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Old 01-12-2007, 10:43 PM   #1
drivercrazy23
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1997 gmc sierra z71

I have a 97 gmc Z with a 3 inch body lift and a 4 inch suspension lift and 35 inch tires. My question is what can i do to get better gas mileage? Im thinking changing the rear end gear ratio, and front. But i dont know the stock ratio. If any of you have any suggestions or info on the ratio. Please let me know. Thanks
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Old 01-12-2007, 11:39 PM   #2
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Re: 1997 gmc sierra z71

I went back and found this thread that should help with your prob.
Quote:
Originally Posted by Redstepside72chevy View Post
Hey guys do yall know how to calculate the rear end ratio

Quote:
Originally Posted by Blackcypress-EB View Post
The quick on the truck way is to mark your tire on the sidewall close to the ground with a grease pencil. Then mark the drive shaft lengthwise as well as a mark on the pinion to line up by. Jack up the truck where the tire will spin. Line up the mark with the mark you put on the pinion. Turn the tire until you make one revolution with your driveshaft. Count how many times and the percentage of one revolution past the mark on the ground the tire turned for one rev. of the driveshaft. That is your gear ratio. This is not extremely precise but will get you within the ballpark of what gears you have. 3.73, 4.10and so on.


Gears out of the pumpkin are easy. Count the number of teeth on the ring gear and divide by how many are on the pinion gear.

Also this link will handle any gear/tire combo ?'s. http://www.izook.com/gearcalc.htm

Lastly, I haven't lifted any of the 88-98 trucks, but I would say 7"'s is possibly excessive for 35" tires. I have adequately cleared 37's with a six inch. You might consider eliminating the body lift. I see a lot of trucks on the road that could easily cut their lift in half and have better road manners and a lower center of gravity. The whole purpose of a lift is to strictly clear a tire size. Any height above that is wasted. I am a fan of low lifts and fender trimming. You can only wheel as deep as your axle allows you anyway. Good luck.
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Old 01-13-2007, 12:02 AM   #3
drivercrazy23
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Re: 1997 gmc sierra z71

Thanks for the info.
About the lift, As it is now, my tires still rub turning lock to lock. but i know i could loose a good inch off the rear end because of the angle that the truck sits at. noses down prety good sitting there. Road manners are good, truck drives great. I dont really have a major problem with anything else except gas mileage.
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Old 01-13-2007, 12:15 PM   #4
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Re: 1997 gmc sierra z71

stock ratio with a Z-71 is 3.73
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Old 01-14-2007, 02:39 AM   #5
drivercrazy23
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Re: 1997 gmc sierra z71

thanks for the numbers. So im thinking some 4.10 gears would bring my truck back to stock final drive and maybe get some better mileage out of it. Whats a good place to get front and rear gears.
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Old 01-14-2007, 08:43 AM   #6
b454rat
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Re: 1997 gmc sierra z71

OPen the gloveobx and look at the RPO codes. Find the ones that start with a "G". 3.73 will be a GT4, GT5 will be 4.10s, and GU6 is 3.42s. Quick and easy way to find what gears you need with a certain size tire, divive the new tire by the old tire, then multiply that by the current gear ratio. So if you have 3.73 with 35s, then a 4.10 would bring it close to stock. But your not gonna get decent mileage out of a 4wd that has bigger tires on it. My old is b!cthing about that now. He has a 98 2500HD with a 454, 6" lift with 35s. It has 3.73s, and still goes OK, but 4.10s would be better. But he still isn't gonna get much better mileage.
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Old 01-14-2007, 09:15 AM   #7
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Re: 1997 gmc sierra z71

Just wanna say welcome to the board from South Mississippi!
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Old 01-14-2007, 03:51 PM   #8
drivercrazy23
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Re: 1997 gmc sierra z71

thanks for the tip. its about 8 degrees outside so im not going to even bother going and checking haha. thank you krue for the welcoming. i have been looking for a forum with good knoledgable(sp) people to help me out with my truck. Another question. start up takes a bit longer then i would expect, it sits and cranks for awhile till it will start up. I didnt know if thats just what the tbi does after it gets over 100,000 miles on it or what. The truck at the moment has 127,000 miles on it. Im not sure of whats been maintnence before i bought the truck so whats good places to check/replace.
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