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Old 01-31-2009, 09:16 PM
 
25,085 posts, read 26,865,379 times
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Okay. I know it's a dumb question. But 18 months ago, a tire dealer put wrong-sized tires on my car. According to another dealer, they're already showing a great deal of wear, even though I keep them inflated. The second dealer claims that, because the wrong-sized tire went onto my car, that's normal and is also screwing up my odometer. Is that really that case?
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Old 01-31-2009, 09:18 PM
 
Location: ID
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Need specifics: What size tires are on now, and what is the correct size?
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Old 01-31-2009, 09:26 PM
 
Location: Blue Ridge Mtns of NC
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You can use this calculator to see what happens to your speedometer reading when you change the size of your tires and wheels.

Tire size calculator

Look on the door jamb to see what size tire/wheel combination the vehicle left the factory with.
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Old 01-31-2009, 09:35 PM
 
Location: ID
2,065 posts, read 4,247,548 times
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Sorry to say it, that Miata calculator is way off on some tire sizes. Which results in inaccurate downstream data.

If you know the brand, subspecies, and size of the original (e.g., BF Goodrich TA/H, 225/60/16) and of the replacement tires, you can get actual diameters and more importantly RPM (revolutions per mile) at Tire Rack.
Plug in your car at the appropriate place, and you can usually find what the OE (original equipment) tires were.

Divide one RPM by the other RPM and you will get the percent difference, which translates to speedo/odo error.

e.g., 800 RPM divided by 770 RPM = 1.03896 = 1.04 = appx. 4% difference. It works either way.
770 / 800 = .9625 = .96 = appx. 4% difference.
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Old 01-31-2009, 09:54 PM
 
2,217 posts, read 450,660 times
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Quote:
Originally Posted by cpg35223 View Post
Okay. I know it's a dumb question. But 18 months ago, a tire dealer put wrong-sized tires on my car. According to another dealer, they're already showing a great deal of wear, even though I keep them inflated. The second dealer claims that, because the wrong-sized tire went onto my car, that's normal and is also screwing up my odometer. Is that really that case?
Yes, it is screwing up your odometer reading.

If the tires are a bit larger than OEM, you're not registering as many miles as you are actually driving. The reverse is true if the tires are smaller than stock.

The wrong sized tires are also messing up the accuracy on your speedometer - and in the same way.


That said, I've often wrong "wrong-sized" tires on vehicles many times. Unless they are VERY wrong, the inaccuracies will be VERY minimal.
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Old 01-31-2009, 10:03 PM
 
Location: Columbia, California
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I used to run larger tires on a 4x4, at 55 mph it showed 48 mph
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Old 01-31-2009, 10:30 PM
 
Location: 77059
7,725 posts, read 18,213,456 times
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I went from a 225/60r16 to a 235/60r16 on my car, which threw off the odometer/speedometer about 1%. Negligible.

I did it for ****s & giggles, but in the end there was nothing positive that came out of doing this. They wore improperly, caused the ride to change (subtly but negatively), degraded emergency handling, and made me lose about 1 / 1.5 mpg city/hwy. (Yes, I added 1% to the mileage readings @ fillup.)

So for the next set, I put the stock size back on and the problems were solved.

As far as I'm concerned, the tires & wheel sizes (and possibly wheel weights) are optimally engineered at the factory for ride, handling, comfort, safety and efficiency. Proper (stone cold) pressure is very important as well. From what I've seen, I believe these modern cars have very tight specifications and I will personally never use a non-OE tire (or wheel set) again. I'll leave experimentation for my project ride.
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Old 01-31-2009, 10:52 PM
 
Location: So. of Rosarito, Baja, Mexico
5,260 posts, read 10,178,983 times
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Back in the mid 70's bought a Cad with less the 50,000 mile that had new tires on it. After a while noticed that they were not wearing on the inside and ouside properly. Soooo while visiting for a wedding was staying at a Motel next to a large chain tire store. I popped in to have the front end checked. They said that I needed new shocks...ball joints and front end work. What the heck...had caddys before and never had problem with front ends. Said to forget it as I would have the Cad dealer back home check it out. (Did not tell them I was a Mechanic)

Long story short...never touched the front end but did later sell it to my dad (also a mechanic) and he looked at the front end and discovered that someone had put one size under on all four wheels which made the car a shade lower but un-noticable to the eye. The weight factor on the smaller tires caused the uneven wear.

Remember that the factory does a lot of testing on size of tires to work the best with the weight being carried and to do the best in handleing.

You can always see these low riders with small wheels or over extended width that they think is COOL but in reality is causing undo wear on the bearings and suspension.

Also more tire on the ground will make the engine work harder and reduce gas mileage.

A word to the wise is sufficient. Steve
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Old 01-31-2009, 10:53 PM
 
2,217 posts, read 450,660 times
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Quote:
Originally Posted by tstone View Post
I went from a 225/60r16 to a 235/60r16 on my car, which threw off the odometer/speedometer about 1%. Negligible.

I did it for ****s & giggles, but in the end there was nothing positive that came out of doing this. They wore improperly, caused the ride to change (subtly but negatively), degraded emergency handling, and made me lose about 1 / 1.5 mpg city/hwy. (Yes, I added 1% to the mileage readings @ fillup.)

So for the next set, I put the stock size back on and the problems were solved.

As far as I'm concerned, the tires & wheel sizes (and possibly wheel weights) are optimally engineered at the factory for ride, handling, comfort, safety and efficiency. Proper (stone cold) pressure is very important as well. From what I've seen, I believe these modern cars have very tight specifications and I will personally never use a non-OE tire (or wheel set) again. I'll leave experimentation for my project ride.
I'm a little surprised that it caused that much of a change for you - especially in light of the fact that your tires were over-sized primarily in width, not height.

I wonder if those changes would have been different on a different vehicle.

Interesting...
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Old 01-31-2009, 11:49 PM
 
8,287 posts, read 22,030,562 times
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I've seen many a car's handling characteristics and tire wear adversely affected by installing the wrong size tires.

Typically, a tire salesman (esp one on commission) will "upsell" you to a larger tire size without even looking at what is the "correct" tire for a car. I've seen it time and time again where a car, let's say a mid-size MB comes in the door with 195/70/R14 tires on it ... and leaves with a 205. The next trip for tires, it again gets an "upsell". "You'll get better handling, better braking, better tire wear because bigger is better!!! ... so they say. The fact is, the car got squirrely, didn't handle correctly, didn't ride as well, and got dramatically shortened tire life because the alignment was all screwed up. I've seen cars come in the shop where you couldn't turn the steering wheel lock to lock because the tires rubbed in the wheel wells. Worst case was my own Alfa spider (OE, 175/R/14 tires) with 215/70's on it ... the front fender wells were bent from the tires rubbing and that was with the manual steering on the car; the car made the "boulevard ride" that little old ladies like in older Cadillacs seem like a Porsche. I bought the car and drove it less than 4 miles to a tire shop to get the tires removed ... even the spare wouldn't fit in the wheel well in the trunk until the correct size tire was installed.

This sales push all started years ago when the American car manufacturers put the minimum tire they could get away with on their cars to save money. In fact, you could get "better" with a next size up tire.

But, in more recent years, the manufacturers of cars (and tires) got more savvy about being sure that the car in the showroom could perform properly on OE tires. Even the tire manufacturers wised up to the policy of putting good tires on the car to begin with in hopes that folks would remember how well the car did with them and come back for the same when they needed new tires.

So, unless you're headed for the track or some specialty driving need (off-road, etc), stay with the OE tire size and quality spec. Granted, the tire manufacturers have come up with a lot of new size/aspect ratio tires that are essentially equivalent, so small changes here can take place safely and without adverse wear, but ... for most folks in normal street driving ... the OE tire is the best all around choice.

FWIW, in Germany, the TUV inspection will fail a car (required for being street legal and your insurance) if it has a tire on it other than the OE spec'd size and quality. Doesn't matter how good it looks, how nice you think it might be, how special you think the handling got improved ... if it isn't what the manufacturer certified the car to have to meet safety and performance requirements, it doesn't pass.
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