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07-27-2013, 06:57 AM
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#1 |
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Registered User
Join Date: Nov 2010
Location: MICHIGAN
Posts: 196
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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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1987 Buick Grand National 2018 Road Glide Special |
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07-27-2013, 07:17 AM
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#2 |
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I know the pieces fit
Join Date: Nov 2007
Location: MONTGOMERY, AL
Posts: 5,523
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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. |
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07-27-2013, 07:25 AM
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#3 |
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Registered User
Join Date: Nov 2010
Location: MICHIGAN
Posts: 196
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Re: Question about gearing
How do I find out what gear is in my truck ? And it does have a 700R4
Posted via Mobile Device
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1987 Buick Grand National 2018 Road Glide Special |
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07-27-2013, 08:08 AM
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#4 |
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I know the pieces fit
Join Date: Nov 2007
Location: MONTGOMERY, AL
Posts: 5,523
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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 |
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