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Backspacing -- The distance from the mounting surface of the wheel to the extreme back side of the rim.

Wheel Width -- The width of the wheel bead seats. Not overall wheel width! Most 6" rims have an overall width of 7"s

Wheel x-section

* * * * *

Here is an explanation of tire fitments and what all the numbers mean, and how they correlate with the old sizes.

First, lets look at metric sizes of today. Ex. 225/70R15. The first number(225) represents the cross section width of the tire at its widest point(sidewall) in millimeters. The second # (70) represents the Aspect Ratio. This a big term meaning that this tire's sidewall height is 70% of the cross section width, or 70% of 225mm. The R means simply Radial tire construction. And finally the 15 represents the rim size.

Now, if you want to go to a wider tire, but you don't want to change the overall diameter/height of the tire the rule of thumb is this:

Take our first example 225/70R15 To go wider you would do this. 235/65R15 wider yet would be: 245/60R15 and one more would be 255/55R15 You see the connection here. When doing what is called a "plus fitment", you increase the width number by 10 and decrease the Aspect Ratio number by 5. That is if you keep the same rim diameter. There are different rules for changing rim size that I won't get into here.

So, if you had a 225/70R15, and you went to a 255/60R15, then you would not have the same overall diameter/height. You would be off 5%.

On tires that came out on these early 60's cars Ex. 7.50/15, the cross section was represented in inches. It was still measured at the fatest part of the tire/sidewall. The tread on that tire was about 5 inches wide. The reason there was no Aspect Ratio number listed is because the sidewalls were always a 1 to 1 ratio. There were no percentages. The sidewall was the same as the width. Then in the late 60' and early 70's they changed to letter designations and used an aspect ratio for the first time. Ex. G-78/15. Remember those? G stood for the width which in this case was about 215mm. Then the 78 was the Aspect Ratio. and you know what the 15 meant. From what I understand the old designations roughly translated like this. F=205,G=215,H=225,L=235. That was if the 78 aspect ratio was used. So a G-78/15 was about the same as a 215/75R15. Those changed if you went to a 60 series or 70 series. The old 78 series matches closely with the 75 of today.

Is everyone totally confused yet? I hope not, I hope that explains tires a little.

           contributed by 62BuickFan

Tire Rack- Revolutionizing Tire Buying

This tire chart is for the BF Goodrich Radial T/A

Tire Size
Serv.
Desc.
Sidewall
MSPN
Stock #
Rim Width
Range
(inches)
*Section
Width
Overall
Diameter
Tread Depth
(in /32's)
Rev's per
mile
Max. load
(lbs.@psi)
P175/70R13
82S
RWL
46604
285-308
4.5 - 6.0
7.0 on 5.0
22.7
12.0
916.0
1036@35
P195/60R13
83S
RWL
81573
265-305
5.5 - 7.0
7.9 on 6.0
22.2
11.0
937.0
1080@35
P205/60R13
86S
RWL
55222
265-309
5.5 - 7.5
8.2 on 6.0
22.7
11.0
916.0
1179@35
P215/50R13
84S
RWL
91781
275-307
6.0 - 7.5
8.9 on 7.0
21.5
11.0
967.0
1091@35
P185/70R14
87S
RWL
50618
285-410
4.5 - 6.0
7.4 on 5.5
24.3
12.0
856.0
1201@35
P195/70R14
90S
RWL
48578
285-413
5.0 - 6.5
7.9 on 6.0
24.8
12.0
839.0
1312@35
P205/70R14
93S
RWL
66748
285-416
5.0 - 7.0
8.2 on 6.0
25.4
12.0
819.0
1433@35
P215/70R14
96S
RWL
50228
285-420
5.5 - 7.0
8.7 on 6.5
25.9
12.0
803.0
1554@35
P225/70R14
98S
RWL
79604
285-459
6.0 - 7.5
9.0 on 6.5
26.5
12.0
785.0
1675@35
P195/60R14
85S
RWL
84497
265-402
5.5 - 7.0
7.9 on 6.0
23.2
11.0
897.0
1135@35
P215/60R14
91S
RWL
98659
265-418
6.0 - 7.5
8.7 on 6.5
24.2
11.0
860.0
1345@35
P225/60R14
94S
RWL
53273
265-434
6.0 - 8.0
9.0 on 6.5
24.7
11.0
842.0
1455@35
P235/60R14
96S
RWL
79644
265-477
6.5 - 8.5
9.5 on 7.0
25.1
11.0
829.0
1565@35
P245/60R14
98S
RWL
54322
265-486
7.0 - 8.5
9.8 on 7.0
25.6
11.0
813.0
1675@35
P245/50R14
93S
RWL
48000
275-536
7.0 - 8.5
10.0 on 7.5
23.7
11.0
878.0
1433@35
P265/50R14
98S
RWL
91337
275-559
7.5 - 9.5
10.9 on 8.5
24.5
11.0
849.0
1642@35
P155/80R15
83S
RWL
TBD
TBD
TBD
TBD
TBD
TBD
TBD
TBD
P215/70R15
97S
RWL
72634
285-639
5.5 - 7.0
8.7 on 6.5
26.9
12.0
773.0
1620@35
P225/70R15
100S
RWL
67244
285-523
6.0 - 7.5
9.0 on 6.5
27.4
12.0
759.0
1753@35
P235/70R15
102S
RWL
61977
285-543
6.0 - 8.0
9.5 on 7.0
28.0
12.0
743.0
1896@35
P255/70R15
108S
RWL
45982
285-561
6.5 - 8.5
10.2 on 7.5
29.1
12.0
715.0
2183@35
P285/70R15
115S
RWL
75701
285-145
7.5 - 9.5
11.3 on 8.5
30.8
12.0
675.0
2679@35
P215/65R15
95S
RWL
78624
265-587
6.0 - 7.5
8.7 on 6.5
26.0
10.5
800.0
1510@35
P195/60R15
87S
RWL
64472
265-487
5.5 - 7.0
7.9 on 6.0
24.2
11.0
860.0
1190@35
P205/60R15
90S
RWL
71072
265-515
5.5 - 7.5
8.2 on 6.0
24.7
11.0
842.0
1301@35
P215/60R15
93S
RWL
67797
265-510
6.0 - 7.5
8.7 on 6.5
25.2
11.0
825.0
1411@35
P225/60R15
95S
RWL
64148
265-534
6.0 - 8.0
9.0 on 6.5
25.6
11.0
813.0
1521@35
P235/60R15
98S
RWL
54712
265-531
6.5 - 8.5
9.5 on 7.0
26.1
11.0
797.0
1642@35
P245/60R15
100S
RWL
60013
265-552
7.0 - 8.5
9.8 on 7.0
26.6
11.0
782.0
1753@35
P255/60R15
102S
RWL
71728
265-562
7.0 - 9.0
10.2 on 7.5
27.1
11.0
768.0
1885@35
P275/60R15
107S
RWL
45185
265-564
7.5 - 9.5
11.0 on 8.0
28.0
11.0
743.0
2149@35
P205/55R15
87S
BSW
88561
270-321
5.5 - 7.5
8.4 on 6.5
23.9
11.0
870.0
1201@35
P265/50R15
99S
RWL
88000
275-744
7.5 - 9.5
10.9 on 8.5
25.5
11.0
816.0
1720@35
P275/50R15
101S
RWL
50381
275-771
7.5 - 9.5
11.2 on 8.5
25.9
11.0
803.0
1830@35
P295/50R15
105S
RWL
53415
275-777
8.0 - 10.0
12.2 on 9.5
26.7
11.0
779.0
2061@35
P275/65R16
111S
RWL
93892
265-240
7.5 - 9.5
11.0 on 8.0
30.1
10.5
691.0
2403@35
P265/60R16
106T
RWL
TBD
TBD
TBD
TBD
TBD
TBD
TBD
TBD
P285/60R16
111T
RWL
93229
TBD
TBD
TBD
TBD
TBD
TBD
TBD
P205/55R16
89S
BSW
70523
270-365
5.5 - 7.5
8.4 on 6.5
24.9
11.0
835.0
1279@35
P195/50R16
83S
BSW
60277
270-555
5.5 - 7.0
7.9 on 6.0
23.7
11.0
878.0
1074@35
P205/50R16
86S
BSW
89159
270-222
5.5 - 7.5
8.4 on 6.5
24.1
11.0
863.0
1168@35
P225/50R16
91S
BSW
67125
270-250
6.0 - 8.0
9.2 on 7.0
24.9
11.0
835.0
1367@35
P245/50R16
96S
BSW
53281
270-803
6.5 - 8.5
10.0 on 7.5
25.7
11.0
809.0
1576@35
P265/50R16
101S
BSW
66711
270-229
7.5 - 9.5
10.9 on 8.5
26.5
11.0
785.0
1797@35
P295/50R16
107S
BSW
51487
270-239
8.0 - 10.0
12.2 on 9.5
27.6
11.0
754.0
2149@35
*The dimensions shown are average values for tires measured on the specified measurig width.
**Section width varies approximately 0.2" for every 0.5" change in rim width.
* * * * *

Convert P-Metric Tire To Tire Diameter Inches

Example: 195/75R15
/   R
Tire Diameter:

 
BFGoodrich Comp T/A Drag Radial by B.F. GOODRICH TIRES
  • Ehanced traction for consistently quick drag strip times.
  • Conforms to irregularities in the pavement surface to maximize longitudinal bite for maximum traction and acceleration.
  • Reduced distortion, squirm and chunking under high-torque loads for more traction
  • High-speed stability and control, plus more cornering performance while being compatible with radial front tires for improved overall handling and balance; reduced internal stresses for longer treadlife compared to bias-ply tires.
  • Reduced risk of punctures, easier mounting with no need for screws; more reliable performance and lower running costs.
  • Tubeless Construction.
SizeMeasuring RimApproved RimSidewall Width
Overall Diameter
Circumferance
Max Load Cap
Tread Depth
P295/65R15
8.5
8.0-10.0
12.4
29.6
93.0
2601 lbs
6/32
P235/60R15
7.0
6.5-8.5
9.5
26.1
82.0
1653 lbs
5.5/32
P275/60R15
8.0
7.5-9.5
11.1
28.0
88.0
2149 lbs
8/32
P315/60R15
9.5
8.5-11.0
13.4
29.8
93.6
2756 lbs
6/32
P345/55R15
11.0
10.0-12.0
14.6
29.8
93.6
2910 lbs
6/32
P275/50R15
8.5
7.5-9.5
11.3
26.1
82.0
1830 lbs
5.5/32
P325/50R15
10.0
9.0-12.0
13.0
27.9
87.7
2447 lbs
8/32
P255/50R16
8.0
7.0-9.0
10.3
26.1
82.0
1687 lbs
6/32
P275/40R17/LL
9.5
9.0-11.0
11.0
25.7
80.7
1433 lbs
6/32
P315/35R17/LL
11.0
11.0-12.0
12.4
25.6
80.4
1433 lbs
6/32
Tire Rack- Revolutionizing Tire Buying

* * * * *
Wondering how far out your speedo will be after the tire size change?

Speedo Converter

Original Tire Diameter:

New Tire Diameter:
Speedometer Reading (MPH): Corrected MPH:

Your new tires didn't quite clear your fenders...:(

1967 GS400
Wheels and tires can be one of the first changes many enthusiasts make to their "toy". It is always a difficult decision. Here is what I did and why. It may help some of you with your choices.

The look I wanted was one that appeared stock, but somehow good. You know how some cars just sit a little nicer than they should.

side view with 15x7 I decided to I wanted to keep the stock wheel appearance, but in 1967 they sat the car on 14x6 wheels with a 3 inch backspacing. It kept them tucked nicely out of the way and surely kept the side of the car cleaner in the rainy weather and on the gravel/dirt roads.

I didn't feel these reasons carried enough importance for what my car was going to be used for,which was definitely not a daily driver. It was only coming out when the weather was expected to be good, and would probably never see dirt and gravel roads.


Buick made the series 57 in a 15x7, but with a 3 1/2" backspacing. If you can find a good set of these, they should fit nicely without any fender mods.
57 Series Buick Style (Chrome)
Part
No.
Rim
Spacing
Bolt
Circle
Rear
Spacing
57-463403214x64 3/43 1/2
57-47340414x74 3/44
57-483404214x84 3/44 1/2
5757340415x74 3/44
57-583404215x84 3/44 1/2
57-50340515x104 3/45
I decided to go with the 15x7... I wanted a little more wheel and I thought the backspacing would allow a decent tire to clear the fenders. The tires I decided to go with were the BF Goodrich Radial TA in a P235/60R15. This gave me very close to an original overall diameter (no messing with speedometer errors). This was going to move the tires 1 1/2" closer to the fenders.

Well, they almost fit!!!

After mounting, there was enough clearance to take it for a test drive and see if it was enough. As with most cars there was about a 1/8 difference from one side to the other. When I hit the right bump, I could get the odd scuffing noise. Oh well, back to the shop for some fitting.


fender with stainless removed In the fender area that required modification, you have the inner and outer fenders overlapping and the stainless covering. I removed the stainless.

I figured about a 1/4" more clearance would be enough. There was two ways to accomplish this. One way is to simply cut or grind away the offending material. It leaves an edge which can still scrap and cut tires should they ever come in contact.

I chose to use a hammer and dolly and roll the lip up. This gave a nice OEM look and a rounded edge that would be much less likely to ever damage a tire even if it came in contact.


Yes, the car used to be blue! Of course they don't make a dolly for this purpose, so I just grabbed a width of flat steel that I thought would give me adequate clearance when the job was done. I used a 5/8". I welded a chunk of scrap to it for a handle and to increase the weight. You'd have a tough time rolling this double lip with a body hammer, I use a modest sized ball pien hammer. Once the lip was rolled to about 45 degrees, stopped using the dolly and continued to tap it till it was verticle. When tapping just use lots of light taps moving along the surface so as to gently roll the lip.


Finished product I then screwed the trim back on and tapped it to show exactly where to continue it roll. I clamped a piece of 1/2" plate in a vise and finished rolling it up, then remounted it and re drilled the two holes which had been moved around the edge.

Now that picture shows how well the fenders are filled!

a wider stance
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