Tire question

The best I can figure the Roadmaster takes a P225/75R-15 radial tire. I've found a set of P 225/70R-15 radials on a good buy. Will they be ok on the Roady? Will they be too small? All help is appreciated. Earl

A P235/75R-15 radial tire (that's 235 not 225) would be most appropriate for the Roady. Any 70 series tire would rub the frame in corners.

http://www.turbinecar.com/tires.htm ... more charts than you need to confuse you some more. Your original size 8.00-15 basically has the same diameter as P235/75R-15 which will keep your speedometer and stance of the car correct.

If you want WWW Coker tire has some choices, http://www.widewhitetires.com/ has Coker tires and other choices with free shipping, http://www.dbtires.com/ is another choice and they can put the correct 2.5" WWW on any tire.

I use bias ply on my cars that I drive less than 15,000 miles in 5 years...radials on the those that go more than 15,000 miles in 5 years. (bias ply will last 15 years or 15k miles; radial will go many more miles but around here they start separating after 5 years)

Willie

Hi Earl,

Here are a couple web sites that may be useful in helping you decide.

Automobile-catalog.com provides a wealth of information on the original specs, and offers some suggestions for alternative tire sizes.

http://www.automobile-catalog.com/tire/1955/292745/buick_series_70_roadmaster_riviera_coupe_dynaflow.html

TeamBuick.com has a calculator page that includes a tire diameter calculator and a speedo converter for alternative tire sizes.

http://www.teambuick.com/reference/calculators.php

The original spec tires for a 1955 Roadmaster Riviera 76R were 8.00 - 15 bias-ply:

[TABLE=width: 100%]

<tbody>[TR]

[TD]Standard tire size:[/TD]

[TD]8.00 - 15[/TD]

[/TR]

[TR]

[TD]Tire width (mm):[/TD]

[TD]203[/TD]

[/TR]

[TR]

[TD]Tire sidewall factor:[/TD]

[TD]90[/TD]

[/TR]

[TR]

[TD]Rim size (in):[/TD]

[TD]15[/TD]

[/TR]

[TR]

[TD]Total wheel diameter (mm / in):[/TD]

[TD]746 / 29.4[/TD]

[/TR]

</tbody>[/TABLE]

So, I guess a perfect radial replacement tire would be a 203/90R-15 -- if it existed!

I have noticed that Coker Tires now offers bias profile radial tires: http://www.cokertire.com/brands/american-classic-tires/american-classic-bias-profile-radials.html

If you haven't already, you may want to check them out.

In addition to the bias profile radial tires available from Coker (and maybe others), possible radial tire alternatives for 8.00 - 15 bias tires include:

225/70R-15 radial tires, which have a total wheel diameter of 27.4". (55 mph speedo reading actually will be 51.3 mph)

225/75R-15 radial tires, which have a total wheel diameter of 28.3". (55 mph speedo reading actually will be 52.9 mph)

225/80R-15 radial tires, which have a total wheel diameter of 29.2". (55 mph speedo reading actually will be 54.6 mph)

235/70R-15 radial tires, which have a total wheel diameter of 28.0". (55 mph speedo reading actually will be 52.4 mph)

235/75R-15 radial tires, which have a total wheel diameter of 28.9". (55 mph speedo reading actually will be 54.1 mph)

I have not found 225/80R-15 radials available anywhere, even though they are listed by automobile-catalog.com as an alternative tire size. I have 235/75R-15 radial tires on my 1957 Roadmaster and my 1963 Electra 225. As with your 1955 Roadmaster, both of these models originally came standard with 8.00 - 15 bias-ply tires. I like the way the 235/75R-15 tires look on my cars. I haven't noticed them rubbing or hitting the wheel arches, even though they are 32mm (~1.26") wider than the original bias-ply tires. Had the bias profile tires been available last year when I was in the market for tires for my Roadmaster, I certainly would have considered them, but I'm happy with the tires I ended up getting.

I hope this helps. Good luck!

Ike

PS:

Willie, why would the 70 profile tires rub and the 75s be OK? Tire width being equal, 70s just have a lower profile and hence a smaller diameter than the 75s. It seems counter-intuitive to me that the 70s would rub and the 75s wouldn't. I would expect the "taller" tire of the same width to be the one with the most potential to rub. What am I missing?

IKE, as I understand tire dimensions, the first three digits describe the tire circumference, while the two-digit profile number is the relationship between height and width. A 70-series tire is 70% as high as it is wide. A 75-series tire is therefore a bit taller but not as wide. Interference with the inner fenders will usually result from a tire that's too wide, not too high.

IKE, as I understand tire dimensions, the first three digits describe the tire circumference, while the two-digit profile number is the relationship between height and width. A 70-series tire is 70% as high as it is wide. A 75-series tire is therefore a bit taller but not as wide. Interference with the inner fenders will usually result from a tire that's too wide, not too high.

Thanks Rob for the correct reasoning. My reasoning is from personal observations on a 55....I had some nearly new 70 series tires from a car destined for the junkyard. I paid for mounting only one to try and it failed. (note: if the idler arm bushing is worn ANY tire will rub).

Willie

IKE, as I understand tire dimensions, the first three digits describe the tire circumference, while the two-digit profile number is the relationship between height and width. A 70-series tire is 70% as high as it is wide. A 75-series tire is therefore a bit taller but not as wide. Interference with the inner fenders will usually result from a tire that's too wide, not too high.

Rob,

You are correct about the meaning of the two-digit profile number; but the first three digits refer to the tire's width, in millimeters, from sidewall to sidewall.

A good explanation of all the numbers is provided here: http://www.goodyearautoservice.com/content/content.jsp?pageName=TireSize

A 70 profile 235 mm wide tire measures 164.5 mm (.70 x 235 mm) from rim to tread. Whereas a 75 profile tire with the same 235 mm width measures 176.5 mm (.75 x 235 mm) from rim to tread. The width hasn't changed; just the tire's height from rim to tread, and therefore it's total diameter.

So, as far as I can tell, a 235/70 has the same width as a 235/75, but it has a smaller total diameter; so why would a 235/70 rub if a 235/75 doesn't?

Ike

.....as I understand tire dimensions, the first three digits describe the tire circumference, .

I don't agree with the above reasoning/understanding. If that was true.....

Circumference = (PI) x (diameter)

235 mm = 3.14 x diameter (X)

X (diameter) = 74.8mm or a 2.95" outer diameter tire.

I agree with IKE.

Al Malachowski

BCA #8965

"500 Miles West of Flint"

235/70R15 will rub, really they do rub on a 55. Maybe because the 55 rim size is 6" vs the recommended 7-8"; the speedometer will be off; the car will look like a local mexican lowrider...

I agree with IKE

Me too. I didn't think my answer through - please disregard.

Perhaps it might be well to define which year of Roadmaster you are concerned with. Old-Tank has the earlier ones nailed-down pretty well, BUT if you're concerned with the last-gen Roadmaster, then things can get a little different.

On that last-gen Roadmaster sedan, the standard tire was P225/75R-15. If you got the "Trailer Tow Package" (the Chevy 'performance suspension" in disguise), then the spec tire was P235/70R-15. Pretty close to the same number of revs/mile, but with a wider section width and tread. Spec rim width for both tires would be 7", I believe.

In the Miata club's website, there's a "tire size comparator" where you choose the section width number, the aspect ratio number, the rim diameter, and it spits out the revs/mile of the particular tire size, PLUS a graphic of what it looks like (in a rectangle shape). When you do the "comparison", it'll overlay the second tire over the first tire, so you can see the differences in width and diameter, plus a diameter % difference. Quite fun! The Miata Club's website is where I found it, but it's in other places too.

Enjoy!

NTX5467

I bought my last two sets of Coker tires from Summit Racing. They are a reseller with a lower price and free shipping. The car drives smooth and the tires don't rub.

Bernie

I think I had better stay out of this one. 'cause they're B-16's (700-16)

And only the LORD knows where there is another one.

Mike in Colorado

... I use bias ply on my cars that I drive less than 15,000 miles in 5 years...radials on the those that go more than 15,000 miles in 5 years. (bias ply will last 15 years or 15k miles; radial will go many more miles but around here they start separating after 5 years)

Willie

Willie,

How much do you inflate your radial tires? I've been inflating my cars' 235/75R-15 tires to ~35psi (cold). The ride is comfortable at this pressure; but I wonder if it should it be less?

Ike

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Perhaps it might be well to define which year of Roadmaster you are concerned with. Old-Tank has the earlier ones nailed-down pretty well, ...

NTX5467

Good point, NTX5467. We should compare apples to apples. Willie is referring to his experience with a 1955 Century, which came standard with 7.60-15 bias-ply tires; whereas I believe that all 1955-57 Roadmasters came standard with 8.00-15 bias-ply tires. By the way, did this difference in tire size necessitate wider rims on the Roadmasters? (Incidentally, it seems that all 1955-57 Supers also came standard with 7.60-15 tires, but 1955-57 Specials came standard with 7.10-15 tires.)

... My reasoning is from personal observations on a 55....I had some nearly new 70 series tires from a car destined for the junkyard. I paid for mounting only one to try and it failed. (note: if the idler arm bushing is worn ANY tire will rub).

Willie

Willie,

I hope you don't mind my pursuing this a little further. Perhaps you can help me figure out why you experienced rubbing with a 70R-15 tire, but not with 75R-15s. :confused:

First, to eliminate the most obvious possibility, by any chance, could the 70R-15 tire that rubbed when tested on your 1955 Century have been a wider tire than the ones you ended up buying? For example, could the tire you tested have been a 235/70R-15, and the tires you ended up buying are 225/75R-15?

Alternatively, another possible explanation might have to do with the fact that you tested only one tire. If the tires on the other three corners were a different size and/or type (e.g. bias-ply instead of radials), could that have shifted the car's stance enough to lead to rubbing on the corner with the 70R-15? Just a thought.

Anyway, thanks for indulging me by reading this.

Ike

PS:

Below, for everyone's convenience, I have attempted to summarize the standard tire size information I found at

http://www.automobile-catalog.com/model/buick/full-size_buick_3gen.html for 1955-57 Specials, Centurys and Supers, and Roadmasters.

Standard tire size for 1955-57 Buick Series 40 Special:

[TABLE=width: 320]

<tbody>[TR]

[TD] Standard tire size:[/TD]

[TD=width: 117] 7.10 - 15[/TD]

[/TR]

[TR]

[TD=width: 258] Tire width (mm):[/TD]

[TD=width: 117] 180[/TD]

[/TR]

[TR]

[TD=width: 258] Tire sidewall factor:[/TD]

[TD=width: 117] 90[/TD]

[/TR]

[TR]

[TD=width: 258] Rim size (in):[/TD]

[TD=width: 117] 15[/TD]

[/TR]

[TR]

[TD=width: 258] Total wheel diameter (mm / in):[/TD]

[TD=width: 117] 705 / 27.8[/TD]

[/TR]

</tbody>[/TABLE]

Standard tire size for 1955-57 Buick Series 50 Super and Series 60 Century:

[TABLE=width: 320]

<tbody>[TR]

[TD=width: 258] Standard tire size:[/TD]

[TD=width: 117] 7.60 - 15[/TD]

[/TR]

[TR]

[TD=width: 258] Tire width (mm):[/TD]

[TD=width: 117] 193[/TD]

[/TR]

[TR]

[TD=width: 258] Tire sidewall factor:[/TD]

[TD=width: 117] 90[/TD]

[/TR]

[TR]

[TD=width: 258] Rim size (in):[/TD]

[TD=width: 117] 15[/TD]

[/TR]

[TR]

[TD=width: 258] Total wheel diameter (mm / in):[/TD]

[TD=width: 117] 728 / 28.7[/TD]

[/TR]

</tbody>[/TABLE]

Standard tire size for 1955-57 Buick Series 70 Roadmaster:

[TABLE=width: 320]

<tbody>[TR]

[TD=width: 258] Standard tire size:[/TD]

[TD=width: 111]8.00 - 15[/TD]

[/TR]

[TR]

[TD=width: 258] Tire width (mm):[/TD]

[TD=width: 111]203[/TD]

[/TR]

[TR]

[TD=width: 258] Tire sidewall factor:[/TD]

[TD=width: 111]90[/TD]

[/TR]

[TR]

[TD=width: 258] Rim size (in):[/TD]

[TD=width: 111]15[/TD]

[/TR]

[TR]

[TD=width: 258] Total wheel diameter (mm / in):[/TD]

[TD=width: 111]746 / 29.4[/TD]

[/TR]

</tbody>[/TABLE]

Willie,

How much do you inflate your radial tires? I've been inflating my cars' 235/75R-15 tires to ~35psi (cold). The ride is comfortable at this pressure; but I wonder if it should it be less?

Ike

Ike, it all depends on the tire. 75 series tires I used in the past had 35psi max inflation and I used ~32 psi. The currently installed Firestone FR380 tires are rated at 44psi max inflation and I use 41psi now. At 32psi the handling was squirrelly and the ride was worse (seemed to slap and kick on small bumps like concrete joints).

Good point, NTX5467. We should compare apples to apples. Willie is referring to his experience with a 1955 Century, which came standard with 7.60-15 bias-ply tires; whereas I believe that all 1955-57 Roadmasters came standard with 8.00-15 bias-ply tires. By the way, did this difference in tire size necessitate wider rims on the Roadmasters? (Incidentally, it seems that all 1955-57 Supers also came standard with 7.60-15 tires, but 1955-57 Specials came standard with 7.10-15 tires.)

Willie,

I hope you don't mind my pursuing this a little further. Perhaps you can help me figure out why you experienced rubbing with a 70R-15 tire, but not with 75R-15s. :confused:

First, to eliminate the most obvious possibility, by any chance, could the 70R-15 tire that rubbed when tested on your 1955 Century have been a wider tire than the ones you ended up buying? For example, could the tire you tested have been a 235/70R-15, and the tires you ended up buying are 225/75R-15?

Alternatively, another possible explanation might have to do with the fact that you tested only one tire. If the tires on the other three corners were a different size and/or type (e.g. bias-ply instead of radials), could that have shifted the car's stance enough to lead to rubbing on the corner with the 70R-15? Just a thought.

Anyway, thanks for indulging me by reading this.

Ike

PS:

Below, for everyone's convenience, I have attempted to summarize the standard tire size information I found at

http://www.automobile-catalog.com/model/buick/full-size_buick_3gen.html for 1955-57 Specials, Centurys and Supers, and Roadmasters.

Standard tire size for 1955-57 Buick Series 40 Special:

[TABLE=width: 320]

<tbody>[TR]

[TD] Standard tire size:[/TD]

[TD=width: 117] 7.10 - 15[/TD]

[/TR]

[TR]

[TD=width: 258] Tire width (mm):[/TD]

[TD=width: 117] 180[/TD]

[/TR]

[TR]

[TD=width: 258] Tire sidewall factor:[/TD]

[TD=width: 117] 90[/TD]

[/TR]

[TR]

[TD=width: 258] Rim size (in):[/TD]

[TD=width: 117] 15[/TD]

[/TR]

[TR]

[TD=width: 258] Total wheel diameter (mm / in):[/TD]

[TD=width: 117] 705 / 27.8[/TD]

[/TR]

</tbody>[/TABLE]

Standard tire size for 1955-57 Buick Series 50 Super and Series 60 Century:

[TABLE=width: 320]

<tbody>[TR]

[TD=width: 258] Standard tire size:[/TD]

[TD=width: 117] 7.60 - 15[/TD]

[/TR]

[TR]

[TD=width: 258] Tire width (mm):[/TD]

[TD=width: 117] 193[/TD]

[/TR]

[TR]

[TD=width: 258] Tire sidewall factor:[/TD]

[TD=width: 117] 90[/TD]

[/TR]

[TR]

[TD=width: 258] Rim size (in):[/TD]

[TD=width: 117] 15[/TD]

[/TR]

[TR]

[TD=width: 258] Total wheel diameter (mm / in):[/TD]

[TD=width: 117] 728 / 28.7[/TD]

[/TR]

</tbody>[/TABLE]

Standard tire size for 1955-57 Buick Series 70 Roadmaster:

[TABLE=width: 320]

<tbody>[TR]

[TD=width: 258] Standard tire size:[/TD]

[TD=width: 111]8.00 - 15[/TD]

[/TR]

[TR]

[TD=width: 258] Tire width (mm):[/TD]

[TD=width: 111]203[/TD]

[/TR]

[TR]

[TD=width: 258] Tire sidewall factor:[/TD]

[TD=width: 111]90[/TD]

[/TR]

[TR]

[TD=width: 258] Rim size (in):[/TD]

[TD=width: 111]15[/TD]

[/TR]

[TR]

[TD=width: 258] Total wheel diameter (mm / in):[/TD]

[TD=width: 111]746 / 29.4[/TD]

[/TR]

</tbody>[/TABLE]

To find out for sure try it on YOUR car. In the distant past I have used 8.00-15 tires on a 55 Special with no issues. The radials that rubbed were 225-70R15 Toyo. I could have lived with that, but the tire looked ridiculous on the the car and measuring masking tape around the circumference of that tire and a worn 7.60-15 it was 10% less. The tires also rubbed the frame of my flatbed trailer which has 7.00-15 tires (I was really trying to get some use out of $400 worth of new tires before I junked the 92 marquis for $400)

Willie

Thanks for the info Willie. I think the max. inflation of my 235/75R-15 tires is 44 psi, so 35 psi should be okay, especially for short drives around town. For extended summertime highway driving at high speeds, maybe I'll lower the cold tire pressure by 2-3 psi.

And thanks also for indulging my curiosity about your experience with the 70R-15 tire. I agree with you that a lower profile and smaller diameter tire doesn't look quite right on old Buicks! It's too bad you weren't able to use those tires on the trailer, though. I hope you managed to sell them for a few bucks.

Ike