Lower load rating question

[threeputtpar]

I'm looking for new tires for my A6 and since I have a speedometer that reads 1 mph fast at 65, I figured I'd move from the stock size of 235/45/17 down to 225/45/17 to correct the problem. My only concern is that the tires that I was considering in the original size are 94 load rating and in the 225 size are only 91 load ratings.

My question is, over the 4 tires the total load rating would go from 5908 lbs down to 5424 lbs on a car that weighs 4000. I am usually the only person in the car as we use our Mountaineer for family outings, so will the drop in load capacity make enough of a difference for me to worry about? There are other options in the 225 size that are XL rated at 94, but they are more expensive. If necessary, I will spend the money to make sure I have a tire that will carry the load properly. Thanks for the help.

 

[Corvette Owner]

What about the maximum tire pressure ratings ? I noticed once, with my Van, that the maximum tire pressure rating went down (from 42 psi to 35 psi) with a lower rated tire. That pressure was below the recommended pressure for my Van in the rear(41 psi) !

Check it out.

Also, remember that maximum load ratings are at maximum tire pressure. So, if your car recommends lower pressures (which is normal) than the load is also reduced.

 

[dparm]

A lower load rating also means the sidewalls are weaker and you'll generate more heat. This increases wear and the chance of a blowout.

I wouldn't do it.

 

[Corvette Owner]

IF your looking at correcting the speedometer, look at tirerack.com for the outside diameter and revs per mile of your original tires and choose new ones that will correct.

Although, I think your 1 mph difference is within tolerances, and no big deal.

 

[Quattro Pete]

Originally Posted By: threeputtpar
I'm looking for new tires for my A6 and since I have a speedometer that reads 1 mph fast at 65, I figured I'd move from the stock size of 235/45/17 down to 225/45/17 to correct the problem.

If your speedo is reading fast, then installing a tire with smaller overall diameter will only make it read even faster, so you're making the problem worse.

 

[threeputtpar]

Wow, thanks for all the fast answers!

One correction, I really meant to say that the actual speed I am traveling is faster than the speedometer reading by 1 mph. I get that backwards everytime I discuss this with somebody. And I used the tire calculator at miata.com and verified that the 225/45/17 will make the reading dead on.

On both the 235 and 225 sizes that I am considering, the max PSI is 50 or 51 so I don't think that that should pose a problem. Typically I run the PSI at what the placard recommends 35F and 35R.

dparm - I thought that since the tire was 10mm narrower, that is what causes the lower load rating not the quality of the sidewall. I could be wrong on that, though.

What I was really trying to find out is when the wight of the car is sbtracted from the total load capacity, will I be OK going from 1908 lbs to 1424 lbs left to account for my weight and the weight of any other passengers and/or gear.

 

[CapriRacer]

You really don't want to be going down in load rating. That is totaly the wrong direction for preventing tire failures - which should be your greatest concern. A 1 mph speedometer error isn't enough to justify making this move.

 

[Corvette Owner]

Did you read my, "Also, remember that maximum load ratings are at maximum tire pressure. So, if your car recommends lower pressures (which is normal) than the load is also reduced." ?

At 35 psi your new tires do NOT support 1424 lbs !

 

[Chris142]

Originally Posted By: threeputtpar


One correction, I really meant to say that the actual speed I am traveling is faster than the speedometer reading by 1 mph.

IMO your being too anal about 1mph. 1mph is more accurate than 99.9% of the cars on the road.

 

[Corvette Owner]

Originally Posted By: CapriRacer
You really don't want to be going down in load rating. That is totaly the wrong direction for preventing tire failures - which should be your greatest concern. A 1 mph speedometer error isn't enough to justify making this move.


I agree totally ! I doubt that your method of measuring (measured mile or GPS) is even that accurate. 1 mph at 65 mph is 1.5% !

 

[Quattro Pete]

If you really want 225/45/17 tire, then just get one with XL load rating (94). Plenty of them out there.

 

[azjake]

Don't go down on the load rating. It's the single biggest safety factor of your tires. Manufacturers build in a cushion on load because they know that motorists may occasionally overload their vehicle, not distribute loads properly, or not keep tires up to the proper pressure, etc. You need that cushion.

 

[Hokiefyd]

Originally Posted By: Corvette Owner
Also, remember that maximum load ratings are at maximum tire pressure.


If you mean maximum sidewall pressure, that is not correct. Technically, a P-metric tire achieves its maximum load at 35 psi, and an E-metric tire (with no preceding P) achieves its maximum load at 36 psi. There are exceptions, such as tires denoted with XL, or Xtra Load, but they truly are exceptions to the rule.

 

[threeputtpar]

Originally Posted By: Quattro Pete
If you really want 225/45/17 tire, then just get one with XL load rating (94). Plenty of them out there.



This is what I knew would be the best possible solution, I just wanted to ask the question so that those with more knowledge on the subject could help me decide.

Also, I'm the type of person that thinks ahead when making a decision so as long as I had the chance to correct my speedometer I thought that I should. I actually had my friend who is a state trooper use his radar gun to tell me if either of my vehicles is correct or not, because the number on the radar gun is the only important one. Thanks everyone for the help and advice.

 

[threeputtpar]

Originally Posted By: Hokiefyd
Originally Posted By: Corvette Owner
Also, remember that maximum load ratings are at maximum tire pressure.


If you mean maximum sidewall pressure, that is not correct. Technically, a P-metric tire achieves its maximum load at 35 psi, and an E-metric tire (with no preceding P) achieves its maximum load at 36 psi. There are exceptions, such as tires denoted with XL, or Xtra Load, but they truly are exceptions to the rule.


CapriRacer, is this true? Is the load rating of a tire measured at 35psi even if the tire is rated at a max psi of 50 or 51?

I will also check your tire tech website and see if I can find the answer there.

 

[Corvette Owner]

Do not go down in tire size for 1 mph accuracy ! Your handling will suffer.

 

[Quattro Pete]

It is true that you'd have to run the XL tires at somewhat higher PSI to achieve that extra load rating. This may of course negatively affect ride comfort, especially that you're already reducing your sidewall height, so you're making your ride firmer.

In general, I agree with others: stick with OEM size. It's not worth messing around with just to correct a 1% speedo error.

 

[Hokiefyd]

Originally Posted By: threeputtpar
I will also check your tire tech website and see if I can find the answer there.


http://www.barrystiretech.com/loadtables.html

 

[threeputtpar]

Originally Posted By: Hokiefyd
Originally Posted By: threeputtpar
I will also check your tire tech website and see if I can find the answer there.


http://www.barrystiretech.com/loadtables.html


Yes, that is the first place I looked. I also read this PDF:
http://www.tiresafety.com/images/Tire Replacement Manual.pdf

Some really good info there. It seems that if I want to go to the 225/45/17 XL tires, I can achieve the same load capacity as the 235/45/17 tires, but I would need to increase psi to 44. I'm sure that would jar the fillings out of my teeth, so it looks like I will have to keep the stock size and be satisfied with being aware that my speedo reads high and call it a day. Thanks again everyone.

 

[CapriRacer]

Originally Posted By: Corvette Owner
.......Also, remember that maximum load ratings are at maximum tire pressure........


Ah.....Mmmmm.....Ah... Actually, No.

You will find info on the sidewall of a tire has 2 forms:

1) Max Load XXXX, Max Pressure YY

These are separate points of reference.

2) Max Load XXXX at YY pressure.

This is a relationship.

Be careful which one you have.