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Changing tire sizes on a CAR?

Old 08-13-20, 03:38 PM
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Changing tire sizes on a CAR?

For years, Hot Rods, Classic Cars, and Pickups have had people regularly swapping tire sizes. Or going from Bias to Radials.

My housemate bought a 10 to 20 year old car with apparently a less common tire size, 205/55R15. The next size up is 205/60R15, but he said he went to a couple of tire shops that refused to sell and mount them.

205/55, profile height: 112.75
205/60, profile height: 123

That gives it about a cm difference in radius. The tire should fit just fine, especially on the rear, with a difference of about the tread depth.

I never quite trust his stories. Perhaps he wanted to buy ONE tire rather than a pair, or all 4.

I know, years ago my Ford Ranger had, I think 215/75R15 tires. The tire shop had no problem putting on one size larger, perhaps 225/75R15 tires. Much better weight rating, and the whole thing drove better.

Has something changed? I'm looking on the web, and there are discussions about tire swapping, sizes, rim sizes, etc. But, not blank refusing to mount tires.

H vs V speed rating? Hopefully he isn't driving > 100 MPH.
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Old 08-13-20, 03:46 PM
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Over the last several years, I've used either Discount Tire or a local used tire shop for car tires.

Discount Tire used to let me mismatch the tires from the car make on their online purchasing software. This past July, their online software refused to let me select the same larger diameter tires than what came from the factory that I bought through their website two years ago. Original sized tires 205/55-16 and the larger tires are 215/60-16. Nothing drastic and they fit inside the fender wells. Sure the speedometer is off a couple of percentage points.

Anyway, I had to get the tire order completed with online chat, or I could have just called the store directly.

From what I was told it's a liability issue as well as Discount Tire not wanting to be stuck with tires that don't fit your car, once they are mounted.

I hate buying tires in person as I'm always being upsold for what's in that store's inventory, instead of warehouse inventory. Considering I had a job as a tire changer in high school, I got to know tires sizes back then when metric tire sizes were taking over.
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Old 08-13-20, 03:51 PM
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Originally Posted by CliffordK View Post

H vs V speed rating? Hopefully he isn't driving > 100 MPH.
H is good enough for most people. Considering speed limits on public roads and all.
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Old 08-13-20, 04:01 PM
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Pretty much like FiftySix said, i'ts mainly a liability issue. I usually buy my tires from Tire Rack then have a small local shop mount them with no questions.
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Old 08-13-20, 04:09 PM
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Some tires I dismount and mount my own.



Setting the bead... after mounting. That is a carry-out trailer tire. They did pull the wrong size once and I didn't notice it until I got home. But, otherwise, I've been mounting them fine (without balancing).

If the housemate had steel rims, I'd probably just pop off the old tires and send him to drop off the wheels to get tires mounted and balanced without telling them what it was for.
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Old 08-13-20, 04:41 PM
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Originally Posted by CliffordK View Post
Some tires I dismount and mount my own.
Away from a tire machine, I used to change my own motorcycle tires. Too much effort using plain tire levers for me these days.

Originally Posted by CliffordK View Post
If the housemate had steel rims, I'd probably just pop off the old tires and send him to drop off the wheels to get tires mounted and balanced without telling them what it was for.
Makes me think you could take the alloy rims off the car and just claim the tires go to a homemade 4-wheel trailer that you don't have the lights working on yet.

Or just find a local non-franchised tire shop that also sells used tires. In my experience, used tire shops don't care what they mount a tire to as long as you pay them.
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Old 08-13-20, 05:36 PM
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Originally Posted by FiftySix View Post
Away from a tire machine, I used to change my own motorcycle tires. Too much effort using plain tire levers for me these days.



Makes me think you could take the alloy rims off the car and just claim the tires go to a homemade 4-wheel trailer that you don't have the lights working on yet.

Or just find a local non-franchised tire shop that also sells used tires. In my experience, used tire shops don't care what they mount a tire to as long as you pay them.
That Harbor Freight tire machine works similar to regular tire machines but using muscles. But, it also has its downsides such as the mounting bar twisting. I've made a few mods to make it work better.

Trailer tires are supposed to be "ST" rated. That won't fly.

But, if one pulled the tires, and brought the rims in, one would just need to find a stock car that uses the desired tire size if they ask.

I sent the housemate to a local tire recycler... They actually had the proper size he needed, but he didn't like the price they were asking. The price was reasonable enough, but he is always broke, and already borrowed to much from me without repaying, as well as draining my gas cans without refilling.
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Old 08-13-20, 05:43 PM
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Is the car 4wd? The store may be telling him he needs to match the size of the other tires so they're all the same.
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Old 08-13-20, 06:58 PM
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Originally Posted by no motor? View Post
Is the car 4wd? The store may be telling him he needs to match the size of the other tires so they're all the same.
Hmmm... I hadn't thought about that. Perhaps AWD?

It sounded like they just wouldn't sell a tire size that they stocked at all. But, as mentioned, he could have been trying to cheap out.
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Old 08-13-20, 07:04 PM
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Originally Posted by CliffordK View Post
That Harbor Freight tire machine works similar to regular tire machines but using muscles. But, it also has its downsides such as the mounting bar twisting. I've made a few mods to make it work better.
That's a lot of work.

The machine I operated when I was 18 was like the one shown below. Almost every tire I changed back then came on steel wheels and the Target I worked at still sold letter sized American tires as well as the "new fangled" P metric car tires.


Up to maybe the mid 1980s, Target stores had an automotive department. I changed tires, oil, shocks, batteries, filters. The busiest days were holidays when sales were in effect. Everyone with a car seemed to have the day off and they all wanted the sales pricing.
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Old 08-13-20, 09:32 PM
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Originally Posted by FiftySix View Post
That's a lot of work.
Mine will fit in the back of a pickup for those road emergencies.

I've looked at the vintage Coats machines, and may get one someday. I have to consider the changes in rim styles, and whether that would impact my choice.

I see a Coats machine on Craigslist now that says it has problems with tires > 17" (many new pickup wheels are 20").
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Old 08-14-20, 06:14 AM
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Those Coats machines were really common back then, and the compressed air really helps. My ex wife brought my car in 1 day so I could change a tire on it after we'd closed and turned the compressor off. I figured I'd just push harder instead of turning the compressor back on and quickly found out how much I needed the help from the air.
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Old 08-14-20, 11:41 AM
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I had made a video of the mods to the HF tire machine. I'll try to track it down and get it posted someday. Needless to say, all the tire levers and bars are now 2x to 3x their stock length, plus a piece of square stock on the mounting bar to help keep it from rolling as badly.

The photo above was using a bike tube to set the bead. Not perfect, but it works, especially with new, never mounted tires that have the beads mashed together.
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Old 08-14-20, 02:55 PM
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Originally Posted by CliffordK View Post
I had made a video of the mods to the HF tire machine. I'll try to track it down and get it posted someday. Needless to say, all the tire levers and bars are now 2x to 3x their stock length, plus a piece of square stock on the mounting bar to help keep it from rolling as badly.

The photo above was using a bike tube to set the bead. Not perfect, but it works, especially with new, never mounted tires that have the beads mashed together.
Could always go redneck on seating a bead.

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Old 08-14-20, 03:48 PM
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I ordered 4 205x60x16 winter tires last year at Walmart shipped to store. When they arrived I went to the store with my car that has 18 inch wheels and tires on it (235/45/18) and took along 4 bare steel wheels + 4 new TPMS sensors in a box + 20 new lug nuts in a box in the trunk. I left with the 18 inch wheels/tires in the car and the new winter tires mounted/balanced on the steelies with my new TPMS and my new lug nuts for around $60. Totally unrelated but it didn't snow once for the rest of the season, oh well, now i have a set ready for next year.

I don't think Walmart cares what you buy. To be fair, I did all the research myself into what steel wheels to buy [1], what size winter tires to buy to closely match the OEM 18 inch size, what TPMS and what lug nuts I needed for the steel wheels and so on. I just needed them mounted and installed.

I have the cheap manual tire "machine" from HF. It's bolted to a large pallet. I use it to take a lot of tires off wheels and I've used it to mount various trailer tires but I never had a desire to use it for car tires with aluminum wheels, specially something under a 60 sidewall.. I guess I could have mounted those above referenced winter tires myself but for $60 and balancing and any TPMS reprogramming, it was worth it to me.

[1] Walked around pick-a part armed with some tools and google search on my phone. Verified fit on a wrecked version of my year car there and got lucky first try. OEM Ford steel wheels but from a different model.

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Old 08-14-20, 03:59 PM
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Originally Posted by u235 View Post
I have the cheap manual tire "machine" from HF bolted down to a pallet. I'll use it to take a lot of tires off wheels and I've used it to mount various trailer tires but I never had a reason to use it for car tires with aluminum wheels. I guess I could have mounted those above referenced winter tires myself but for $60 and balancing and any TPMS reprogramming, it was worth it to me.
Spin balancing is worth it every time. If you've ever lost a significantly sized wheel weight on a car or truck tire, you know what an unbalanced tire can feel like while driving down the highway.
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Old 08-14-20, 04:01 PM
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Originally Posted by u235 View Post
I have the cheap manual tire "machine" from HF bolted down to a pallet. I'll use it to take a lot of tires off wheels and I've used it to mount various trailer tires but I never had a reason to use it for car tires with aluminum wheels. I guess I could have mounted those above referenced winter tires myself but for $60 and balancing and any TPMS reprogramming, it was worth it to me.
I machined down an aluminum spacer/washer to mount my Blazer aluminum rims on the Harbor Freight machine. It worked reasonably well.

I have my tire machine mounted on a 2x2 square tube which slides into a hitch receiver, but is a bit of a pain if I have to disconnect my trailer to use it.
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Old 08-14-20, 05:46 PM
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Originally Posted by FiftySix View Post
Spin balancing is worth it every time. If you've ever lost a significantly sized wheel weight on a car or truck tire, you know what an unbalanced tire can feel like while driving down the highway.
Agree 100%. I don't balance any of my trailer tires and don't expect any issues but I've been tempted to get a cheap bubble balancer just to try it out.

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Old 08-14-20, 06:17 PM
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Originally Posted by CliffordK View Post
Mine will fit in the back of a pickup for those road emergencies.

I've looked at the vintage Coats machines, and may get one someday. I have to consider the changes in rim styles, and whether that would impact my choice.

I see a Coats machine on Craigslist now that says it has problems with tires > 17" (many new pickup wheels are 20").

I picked up a hydraulic scissor lift from a local Sears Auto closing recently (RIP Sears). They had a few tire machines but I couldn't justify it even though they were relatively cheap.

Last edited by u235; 08-14-20 at 06:24 PM.
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Old 08-15-20, 06:35 AM
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Legal issue. You are allowed to go smaller. You aren't allowed to go bigger.
Why?
So that your speedometer doesn't tell you that your going SLOWER than you really are. Your speedometer IS allowed to tell you that your going FASTER than you really are.
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Old 08-15-20, 06:57 AM
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Lots of reasons besides liability. 4WD/AWD almost always require the same size tires all around. Tire shops also consider the size of the wheel well and suspension travels the tires won't hit the fenders when going over a bump or turning. Also consider sidewall size as that'll make the car more unstable if there is too much height for the size of the rim (you'd have to buy larger rims). And some manufacturers will void the car's warranty if different size tires are put on a car, especially a larger one on a stock rim.
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Old 08-15-20, 11:52 AM
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Originally Posted by AnthonyG View Post
Legal issue. You are allowed to go smaller. You aren't allowed to go bigger.
Why?
So that your speedometer doesn't tell you that your going SLOWER than you really are. Your speedometer IS allowed to tell you that your going FASTER than you really are.
I'd forgotten that aspect. Cabs had to use the standard sized tires to keep the mileage accurate for determining fares when I was in the tire business.
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Old 08-15-20, 12:23 PM
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Originally Posted by AnthonyG View Post
Legal issue. You are allowed to go smaller. You aren't allowed to go bigger.
Why?
So that your speedometer doesn't tell you that your going SLOWER than you really are. Your speedometer IS allowed to tell you that your going FASTER than you really are.
That would make sense, although smaller tires often have a lower weight rating.

However, if one chooses to replace tires in pairs, then the speed sensors are almost never on the non drive tires. Not sure about other sensors like ABS.

There is a fudge factor built into speed limits (which is part of the reason for the +5 or +9 over general rule).

So, back to the original example:
15" = 381mm
205/55R15, profile height: 112.75: 381+2*112.75 = 606.5
205/60R15, profile height: 123: 381+2*123 = 627

627-606.5=20.5
20.5/606.5 = 3.4% change in the speedo.

Assuming the speedo was previously accurate, I think I read the legal limit for speedometer accuracy was 5%, so it may be within the limits.

At 70 MPH, that would put one's speedo off by about 2.4 MPH, so if it was reading 70MPH, one would be actually going at about 72.4 MPH.

The latest thing for hot rods is GPS speedometers, making things drivetrain upgrades easier.
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Old 08-15-20, 01:43 PM
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Just think, all this potential hand wringing about car tire sizes being changed, but we've got lifted 4x4s, lifted trucks, and slammed cars of which almost none of them are wearing tire sizes that were original to the vehicle.

Plus, if motorcycles are still the way they were years ago, every speedometer reads optimistically compared to actual speed.
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Old 08-15-20, 01:48 PM
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Originally Posted by CliffordK View Post

At 70 MPH, that would put one's speedo off by about 2.4 MPH, so if it was reading 70MPH, one would be actually going at about 72.4 MPH.
Thanks to my slightly larger tires, my car is estimated to be doing about 73.7 mph when indicating 70 mph on the speedo. It's not hard for me to just go down the road at a slightly less indicated speed. Plus, with all the radar/license plate readers around my state, it's pretty easy to see what my actual speed is as everyone else is flying past me.
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