Question about gearing
 

Assuming everything is stock on my truck (91 full size jimmy) and I want to add larger tires and possibly a lift, at what point would I need to change my gear ratio and to what gear ? Truck will be a daily driver and see some highway use.
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Re: Question about gearing
 

Use this calculator: http://www.wallaceracing.com/calc-rpm.php

Your overdrive ratio is .70 assuming it has a 700R4. If you have 3.42 gears you could get away with running 33" tires and not doing a gear swap. If you have 3.08 gears you'll need to swap gears. You want at least 1700 rpm @ 70 mph. Or I would. Something between 1700-1900 would be pretty good. You wouldn't want the rpms so low that the R4 starts gear hunting, it's not the strongest tranny.

Re: Question about gearing
 

How do I find out what gear is in my truck ? And it does have a 700R4
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Re: Question about gearing
 

Look in the glove box for one of these codes:

G80 : AXLE POSITRACTION, LIMITED SLIP
GU4 : AXLE REAR, 3.08 RATIO
GU5 : AXLE REAR, 3.23 RATIO
GU6 : AXLE REAR, 3.42 RATIO
GT4 : AXLE REAR, 3.73 RATIO
GT5 : AXLE REAR, 4.10 RATIO