I have a 1986 Dodge D150 pickup with the 225 cu in six cyl engine and automatic transmission. I have changed the tires to a larger size and need to know the rear end ratio so I can get the correct speedometer gear. I have the Service Maual and the Parts Catalog and can't find a listing for the rear axle code on the EIP. The rear axle code is 058. The Parts Catalog doesn't even list a D150 with the 3.7L engine.

should be a tag on one of the rear cover bolts

Tags get removed.

If rear is a peg-leg jack up one wheel, if Suregrip jack both off the ground.

Mark inside of tire rotate tire exactly two complete turns while counting each ¼ turn of drive shaft’s rear “Uâ€￾ joint. (Hint; start with one of “Uâ€￾ joint’s caps pointing directly at ground before marking tire).

Divide number of ¼ turns by 8. This will give you your axel ratio.

Example:
When conducting this procedure on my car I counted 28.5 1/4th turns of the “Uâ€￾ joint. 28 / 8 = 3.56… Or 3.55

Look at Year One’s speedo gear selection chart to find which gear you need, than go shopping.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC