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Discussion Starter #1
Help, I went from the stock "245" tires to "315 BFG's" (Hummer H2) and I would like to know what gear ratio to use to correct the change, and also where to get them. I have called Richmond Gear and they don't make a gear set yet for the GM14X11.5 rear-end. I have the Hypertech to adjust the speedo, works great, but I would like the ALL of the power back for towing. THX
 

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I believe your at a 3:30 -3:32 from a 3:73 and aprox 12.6mph slow at 60mph I forgot what they figured out to on my truck the 4:10's would put you right around 3:69-3:70's if I remembered right. I know Randy's ring gears has them they can get expensive in my oppinion though.
Jon
 

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The correct ratio would be a 4.56 to bring it back to close to original. It is about $900 for the rear and $600 for the front from GM, parts only. I have spent a lot of time trying to find a less exspensive alternative and I am still looking. Also the correction factor is actually 13.6% so multiply your speedo by 1.136 for correct speed. Also multiply odo for same.
 

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The perfect ratio to get you back to original would be 4.23. You could use 4.10s or 4.56s. 4.10 is a factory ratio for the 11.5" axle, so those gears are available from the dealer. Randy's does carry 4.56 gears for the big axle as stated above. Randy's
 

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All you folks are very close to the right number. When you're talking the difference between 12.6% and 13.6% that's not a whole lot! I've been looking at changing gears for a while now and I was hoping I could find a 4.27 gear, but they don't make that for the 11.5" so I guess I'll be going with the 4.10's instead. The 4.56's just put my RPM a little too high in my opinion. I suppose if one was to do any amount of serious towing, then the 4.56 gears might be a better choice. Of course I'm just throwing these numbers around assuming going from the stock 245/75R16 tires up to 315/75R16 rubbers. I know the H2's are 315/70R17 so the calculations are going to be just slightly different.
 

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I would do 4.10 for the 315.


the 4.56's will run a little high in RPMs when not towing.


just my .02
 

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Hummers come stock w/ 4.10 gears and they are OK.


Come to think about it, I ought to race the wife sometime and see what happens.
 

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Discussion Starter #9
The low interstate rpms are good, but I'm concerned about the transmission's (Allison) life span as the dealer says "NO mods to tire size".


I have found a 4.10 rear ring and pinion from: http://www.samtaylor.com/html/parts.html talk to Jason, for $530.00. Luckily I have a 2wd and only have to do the rear!!


Thanks all for your help with this......Kirk
 
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