TIRE & PROPSHAFT SPEED REFERENCE CHART-LIGHT TRUCKS

MODELS: ALL LIGHT DUTY TRUCKS

Reed Tachometer usage requires calculating the speed of rotating components on a vehicle and relating it to the frequency of a vibration (reference GMC Truck Service Bulletin 87-T- 9/Chevrolet Dealer Service Bulletin 87-139 and film #87-404 for Reed Tachometer usage instructions). The chart below was developed to simplify calculating driveline and wheel and tire RPM.

The procedure for using this chart is as follows:

o Determine the vehicle tire size and rear axle ratio.

o Drive vehicle and note the speed range that the vibration occurs at and the vibration frequency (using a Reed Tachometer).

o Locate the tire size in the first column that corresponds to the vehicle tire size. Select the speed range that the vibration occurs in and note the tire RPM. For example, a P215/60RI4 tire turns 792 RPM at 55 mph.

o Calculate the propshaft RPM by multiplying the tire RPM by the rear axle ratio. For example, 792 (tire RPM) X 3.08 (axle ratio) 2439 propshaft revolutions per minute.

o Relate the vibration frequency to the speed of the propshaft or wheels and tires. (For example: A first order tire vibration in the above example would excite the 800 CPM reed on the Reed Tachometer.

TIRE TIRE TIRE TIRE TIRE TIRE rpm @ rpm @ rpm @ rpm @ rpm @ rpm @ TIRE SIZE 45 mph 50 mph 55 mph 60 mph 65 mph 70 mph --------- ------ ----- ----- ------ ------ ------ P155/80R13 686 762 838 914 991 1067 P175/70R13 691 768 845 922 998 1075 P175/80R13 653 726 799 871 944 1016 P185/80R13 637 708 779 850 920 991 P195/70R13 659 732 805 878 952 1025 P205/70R13 648 720 792 864 936 1008

P185/75R14 626 696 766 835 905 974 P195/75R14 610 678 746 814 881 949 P195/70R14 626 696 766 835 905 974 P205/75R14 599 666 733 799 866 932 P205/70R14 616 684 752 821 890 958 P205/60R14 659 732 805 878 952 1025 P215/60R14 648 720 792 864 936 1008

HR78-15D 565 628 680 742 803 865 P195/75R15 589 654 719 785 850 916 P205/75R15 578 642 706 770 819 899 P215/75R15 567 630 693 756 897 802 P215/60R15 621 690 759 828 866 966 P215/65R15 599 666 733 799 866 932 P215/56HR15 599 666 733 799 866 932 P225/70R15 572 636 670 763 827 890 P225/70HR15 572 636 670 763 827 890 P225/75R15 551 612 673 734 796 857 P235/75R15 540 600 660 720 780 840 P255/50VR16 599 666 733 799 866 932 P245/50VR16 610 678 746 814 881 949 P235/85R16 491 546 601 655 710 764

LT215/85R16 513 570 627 684 741 798 LT235/85R16 486 540 594 648 702 756 LT225/75R16 531 590 649 708 767 826 LT245/75R16 504 560 616 672 728 784 LT265/75R16 486 540 594 648 702 756 7.50-16 486 540 594 648 702 756

NOTE: The referenced tire, wheel and vibration diagnosis film #VC 87-404 was mailed to all GMC dealers on February 23, 1987.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.