View Full Version : Rear-end gears
Hey kids, PLD here. I have an 81 K-10 with a 350 motor, Turbo 350 tranny and 3.08 gears. I get about 9 mpg. 99% of my driving is around town under 55mph, and alot of stop/go action. If I were to switch to, say, 3.73 gears, would it help my mileage or hurt it? I seem to think it would help because it would require less throttle to take off from a dead stop( I think). I'm just a dumb kid though, and I'm sure you guys would know what to do.<br> Thanks,<br> PLD
03-12-2000, 02:31 PM
what you might have as an option is swap out a later model chevy 4x4 83-86 with the 700R4 4spd. automatic, the transfercase will bolt on to the back of the adapterof the tranny, plus you'll have to redo the fount and rear drive shafts and for rear end gears go for either 3.23 or the3.73 might help <br>Vance
03-12-2000, 04:58 PM
What sixe tires do you have on the truck? that will a huge factor when deciding what geasr to get. Unless you have a buddy that can set them up for you, the two ring and pinon sets will set you back quite a bit. At least 6-800 plus labor to install. It will take quite a while to recoup the cost.<br>Dino
03-13-2000, 09:39 AM
mileage will not improve much,acceleration will though.Like Dino said,go out and measure your tire height,it should be around 29-31"stock.Your truck must be a dog until you get moving with your setup,If your truck is worth it,you might be better off buying a wrecked truck with 3.73 gears or better yet a 3/4 ton LD.Around here very few (mechanics)can setup rears correctly,and they wine and hum,and clunk.I have experimented on my own and have about a 75%success rate,which is better than average here,but I wouldnt do it for anyone else,to risky.Plan on 300 each axle for parts and another 300 labor each end for installation,thats what you'd pay around here.Thats why I suggest a rusted out or hit parts truck.Then youll have spare parts too.I am not sure with 8.5,3.08 but the 7.65 S10 rears have 2 diff carriers a 3.08 and down and a 3.23 and up carrier,I dont know about the 8.5 10 bolt,check it out first to save a lot of aggravation and wasted money.I think if your happy with acceleration forget the swap,you'll never recoup the money with mileage savings.<p>----------<br>John D<br>
03-13-2000, 08:57 PM
Depending on where you are I have a pair of 1986 3/4 ton axels with 4.10 gears. I am swapping in one ton axels. Also the 700r4 wont do him any good. the 1-3 gears are the same ratio as the turbo 350 he has now. the over drive will just hurt him further, it will never lock up till about 70 on the high way, then when he gets to a hill the tranny will downshift. <br>Now 4.10's and overdrive with 31" tires will do fine.<br>Or trade the 31x10.50's for a p-metric tire that is as tall but not as wide. With less rolling resistance that may help acceleration and milage.<br>Dino<p>----------<br> Professional Ice and Snow Management <br>Products:Services:Equipment www.sima.org
i have a 77 1/2 ton with a 350,350 trans<br>& 3.07 gears. i thought my mileage was<br>bad at 11.5 but not anymore. i run 32x11.50<br>A/T tires on my truck. the engine is stock<br>except for a dual exhaust. i've heard good<br>things about jacob's ignition systems from<br>some of the other guys that use this site<br>and i'm planning on getting one, it should<br>bring mileage up a couple of points. i agree<br>with dino on the 700 swap, w/3.08's & od<br>your truck would hardly ever go to 4th.<br>4LOW
03-17-2000, 07:50 AM
Another option contact Boyce Equipment<br>(1 800 748 4269 ) and get a rebuilt Dana Dana and bolt them in should cost you about about $200 more than the gear swap but a lot less down time
04-01-2000, 10:33 PM
Hey plowking, the 700 r4 does not have the same 1-3 gear ratio by far as the th350 or 400. A 700 r4 swap WOULD create more torque off the line 3.06 2.61, and 1.00 ratios as compared to 2.52, 1.48, 1.0 of the TH350, 400 having a 2.48 first gear. I know the 350 in my truck replacing a 700r4 lost noticable jump.
Yep yep, turbo 700R4 does have different gears, as doug said, also the 8.5" 10 bolt has the same carrier for 2.73 and up, so you're all good in that regaurd. If you're running between 31 and 33 inch tires, I'd go with a 4.10 set of ring and pinions. If you have bigger tires than that, better go get some bigger axle's, because you're just going to break them. Be careful about buying a used 700R4 also, if you go that route, becuase the early ones aren't worth beans. Good luck.
04-04-2000, 08:27 PM
What size tires are you running? If there big. you could go to the size that truck was built for, I know that it won't look nearly as good. but its a lot less hassle. I have seen some pretty small tires on these types of trucks. some thing like a 225/75R15. your local tire dealer should have a listing.<br>Good Luck
I have a 78 k20 chevy and it has 4.10 rear end in it along with a 400/th400, np203 conversion to part time, but anyway, my truck has the ol original size tires 16.5 x 9.5 on it, orig, it came with mudders, and my spare is the orig. with the truck. (dad was orig owner) so i have pleanty of photos and all of the orig paperwork for it, but anyway where I was going before i got into my novel is that when i ran 32" tires on it i lost fuel economy somethin fierce, dropped from 12mpg to around 8 mpg, so going back to the orig size may be all you need to do to regain some of that lost fuel economy<p>by the way<br>140,000 orig miles on the sm block 400<br>no rebuilds, orig plug wires, still gets 12 mpg! when someone knows what to feed the rust monster let me know!<p>-abe<br>
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