Lower profile tires in my car - Odometer, Speedometer, ABS, Tire Pressure problems?

Question

.........
Guys,

Help me out here.

I bought this used 99 Accord EX.
Honda recommends 195/65R15 89H tires.
The car has two 195/60R15 87H (front tires) and 195/60R15 88H (rear tires).

With the lower profile tires (other dimensions being same),
1) Do I have faulty Odometer/Speedometer readings?
2) Any concerns on ABS?
3) Honda recommends 30psi pressure on tires. What should I use?

Thanks in advance

Answer 1

How interesting. I had a Nissan Altima in the shop today that had almost exactly the same problem.

Hold on, this is going to get kinda long.

The size recommended by Honda is actually P195/65R15 89H (the "P" is important - more in a moment). the tires you have on there are respectively P195/60R15 87H (note the P in the size) and 195/60R15 88H (no P means this tire uses a different load and inflation table than the ones with the P).

The air in the tire is what determines the weight the tire can carry, or its load capacity. Up to a point at least the more air the more load. Tires of different sizes though do not carry the same load at the same pressures. Therefore the key to determining proper air pressure when using a different sized tire than what originally came on the vehicle is to match the load capacity of the new tire to the OE tire.

OK, how do we do that? Well, you need a load and inflation table, which you can find here: http://www.tiresafety.com/images/Tire%20Replacement%20Manual.pdf I'll walk you through it.

A 99' Honda Accord EX 4-cylinder with P195/65R15 89H tires has a recommended inflation pressure of 29 psi front and rear according to my sources (check your door placard to be sure). At 29 psi this tire has a load capacity of 1,168 lbs. That's the number we have to match.

At 29 psi a P195/60R15 87H tire has a load capacity of 1,091 lbs. We need to go higher. Referring to page 32 of the above link we find that 34 psi will get us 1,174 lbs. load capacity so that is our new pressure.

At 29 psi the 195/60R15 88H tire (notice no "P" and a different service rating - 88 instead of 87) has a load capacity of 1,036 lbs. You'll note that this number is lower than the P-metric tire even though this one has a higher service description. Yeah, it's complicated. Anyway, if we go to the table we find that 34 psi will get us nuts on at 1,168 lbs load capacity.

Thus, you need to run 34 psi front and rear.

What can happen if you don't? Remember that car I mentioned at the top? They were running the factory recommended 29 psi and like you had the exact same size differential (195/60R15 88H instead of the correct P195/65R15 89H). The tread in the middle of the tires was about 8/32" deep, which is about 20% worn. The edges however were worn down smooth, almost to the secondary rubber. The tires were effectively bald and it was because they should have been running 5 psi higher pressures. Instead of the 60,000 miles they should have got out of this tire the owner got about 20,000 miles. Now they are junk. The owner did not compensate for the reduced load capacity of the tires they had and it cost him.

As for your other questions, yes, your speedometer will read fast as the 195/60R15 tire rotates 50 to 60 times per mile more often than the 195/65R15 - about 890 revs per mile compared to 830 per mile for the OE tire. Your odometer will also rack up a greater mileage than you have actually driven.

Your ABS should be fine since the tires are all the same dimensions. A mismatch in tire dimensions could cause problems but your tires, while technically different sizes because they have different service description are dimensionally the same.

ASE Certified Automotive Service Advisor with 5 years experience in the tire industry

Answer 2

Air pressure fluctuates when the temperature is cold or hot. This means that your tire pressure was probably good in the warmer months and since it got colder the pressure doesn't push out on the tires as well, which might explain why your light came on and the tires were not up to par. This is not nescisairly a bad thing if you have ice or slick roads because lower tire pressure increases how much your tire touches the road which gives you better traction. However your mileage will go down from under inflated tires. But all in all this is a normal thing if you have a climate that has temperatures that fluctuate throughout the year

Answer 3

Your speedometer will be off, if the diameter of the tire is different from the stock ones. You would need to get a programmer and reprogram the tire size in to correct the speedometer. The ABS should be ok if the front and rear tires are the same diameter. I run my tires at about 2 or 3 pounds of air less than the max pressure rated on the tires. this will help with the gas mileage, but you give up a little on the ride. It's a little harder. good luck

Answer 4

You can calculate your speedometer error with this calculator:

http://www.miata.net/garage/tirecalc.html

ABS is not affected. The only other issue is that the new tires have slightly less load carrying range than stock. Unless you're hauling bricks, it's not enough of a difference to worry about.

Use the stock pressure.