tires are on the same end?

Thank your for your help,
Jim

ANSWER: Jim,

If the tires on your daughter's car are the same as the original (as indicated on the placard), then use the placard pressure.

But from the partial tire size you gave me, I don't think that is the case.  So I will need to know what the placard says (size and pressure) and what the size of the current tires are.

And I agree, tires should be paired on an axle - not side to side.  And I can't tell if it would be better to get this fixed unless you tell me what the tread depths of the tires are - and whether or not the tires are directional (arrows on the sidewall) or asymmetrical  (Says "This Side Out" on the sidewall).

---------- FOLLOW-UP ----------

QUESTION: Barry,
The full sizes and manufacturers are:
Driver's Side: 215/35 R18 80H by WHANKOOK Radial 106
Passenger Side: P215/35 ZR18 80W Continental ContiExtremeContact

I know this is also a problem since one set is P-Metric and the other is ISO Metric. So much for trusting their tire guy.

I looked at the tread on all four tires. The only one with noticible wear is the left-front. It is worn on the outer edges to about half the depth of the same tire in the rear. The inner edge is a bit more worn than the outer one. The overall wear on the rest of the tires looks ok. I can't find my micrometer but they are all at least up to Lincoln's eye on the penny test.

Unfortunately I beleive that all four are directional. The WANKOOK's definitely are since I found the indicator. All of the information I have found on the Continentals is that they are non-directional.

The fact that they have different speed ratings makes me a little nervous as well.

So I guess this adds a third question. What are the pros and cons to moving the directional WANKOOK to the opposite side rear (running backwards)? Does it make any sense to take it to a shop and have it reversed on the wheel?

Thanks for your quick response,
Jim

ANSWER: Jim,

I've seen your other postings and I'm going to push the "duplicate question" button when I get through answering this one.  Don't be alarmed when you get that response.  I have to do this or I keep getting requests to answer those questions.

First, please check the tire size again.  My book doesn't list a P185/60R14 at 29 psi for any Honda - and that concerns me.  While you are at it, it would be helpful if I knew the year and model.

Running a directional tire backwards isn't as bad as some folks think.  It definitely will reduce the wet traction properties, but the reduction is fairly small.

However, the unbalance might be a problem as cars tend to pivot around odd tires when heavy braking is involved (severe cornering, too!)

The difference in speed rating is a concern - and the W rating should probably go on the back.

In the meantime let's also get a tread depth measurement on all the tires.

---------- FOLLOW-UP ----------

QUESTION: Sorry about the duplicates.

The car is a 1995 Honda Civic EX. I checked the plackard in the glove compartment and it says P185/60 R14 82H and specifies 29psi for both front and rear. I also looked in the owner's manual and it has two specs. One labeled U.S.: EX, Canada Si with the specs above. The other is labeled DX and the specs are: P175/70 R13 82S with 32psi front and rear.

Now that she has moved the weather is pretty dry (southern California). I was actually thinking the opposite for the speed rating. My logic was that the W rated tires are in the best shape and most of the weight is up front. Having the one backwards tire up there might cause some steering or tracking issues. I was also thinking along the same lines you were regarding breaking and thought it would be best to have the two most like tires where most of the breaking is done.

At the moment I don't have a better tread measurement than the penny test. I have checked the center and edge of each tire and except for the right-front the wear is fairly even at Lincoln's eye. The one with the wear is just a little deeper than the top of his hair in the low spots.

-Jim

Answer
Jim,

Thanks,

I looked in an older book and verified the information about the tire size and the pressure.  That makes me feel more comfortable.

So based on that, the 215/35R18 are Standard Load (SL) tires.

The rule of thumb is that if you have the same type of tire - in the case SL vs SL you should add (or subtract) 1 psi for every Load Index (LI) increment - so that means to get the same load carrying capacity as an 82 LI in a 80 LI, you need to add 2 psi.  I also think it is a good idea to use 3 to 5 psi more for the low aspect ratio in order to help prevent impact damage to both the tire and the rim - So I'd recommend a minimum of 31 psi, but I think 34 to 36 would be better.

In spite of the fact that the front tires do most of the work - especially in braking - the rear tires become important for vehicle stability.  So the generally accepted thinking is to have the most capable tires on the rear to prevent oversteer - especially when it comes to stiffness.  The idea is to get the rear end to stick best, because once you lose grip in the rear, you've basically lost control of the car - where if you lose grip on the front, you may gain control of the car when it slows down - and you're at least pointed the direction you are traveling.

So I would make sure you get the direction of rotation correct on all 4 tires.  BTW I looked up the ContiExtremeContacts and they look very directional to me.  Be sure to check the sidewall for and arrow.

OK, it seems to me we've covered everything.  Good Luck.