Wheel & Tire Specifications - Roadster

Optimum Backspace Calculation Chart
Vehicle Position On Vehicle Backspace Equation Maximum Width Pin-Drive Backspace Equation
Roadster Front (width/2)+1.25"=B.S.* 9" (width/2)-0.375"=B.S.
79’-93’ Rear Axle or IRS (width/2)+1.25"=B.S. 10" (width/2)-1.25"=B.S. [IRS only]
94’-98’ Rear Axle (width/2)+2.00"=B.S. 10" N/A
99’-present Rear Axle (width/2)+2.875"=B.S. 10” N/A

Coupe & Spyder Front (width/2)+0.75"=B.S. 10" no change
79’-93’ Rear Axle or IRS (width/2)+0.25"=B.S. 12" (width/2)-2.25"=B.S. [IRS only]
94’-98’ Rear Axle (width/2)+1.00"=B.S. 12" N/A
99’-present Rear axle (width/2)+1.875"=B.S. 12” N/A

* 9" wheel would be desirable to have a 6" B.S.
79’-93’ 4-lug Rear Axle is 59.25" hub to hub (4.25” 4-lug bolt pattern)
94’-98’ 5-lug Rear Axle is 61.125” hub to hub (4.5” 5-lug bolt pattern)
99’-present Rear Axle is 62.52” hub to hub (4.5” 5-lug bolt pattern)

To run 315/35R-17 tires on an MKIII Roadster it would be desirable to have the 59.25” width rear axle. 10.5” wide wheels with a 6.85” B.S. fit well with the 315’s, and are easily attainable in the aftermarket from several of the OEM reproduction wheel suppliers. With custom wheels, use the same backspace, or with an 11” wide wheel, run a 7.1” B.S.

Pin-drive IFS hubs are 1.625” further in per side and the IRS is 2.5” further in per side
Pin-drive adaptors are 0.375” thick

Maximum Tire Sizes
Wheel Diameter Front Tire Sizes Rear Tire Sizes
Roadsters 15" 235/60R-15 275/50R-15
16" 255/50R-16 255/50R-16
17" 245/45R-17 or 255/40R-17 275/40R-17 *

18" 245/40R-18 275/35R-18

Coupe & Spyder 15" 255/60R-15 295/50R-15
16" 255/50R-16 255/50R-16
17" 275/40R-17 335/35R-17
18" 275/35R-18 295/35R-18



*For a Mk3/Mk4 Roadster you can run a 315/35R-17


Alignment Specifications Suppliers of Cobra R’s and Modern style wheels
Alignment Spec Value Unit
Front For All Vehicles Caster 3 degrees
Camber - 1/2 degrees
Toe 1/16 inch

IRS All Camber -1 degrees
Toe 1/16 inch
Tire Rack 888-541-1777 tirerack.com
Diviersified 734-459-0130 buyfordmotorsport.com
Discount Tire Direct 800-589-6789 tires.com
*Performance Wheel Outlet 909-825-5242 performancewheeloutlet.com
*AFS 818-340-6181 afswheels.com
*Performance Tire 832-567-5265 wheelreplicas.com *(OEM reproduction wheels)