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Bias Tire Conversion and the Difference in Sizes


Fleetwood Meadow
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It’s time for me to replace the tires on my cars and the old debate pops up. Which tire size should I get? My ‘51 Dodge Meadowbrook originally had 7.10-15 bias tires on it. The guy I bought it from out 225/75/15 on it. So when I changed them I put them on too. As I’m looking at different conversion charts they are saying 215/75/15 or 225/70/15. Being a little green on tires I’m not sure exactly what makes one tire better or worse than the other. I understand the fundamentals of the tire sizes but not necessarily the functional difference between them. Anyone willing to shed some light and their thoughts on this would be great. Does one provide a better comfort when riding down the road than the other?

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The 70 vs 75 is the sidewall height, the lower the number the smaller the sidewall height which translates to a stiffer ride.  For performance handling you want a low number, for comfort you want a higher number.  The taller sidewall looks better, in my opinion, on older cars.

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I've found most conversion charts to be completely inaccurate. This is why I prefer to search for the original tire dimensions and select an equivalent new size myself. My info shows that 7.10-15 tires are 28.88" in diameter with a 4.50" tread width and a 7.70" section width. Neither of your two suggested modern replacements is even close. 215/75-15 tires are over an inch smaller in diameter at 27.70" OD and are 3/4 inch wider at 8.46" section width. The 225/75-15s are almost 1.5" smaller in diameter at 27.40" OD and have an 8.86" section width. If you can find them, 225/75-15 are 28.29" diameter. 235/75-15 tires are exactly the same 28.88" OD as your originals but are about 1.5" wider at 9.25". I have Hankook 235/75-15 tires on one of my cars and they're fine. If you can handle the extra width, they are probably the best modern match.

Edited by joe_padavano (see edit history)
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Tires of the 60s and earlier have an aspect ratio of either 100%+, 90% or 80/82% depending on when they were made, when the size was introduced, what digit the size ends in (with a couple exceptions), and so on. The last change was in 1965, and somewhere packed away I have a poster from that year explaining the new sizes. The other change may have been in 1948, but don't quote me.

 

It is clear as mud.

 

What you can take away is that if the aspect ratio of the new replacement tires is not 80 or higher, it isn't even going to be close.

 

In the US, the selection of modern tires with an 80 or higher aspect ratio ranges from dismal to non-existent.

 

So, what you do is take Joe's advice and use the actual measurements. In practical terms this means if you pick something that is narrow enough to fit, the outer diameter is going to be much smaller. It lowers the car a little, and on some cars it looks good, but also lowers the gearing. If the car is geared too low in the first place, lowering it more can be a problem.

 

Tires wont always match their marked size, but are usually close. Example for a metric tire, 215-75-15:

 

215 = section width in millimeters. This is the width at the widest part of the tire, measured on whatever rim width the tire designer used when he designed the tire. It will vary some depending one what rim width you mount it on. 215mm = 8.46 inches.

 

75 = aspect ratio. This is the percentage of the section width that the sidewall measures, from the bead seat on the rim out to the edge of the tread. 215mm x .075 = 161.25mm = 6.35 inches.

 

15 = rim diameter, measured at the bead seat, in inches.

 

15" + 6.35" + 6.35" = 27.7"

 

27.7 inch tall tire, 8.46 inches wide at the widest point.

 

.

Edited by Bloo (see edit history)
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Personally, I hate it when the tires are too small in diameter and there's waaaay too much daylight between the tire and the wheel lip, but that's a matter of personal preference. As Bloo noted, you have to consider both diameter and width for clearance. I've found that an extra inch or so of section width usually isn't a problem, but it might be on certain cars. Aspect ratio on those older bias ply tires were probably closer to 85 or 90 than 80, to be honest. In fact, if you do the math on that 7.10x15 tire, you get a 6.94" sidewall height and a 7.70" section, which is a 90 aspect ratio.

 

In any case, the actual section width will vary with wheel width; manufacturers typically specify tire dimensions on a specific wheel size. A narrower rim pulls the section in; wider pushes it out. Also keep in mind that for speedo/odo purposes, the dimension that really matters is the rolling radius and resulting revs per mile. Since radial tires have more flexible sidewalls than do bias ply tires, a radial with the same theoretical outside diameter will actually have a smaller rolling radius due to sidewall deflection and thus will have more revs per mile than the same diameter bias ply.

Edited by joe_padavano (see edit history)
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58 minutes ago, Bloo said:

I think the new sizes introduced in 1965 are the ones that had an aspect ratio of about 80.

 

 

I believe you are correct. For example, a 1960s-vintage 8.85-15 tire has the same outside diameter as the OP's 7.10-15 but an 8.80" section width, yielding an aspect ratio of 78.

 

 

1970trapage106.jpg

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18 hours ago, joe_padavano said:

Also keep in mind that for speedo/odo purposes, the dimension that really matters is the rolling radius and resulting revs per mile.

 

Expanding on this for maximum clarification, a "smaller" tire is like putting a lower rear axle ratio in your car. (e.g higher numerical ratio)

 

Old cars typically have ratios that are barely compatible with modern driving conditions. No way would you want to make things worse. 

 

Key Concept: A bigger (diameter) tire will lower your engine revolutions at highway speeds. (kind of an overdrive) 

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2 hours ago, m-mman said:

A bigger (diameter) tire will lower your engine revolutions at highway speeds. (kind of an overdrive) 

 

This is true, but the reality is that practical changes in tire size make a minimal difference in effective final drive ratio. For example, the stock 7.10-15 tires on the OP's car have a 28.88" diameter. The suggested 215/75-15 have a 27.70" diameter. This results in a 4% difference in effective final drive. If the car has 3.90:1 rear end gears, the small diameter tires would be equivalent to about 4.06:1 gears with the original size tires.

 

Edited by joe_padavano (see edit history)
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1 hour ago, joe_padavano said:

This is true, but the reality is that practical changes in tire size make a minimal difference in effective final drive ratio.

 

Every little bit counts.

I have benefited from having the correct (large) tires on several of my cars.

My 1959 Lincoln calls for 9.50-14.  Getting the correct 14" is not cheap or easy. I have had and seen these cars operating with very tiny 14" radials ("That's all they had at the tire store") installed and they not only look like a low rider, they drive like one too. 

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26 minutes ago, m-mman said:

Getting the correct 14" is not cheap or easy. I have had and seen these cars operating with very tiny 14" radials ("That's all they had at the tire store") installed and they not only look like a low rider, they drive like one too. 

 

Which is what I said in my first two posts in this tread and why I pointed out that neither of the two tire sizes the OP asked about were even close to being correct. I was addressing the post that suggested going to taller tires to make the car effectively geared "higher". The fact remains that you won't feel any meaningful acceleration or RPM difference from a 4% difference in effective gear ratio.

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