
Wheels & Tires for 19851987 Turbo Regals 
Greg Wentz
gwentz@rt66.com 
"THE TIRE SOME DETAILS" 
The majority of information presented in this page was reprinted with permission from The TURBO BUICK NEWSLETTER (TBN), published and edited by Daryl Riegel. The information presented below is intended to help one gain a basic understanding of wheel and tire technobabble so that you might make a wiser decision when replacing your OEM wheels and tires.
The way it was...
Turbo Regals with the exception of GNXs were factory equipped with P215/65R15 Goodyear Eagle GTs. These tires were nonspeedrated and were NOT allseason radials. White letter Goodyear GTs of the same size were an option. The tires were mounted on either chromeplated steel wheels ('86/'87 GN) or ribbed aluminum wheels ('84/85 GN, '86 TType & '87 TurboT). There were other wheels available for other turbopackaged Regals but these are the two most common styles. Both styles were 15"x7". '86/'87 GN wheels had a 3mm negative offset and 4" of rear backspacing. The aluminum wheels had 3 3/4" of rear backspacing. The circle bolt pattern was 5 X 4 3/4 inches.
Grand NationalTType
The GNXs came with P245/50VR16 Goodyear Gatorbacks on the front and P255/50VR16 on the rear. They were black letter, NOT allseason, and mounted on 16'x8" modular style wheels with a black center section and polished rims. The front had a 15mm positive offset and the rear, a 23mm positive.
Tire Nomenclature
Currently there are several classifications and designations of tires. To be knowledgeble about tires, you must be familiar and understand what all of the markings mean. This is the only way to achieve the right match of tire and rim.
There are at least four systems used to identify tires:
PMETRIC
Code Schemes: W/ASCD LS & W/ACD LS Example: 215/65HR 85H 215/65R 85H W  Tire section width (millimeters) (ex: 215 mm) A  Aspect ratio (percentage) (ex: 65%) C  Construction (ex: R  radial) D  Diameter (inches) (ex: 15 inches) L  Load index (see chart) (ex: 85) S  Speed rating (see chart) (ex: H)
NUMERIC SYSTEM
Code Scheme: WD Example: 8.2515 W  Tire section width (inches) (ex: 8.25 inches) D  Diameter  Rim (inches) (ex: 15 inches)
W  The tire section width is the total width of the tire including sidewall bulge. Do not confuse this with tread width. The tread width (footprint/contact patch) is significantly less. NOTE: The section width increases .2 inches for every .5 inch increase in rim width. Please be sure to compensate for this in ALL your computations. A  The aspect ratio is a percentage of the tire section width (W) that makes up the tire's heigth. If the width were 215mm and the aspect ratio were 65, the tire section heigth (sidewall) would be (65% of 215) or 139.75mm. C  The construction is of type: R: Radial ply B: Biasbelted D: Diagonal or Bias ply LS  The service description consists of a load index (L) and a speed rating (S) which is further explained below. L  The load index ranges from 0 to 279. Passenger car tires usually range from 75 to 100. Divide the Gross Axle Weight Rating (GAWR) by two. It's usually on a sticker located on the side of the driver's door (GAWRFRT front) and (GAWRRR rear). For example, a front of 2169 and a rear of 2278 equates to 84 & 86 (minimum) respectively (2169 divided by 2 and 2278 divided by 2).
Load Index   Load lbs./wheel  GAWR
S  Speed ratings (S) range fron A1 (3mph) to Z (149+mph). Passenger car tires maximum speed ratings are as follows: P  94 mph U  124 mph Q  100 mph H  130 mph R  106 mph V  130+ mph (w/o load indexes) S  112 mph V  149 mph (w/load indexes) T  118 mph Z  149+ mph (by manufacturer)
Speed rated tires usually can be repaired and retain their speed rating, although H, V, & Z rated may only have one .25 inch puncture. Any more, and the tire cannot retain the orginal speed rating.
Older tires, without speed ratings such as stock GOODYEAR EAGLE GT 215/65R15 have a rating of less than 113 mph. This is interesting since the stock computer chips allow turboBuicks to acheive 124128 mph. The stock tires are considered "safe" because noone usually maintains these speeds for any length of time. (The rating is based on a ten minute period.)
Max Load & Max Inflation
PMetric system tires have a load range: P  passenger (standard) usually 4ply (35 psi) XL  Extra load 6ply (4144 psi) AlphaNumeric and Numeric system tires use different load ranges. B  standard 4ply (32 psi) C  extra load (36 psi) D  extra load (41 psi) LT  light truck
Ply Information
There is ply information on the tire which shows type of construction. ex:Tread Plies: 2 Polyester + Steel + 2 Nylon Sidewall Plies: 2 Polyester
UTQGL  Uniform Tire Quality Grade Labeling
Treadwear
The higher the rating, the longer the projected tread life under optimal conditions. Driving style, road conditions, alignment, rotation, and other factors can affect a tire's life expectancy. For comparison purposes, the index is a percentage grading based on 100 being average. The treadwear is rated against other tires from THE SAME manufacturer. For example, a treadware rating of 150 from company X means that tire will wear 1 & 1/2 times as well as a tire from that company with a rating of 100 given the driving conditions are constant. As another example, B.F. GOODRICH Radial T/A HR4 has a rating of 310 and B.F. GOODRICH Comp T/A HR4 Has a rating of 340. The Comp T/A HR4 should last 1.096 times as long as the Radial T/A. The actual treadwear warranty is a more easily digestible figure that indicates the number of miles a consumer could expect from a properly maintained tire. Many tires carry a 40,000 mile warranty, which translates to an index of approximately 360.
Traction
The traction index rates STRAIGHTLINE stopping ability on wet concrete and asphalt surfaces using the A, B, and C scale, with A being the (best) and C the (worst). Purchase decisions shouldn't be based strictly on this rating, as it doesn't take into account handling on other surfaces such as dry or snow covered pavement, nor does it rate cornering and acceleration abilities.
Temperature Rating
A rating of the tire's ability to dissipate and resist heat, on a scale of (from highest to lowest) A, B, and C. To earn the highest grade of "A", a tire must withstand a half hour run at 115 mph without failing. Like treadwear and traction index ratings, the temperature index can only be used to accurately compare within a manufacture's product line. All passenger tires must achieve a rating of C. A & B represent higher levels of performance than are required by law.
AllSeason
M/S, M+S, or M&S states the tire meets the Rubber Manufacture's Association (RMA) definition for mud and snow. Current trends are the use of a "4" in the tire model's name.
NOTE: Stock GOODYEAR Eagle GTs are NOT allseason tires!
Radial Construction
They must state "RADIAL" on the tire. Radial ply construction tires comprise the vast majority of the passengercar market.
TPC Spec
The TPC spec states the tire has met the General Motors performance specifications.
Tubeless
They must state "TUBELESS" or "TUBETYPE".
Department of Transportation
DOT XXXX XXX  "DOT" means the tire meets the Department of Transportation safety standards. The code identifies the tire manufacturer, plant, type of tire construction, and date the tire was made. The last three numbers indicate the week and the last digit of the year the tire was made.
Wheel & Tire Formulas
NOTE: Each tire's section width is based on a particular rim size and construction. Compensate for differences. Increase section width .2 inches for every .5 inch increase in rim width. For example, if the tire's section width is 9.14" on a 6.5" rim and is to be mounted on a 7" rim, the section width would be 9.34". Section width (sidewall flex) will vary by construction. ANY variance will affect calculations!
Radius  Diameter  Circumference
To compute the diameter of a PMetric tire, multiply the section width by the aspect ratio (as a percentage). If necessary, divide this number by 25.4 to convert millimeters to inches. For example, a 215/65 equates to a tire height of 5.5019685 inches (215 x .65) divided by 25.4. Multiplying this by 2 for the top and bottom sidewall height and adding the rim size will get the overall tire diameter. For example, if the tire is on a 15 inch rim, it is approximately 26 inches ((5.5 x 2) + 15). The radius is half of the diameter. Multiplying the diameter by pi (3.1415927...) will get the circumference.
Tire Revolutions Per Mile
To compute the number of tire revolutions per mile, divide 20,168 by the diameter. 20168  = Tire Revs/Mile (Tire Heigth X 2) + Rim Size For a 215/65R15: 20168  = 775 (5.5" X 2) + 15 For a 255/60R15: 20168  = 747 (6" X 2) + 15
Speed
By dividing the original tire revolutions by the new tire revolutions and multiplying by the indicated speed on the speedometer, we arrive at the new actual speed.
(Orig. Tire Revs/Mile)  X Indicated Speed = Actual Speed (New Tire Revs/Mile) Using the above tire sizes: (775)  X 60 mph = 62 mph (747)
Rear Axle Ratio
Changing tire diameters will also change the effective rear axle gear ratio. Divide the new tire revolutions by the original tire revolutions and multiply by the original axle ratio. For example, a turbo Buick that had 215/65s and a rear axle ratio of 3.42 (what else?) and moved to a 255/60 will net an effective ratio of 3.30.
(New Tire Revs/Mile)  X Orig. Axle Ratio = Effect. Axle Ratio (Orig.Tire Revs/Mile) (747)  X 3.42 ratio = 3.30 ratio (775) As you can see, this will theoretically raise the top speed of the vehicle but hurt its acceleration. To regain the original effective ratio with the new tires, a 3.55 ratio is required. (Orig.Tire Revs/Mile)  X Orig. Axle Ratio = Equiv. Axle Ratio (New Tire Revs/Mile) (775)  X 3.42 ratio = 3.55 ratio (747)
Maximum Speed
To calculate the maximum speed of a vehicle, take:
Overall Tire Diameter X Achieved Engine RPM (redline)  = Max Speed Differential Ratio X Top Gear Ratio X 336 26" X 5250  = 177 mph 3.42 X 0.67 X 336 This differs from actual speed. A stock turboBuick's actual top speed is in the 152 to 162 mph range. The aerodynamics on these cars are nothing to write home about.
Lateral Acceleration
To measure lateral acceleration (cornering ability), use:
1.227 X R g =  (T X T) R is the radius of the turning circle and T is the time (in seconds) required for one lap. Stock turboBuicks usually can achieve .79 to .80g.
Tire Mixing
It is always best to have all four tires the same size and construction. If mixing is necessary, install radials on the rear and nonradial on the front and/or wider tires on the rear than front. Never mix constructions and/or sizes on the same axle.
"Plus One / Plus Two Sizing Concept"
This allows vehicles to use lower aspect ratio (lower profile) tires by increasing wheel rim diameter (+1/+2 inches), rim width, and tread width to gain handling and traction. Choose larger diameter tire which retains revs/mile and load capacity.
Rim Width Recommendations
Where ranges are specified, V & H speed rated tires require and can accomodate a wider rim. Check with specific manufacturer!
50Series (P)195/50xR15 5.5  7.5" (P)205/50xR15 5.5/6.0  8.0" 225/50xR15 6.5  9.0" P245/50xR15 6.5  9.5" P265/50xR15 7.0/7.5  10.010.5" P275/50xR15 7.0  11.0" * P285/50xR15 7.5  11.0" P295/50xR15 7.5  11.011.5" * P295/50xR15 8.0  11.5" 60Series 195/60xR15 5.0  7.5" 205/60xR15 5.5  8.0" 215/60xR15 5.5  8.5" 225/60xR15 6.0  9.0" * 225/60xR15 6.0  8.0" (P)235/60xR15 6.0  9.0" *(P)235/60xR15 6.0  8.0" P245/60xR15 6.5  9.5" (P)255/60xR15 6.5  10.0" *(P)255/60xR15 6.5  9.0" *(P)265/60xR15 7.0  10.0" P275/60xR15 7.0  11.0" 65Series 215/65xR15 5.5  8.0" (P) same for passenger and nonpassenger tires /  denotes manufacturer model construction variance *  denotes tire designation conversion estimates computed from the Tire and Rim Association Inc. NOTE:Within a rim range, a middle width should be used for normal driving and a wider rim width should be used if improved handling is preferred.
UNDERSTANDING WHEEL TERMINOLOGY
The following is a list of defined terms which need to be understood before aftermarket wheels can be intelligently purchased.
RIM WIDTH  The distance between bead mounts (where the tire attaches to the rim)
CENTERLINE  Half of the rim width (center)
OFFSET  The distance from the centerline to the hub mount pad (where the wheel attaches to the hub)
POSITIVE OFFSET  Centerline is outside of hub mounting pad (increases vehicle track)
NEGATIVE OFFSET  Centerline is inside of hub mounting pad (decreases vehicle track)
Note: Using wheels with greater negative offset than stock can increase strain on wheel bearings and spindles. Inspect more frequently. Be sure to check clearance with frame and inner fender UNDER CORNERING SITUATIONS!
REAR SPACING / REAR BACKSPACING / BACKSPACING
The distance from the inner edge of the wheel to the hub mount pad
Note: Sometimes the distance from the outer edge of the wheel to the hub mount pad is referred erroneously to as offset. BE CAREFUL!
BOLT PATTERN:
 Number of lugs
 Bolt Circle  The diameter of the imaginary circle which intersects the centers of the lug holes.
GENERAL WHEEL FACTS
Look for the lighest weight wheel (least unsprung weight requiring least energy to get them moving) yet having the strength required by your type of driving. Brake cooling fins are a plus.
Steel:(sheet metal rolled rim welded to a stamped center)
Pluses: inexpensive, very strong, many sizes & offsets
Minuses: heavy, not much lateral (cornering) support
Aluminum:(usually light alloy)
Pluses: fairly light, fairly strong, improves handling, fairly inexpensive
Minuses: can crack or break them more easily than steel, fewer sizes and offsets than steel
Cast:
Lowpressure: inexpensive, more prone to damage
Highpressure: more strength
Highcounterpressure: more expensive, more strength
Rolled,Stamped: common
Forged: ultimate strength
Magnesium: (aluminum with some magnesium)
Pluses: very light
Minuses: Corrodes easily and requires constant care, expensive, not recommended for everyday driving
Usually the more pieces (1,2,3piece) there are to a wheel, the greater its strength and price. One piece wheels accept wider tires for improved traction and handling.
BALANCING
There are three ways to balance a wheel: static (bubble), dynamic (spin) and on the car spin. Bubble balancing is an older method. Spin balancing is done with a machine which electronically marks the wheel for balancing. Onthecar spinning is the most precise because the car spins the wheel while mounted and a strobe light marks the wheel for balancing. The weight of the brake drum and rotor is included.
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