Tire size question for p-15

[Redmond49]

I've been running what I believe are the recommended tires for my First Series '49 P-15 --- according to the tech reference, 6.70-15 is the correct size. I'm currently running 215/70R15 tires. According to this tire size conversion chart, the 215/70R15 tires are the right size: http://www.vintagecarconnection.com/vintage_tire_size_conversion_chart.htm.

 

However, my speedometer reads way optimistic, and at an indicated 50 or 60 mph, the engine is turning over pretty loudly.

 

I have 16 inch wheels on my '46 P-15, and they have 6.00-16 Auburn Deluxe radials from Diamondback on them. I put these on the '49 just for fun a few weeks back and the speedometer read true, the engine was more relaxed at speed, and it seemed to accelerate better.

 

I talked with Diamondback today about this, and their rep said the later P-15s did indeed come with smaller tires. 

 

Did the switch to different tires for the later cars also involve changing rear end or transmission gearing? Or is my speedo just out of adjustment? 

[Doug&Deb]

They did indeed change size in late 47 I believe. It seems odd that the speedo reads correctly with the 16s on. I had the same problem with my D24 when I put radials on the stock wheels. I’m not aware of any changes to the gears or speedo 

[YukonJack]

I'm confused. Wouldn't the the 670-15 be taller than a 600-16?

[P15-D24]

Following the same thought as YK, you need to look at the overall heights of tire to figure out what is going on. 

[kencombs]

According to my calculation, a 6.70x15 is about 2inches taller than the 215-70r15.   And even then, it will vary a bit by maker.

 

6.70 x 2 + 15 = 28.4.   215 in inches is 5.9.  ((5.9 x 2)x.7) + 15 = 26.8.   A 225 would be closer. 

 

BTW a 6.00 x 16 should be exactly 28 diameter.  Just a hair smaller than the 670x15.  I think the difference is in maker as noted above.  That shouldn't be true but unfortunately not every tire company adheres to the size printed on the tire.  Especially the oriental makers.

 

EDIT to add this copied from Coker's web site:  Numeric sized tires (such as 6.00-16 or 6.70-15) that end in zero feature a 90-series aspect ratio, while numeric sizes that end in five (such as 6.95-14) feature an 80-series aspect ratio.

 

I used 100 in the above calculation, as i've found on 4 1/2" wheels at 32PSI they are always over 90.  On wider wheel and 28PSI they are closer to  90 .

Edited October 13, 2018 by kencombs

[Plymouthy Adams]

older tires was often 80 series and as stated some makers used the 83 series standards......the wheel size and tires as outfitted by the factory WILL HAVE the correct speedometer pinion gear installed.  IF CHANGED over the years by PO ie rims and tires, you basically need to establish your speed by GPS or by mile marker average and calculate the % difference and adjust by installing correct tooth count speedo gear.  At one time this chart was posted here, I believe Bob Toft posted it...I saved it but that would be some 4 computers back...lol  I will still have it, but who know how long to find it...search here would be faster.

[YukonJack]

I think wheel width also impacts tire height. Wouldn't a 215 be taller on a stock 4" wide wheel than on a later model 6" or 7" wide wheel?   

Edited October 12, 2018 by YukonJack

[Redmond49]

I had the Firestone 215/70R15 tires and the Auburn Deluxe radials side by side and the 6.00-16s are definitely taller tires, by more than an inch. 

[9 foot box]

   It's the aspect ratio from 70 to 85,that would give you taller tires. A 215/85r15 is 29.4" high, 8.5" wide and a 215/70r15 is 26.9" tall.

[Redmond49]

I did some checking on the Coker site tonight - their modern sized radials for this car are all around 27 inches tall, but their classic bias-ply tires and classic-look radials are all 28 or even 28.5 inches tall, so I think a switch to the classic style 670-15s will work just fine and probably produce the result I'm looking for.