N/A Tire Question

Crashnburn

Crashnburn

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Crashnburn
My Firebird needs new shoes. The tires are way past their good till date, and tread is coming off of them.

I looked at the current tires; they are P235/60 R15. America’s Tires has one stock size for them: P225/70 R15.

I’m not a tire guy; but are my current tires lower profile than stock? Would that be enough to make the speedometer read about 5% slow?
 
Matthew Rogers said:
If the OP's tires are undersized, does that mean he gave his car a negative tip at the last tire change?
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So if I pump vacuum into my tires, it should float, right?

Will a 225/70 R15 fit on the same wheel as a P235/60 R15?

BTW, it’s a weekend driver. And less expensive than ride share if my SUV is in the shop.

I was thinking America’s Tires, but their tires are boring. I found a couple different brands of muscle car tires at America’s Tire Depot. Both have white lettering.
 
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Crashnburn said:
My Firebird needs new shoes. The tires are way past their good till date, and tread is coming off of them.

I looked at the current tires; they are P235/60 R15. America’s Tires has one stock size for them: P225/70 R15.

I’m not a tire guy; but are my current tires lower profile than stock? Would that be enough to make the speedometer read about 5% slow?
Click to expand...

Yes it would. Assuming the 225/70 R15 is the stock size, I'd go back with that. If you want to get the same diameter in a 60 profile tire, you'd have to go up a couple of sizes, which would probably hit the fenders. The 70 series tire would probably fill your wheel well better than the lower profile 60 series tire.
 
I like TireRack.com. I've been using them for a while and it's always smooth.

Can't help with the sizing question though. :(
 
Thank you all. Mystery solved. My wife says get the tall ones because the car will sit a little higher.
 
Crashnburn said:
So if I pump vacuum into my tires, it should float, right?

Will a 225/70 R15 fit on the same wheel as a P235/60 R15?
Click to expand...

Yes, that won't be an issue. The slightly wider and slightly lower profile should help handling, but I agree that going with the stock size will likely look "right". I've never seen a car that looked better with smaller diameter tires than stock.

luvflyin said:
So it looks like size changes the speedo
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Correct. Production cars determine vehicle speed by some kind of sender/sensor/mechanical cable either on the transmission or else from a wheel speed sensor in later cars. So if you change the diameter of the tire, you'll travel a different distance per revolution.
 
Ted said:
Yes, that won't be an issue. The slightly wider and slightly lower profile should help handling, but I agree that going with the stock size will likely look "right". I've never seen a car that looked better with smaller diameter tires than stock.



Correct. Production cars determine vehicle speed by some kind of sender/sensor/mechanical cable either on the transmission or else from a wheel speed sensor in later cars. So if you change the diameter of the tire, you'll travel a different distance per revolution.
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Yeah. It was a punchline though. I didn't spell speedometer all the way out for a reason
 
Crashnburn said:
Thank you all. Mystery solved. My wife says get the tall ones because the car will sit a little higher.
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And the speedometer will lie to you. You will be going faster than it says you are. Verify the amount with the GS from a GPS.

In my experience, new vehicles are routinely delivered with tires too small/consequently speedometers that read too high by a few percent. IMO this is a CYA from the manufacturers who do not want to be blamed for a speeding ticket.

I check my speedometer with a GPS, then look up my tire diameter on the internet. From the speedometer error and the tire diameter I can calculate the diameter that will give me correct speeds. I buy those. This has always been a small tweek to the tire size or aspect ratio.

With big size or aspect ratio changes you potentially run into issues with tire clearance at full bump or full steering angle, but for small corrections I have never seen it.
 
https://americantiredepot.com/tires-cooper-cobra-radial-gt-225-70r15-90000002531.html

I’m getting these Saturday. The current tires are lower profile so the speedometer reads too fast by about 5%. Going to the stock size should get the speedometer closer to true. Coincidentally, that’s about the same error you’d get by changing to the next higher differential gear ratio.

I wouldn’t have known about the speedometer error with the current tires if my wife hadn’t checked the speed with her GPS. She’s always worried I’m driving too slow.
 
Crashnburn said:
https://americantiredepot.com/tires-cooper-cobra-radial-gt-225-70r15-90000002531.html
I wouldn’t have known about the speedometer error with the current tires if my wife hadn’t checked the speed with her GPS. She’s always worried I’m driving too slow.
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You married well.

If anyone was wondering what those numbers mean, the first one is the width of the tire (called the section width by the tire industry) in millimeters, the second is the height of the sidewall expressed as a percentage of the section width, the "R" is for Radial, and the last one is the diameter of wheel in inches. So, for the tires you're buying, the section width is 225 millimeters, the sidewall is 70 percent as high as the sidewall, which comes to 158 mm, and it fits on a 15 inch wheel. Your current tire would have a 141 mm sidewall, so that's a good bit shorter.
 
Crashnburn said:
... I’m getting these Saturday. The current tires are lower profile so the speedometer reads too fast by about 5%. Going to the stock size should get the speedometer closer to true. ...
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Well, if they are 5% larger in diameter than the current tires, "should" becomes "will." If something other than 5%, I'd look for another tire.
 
FormerHangie said:
You married well.

If anyone was wondering what those numbers mean, the first one is the width of the tire (called the section width by the tire industry) in millimeters, the second is the height of the sidewall expressed as a percentage of the section width, the "R" is for Radial, and the last one is the diameter of wheel in inches. So, for the tires you're buying, the section width is 225 millimeters, the sidewall is 70 percent as high as the sidewall, which comes to 158 mm, and it fits on a 15 inch wheel. Your current tire would have a 141 mm sidewall, so that's a good bit shorter.
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Thanks, we met cute. I’m going back to stock so I expect the speedometer to be more accurate.
 
My baby got her new shoes yesterday. I got the 225/70 R15s. According to the GPS, the speedometer is just about dead nuts accurate. It rides smoother, probably because the new tires aren’t missing any tread. It seems there’s more available power because there’s less rubber on the road, which also means the handling isn’t as good. I’m no Richard Petty nor Mario Andretti but I could tell the difference on the way home. Still, I don’t expect to do much canyon carving nor Skyline skipping so that’s not a big deal.

I have a small V8-4.9L and there aren’t a lot of CA legal upgrades for it, so I’ll trade a little bit of handling for more available power.
 
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Crashnburn said:
I have a small V8-4.9L and there aren’t a lot of CA legal upgrades for it, so I’ll trade a little bit of handling for more available power.
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Do you still have to get an inspection on a car of that age in California? I'm more wondering how much the "CA-legal" aspect truly matters.
 
Ted said:
Do you still have to get an inspection on a car of that age in California? I'm more wondering how much the "CA-legal" aspect truly matters.
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Smogging is necessary for anything 1976 or later. You can't make unapproved mods to cars subject to it. If his 301 (aka 4.9L) was original to the car, he's got a 1977 or later model.
 
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