I have an 88 K-5 Blazer and it has 3:73 gears in it and i would like to put 4:56 instead. I run the truck in the mud and it has 35 inch tires but will be putting on 38's soon. I want to know if 4:56's would be a good set up for what i want to do and will there be any problems that i will have to look foward to? Any input would be great. Thanks  

Right now you should be running 4.56 minimum with 35's (a 4.88 would be better) 38's would be in the 5.13 range. These gears are available for a 10 bolt. The real problem is axle strength. 35's is kinda the limit on 10 bolts of full sized trucks. With 38's in mud and such you stand a good chance of starting to break things. I would regear axle to 4.88's and keep 35's. If you want to go bigger and have it stay together, you need bigger axles. Do not let the number scare you, even with 4.88's and 35's you will only be turning about 2100 RPM in OD at 65 which is not much at all but at least OD will work too. I am kinda surprised that you have not toasted tranny yet with 3.73's and 35's. DO NOT use OD with current tire size/gear ratio setup if you value your tranny.  

We have put a Turbo 400 in the truck so im not to worried about the tranny. But someone told me i would have to change out the carrier to put 4.88 in it. Is this true? The 1/2 ton set up has worked really well for us so far. I have thought about up grading to 3/4 Ton axles but they are more expensive then it would be to change the gearing. I really want the tires to light up by just burping the throttle. Any Feedback is appriecated.  

wooddog645:
We have put a Turbo 400 in the truck so im not to worried about the tranny. But someone told me i would have to change out the carrier to put 4.88 in it. Is this true? The 1/2 ton set up has worked really well for us so far. I have thought about up grading to 3/4 Ton axles but they are more expensive then it would be to change the gearing. I really want the tires to light up by just burping the throttle. Any Feedback is appriecated.

I would not retro to a THM400 as that is plain silly!!! Nothing wrong with a 700R4 design, only how it is used sometimes. What fries 700R4/4L60's is stock gears with big tires which forces tranny to ride the torque converter stall in upper gears. In your case with your current setup you are forcing engine to about 1600 in OD at 65 MPH which is slow bake riding stall or badly straining OD clutch. The 700R4/4L60 has a much deeper first gear than THM 400 (3.06 vs 2.48) and this deeper gear helps launch these tall drive ratio (big tires stock gears) but then it cooks tranny in upper gears riding stall. 700R4/4L60 also has a deeper reverse gear too than THM 400 (2.29 vs 2.07). Lets do some numbers. If you installed a THM400 and say 4.10 gears (to try to keep cruise RPM down) you would have a 65 MPH cruise of about 2700 RPM (THM 400 converters slip some as there is no lockup in them) and your start out ratio is 2.48 x 4.10 or about 10.2 to 1 plus a 2 to 1 one converter stall (or about 20 to 1 torque multiplication total at stand still) Now with a 700R4/4L60 lets run the numbers with 35's and 5.13 gears. At 65 MPH you will cruise at about 2200 RPM in OD and your start out ratio is 3.06 x 5.13 or 15.7 to 1 plus a 2 to 1 one converter stall at stand still for about 32 to 1 torque increase. Now which one is going to run and be a stump puller too??? BTW 2nd gear overall effective gear ratio in the 700R4/4L60 combo is nearly as deep as first gear in THM400 combo. (8.2 vs 10.2)  Also reverse ratios would be 2.07 x 4.10 or 8.4 to 1 for THM 400 setup above or 2.29 x 5.13  or 11.75 to 1 for 700R4/4L60 setup above (about 40% MORE backup torque than possible with thm400 combo) When you look at a 700R4/4L60 as a true 4 speed tranny and properly gear axles so that you have a 2100 rpm or better cruise at 65 in 4th gear, you will not have any problems with them. It is drivers that run big tires and stock gears that toast tranny and then blame tranny not the driver that installed big tires and did not regear (it is always trannies fault it seems, never drivers) One more thing, the carrier break on a 10 bolt is at 2.56/2.73 so it is not a issue for you at all and deepest ratio I find listed for a 10 bolt is a 5.57.  

I really appriatiate the feedback about the tranny and gearing issue. I have notices that the TH400 isn't as low as the 700R4. We also put a NP 205 transfer case to the 400 and i noticed in 4 low it isn't that low. I have read in past issues about the "Bomberproof Blazer" with a TH400 and NP 205 thinking it was going to be the best set up but have noticed that the original set had a much lower gear set. I have already invested a good some of money in the 400 205 set up and by gearing it to 4.88 or 5.13, will it bring it back or make it lower then the original set up? Also the TH400 shifts so much nicer than the 700 thanks for reading.  

You are really kinda in a pickle with gearing now. If you install the gears you need for your tires, you are going to be screaming going down the road in drive (about 3200 RPM in drive @ 65 MPH with 4.88)just to get the same overall start out ratio in first (axle and tranny combo) as you had with 700R4 and 3.73 gears. Generally people that recommend the THM400/205 as the bullet proof combo are the same ones that do not believe in properly regearing axles. If you gear axles properly, you will not have problems with 700R4 or 208 (I have a 700R4 in a heavy 89 burb that I bought new and it has 180K and tranny and still works like a champ) As you stated you also discovered the low range difference. A 205 has a 1.96 to 1 low range and a 208 2.62 to 1 and NP231's and 241's are 2.72 to 1. Before your conversion, your crawl ratio in low range was 3.06 x 2.62 x 3.73 or 29.9 to one. Now it is 2.48 x 1.96 x 3.73 or 18.1 to one, a BIG difference. Even a 5.13 would only increase it to about 25 to 1. (5.13's with old tranny/Tcase combo would be 41.1 to 1). So now we are in the damage control mode trying to figure the best way to fix your truck. The best way would be to go back to original setup and regear but you do not want to do that so we go to plan "B". This would be to install the 203/205 doubler mod where you take a planetary reduction unit from a 203 and add it to 205 and then have a 3 speed Tcase with 1 to 1, 2 to 1 and 4 to 1 approx ranges. This is a double edge sword though because you can make to much torque for drive shafts and frame mounts (counter torque) if you use this combo to over come tall axles and big tires. In your case I would do the 203/205 (if you want to keep current tranny and Tcase) and do a 4.56 in drive axles (since you have no OD) as a trade off between power and cruise RPM. I personal place a lot of importance on axle ratios but it is not just for power but it also limits chassis torque and twist load on drive line. The taller the axle ratio the more torque you have to put into drive shaft and the more counter torque the frame/chassis and mounts have to absorb and then mounts break and frames crack when pushed to far. Below you will find a link with some info about the 203/205 conversion I mentioned (though again I must say if it was mine I would go back to old setup with 5.13's and be done with it and truck would weigh less too)

Click Here  

Snoman has done his research.  I too have been running all the gear calcs and comparing tire sizes.  I have an 82 Blazer, still has 10 bolt front and rear, with 3.73 gears, well built (fortunately because of 400HP crate 350) 700r4, and 32" tires.  It runs great on flat highway, with great milage (relatively speaking - 16mpg), but going up and down Colo. mountain passes, it's a dog.  I have been upgrading everything in this truck for years, and the next step is some 37" tires, and a 14 bolt FF rear. To put 4.56 gears into that setup, would be numerically the same as the 31-32" tire @ 3.73, a 4.88 gear would be slightly more torquey.  To maintain driveability (mpg's) and useful trail torque, there is no other choice than a 5.13 gear with a 37" tire, and maintain that low 1st gear, and nice 30% overdrive that the 700r4 has, and not to mention the top speed on the highway all the way out to 117 mph (vs 82mph w/ TH400).  Now introduce the need to roast the 39" tires that was mentioned earlier, again, no choice but to utilize the very low 1st gear of a 700r4, and the tallest axle gear possible... no problem, now you need new axles, and just don't hope to go much faster than 60.  

bikenut007:
Snoman has done his research.  I too have been running all the gear calcs and comparing tire sizes.  I have an 82 Blazer, still has 10 bolt front and rear, with 3.73 gears, well built (fortunately because of 400HP crate 350) 700r4, and 32" tires.  It runs great on flat highway, with great milage (relatively speaking - 16mpg), but going up and down Colo. mountain passes, it's a dog.  I have been upgrading everything in this truck for years, and the next step is some 37" tires, and a 14 bolt FF rear. To put 4.56 gears into that setup, would be numerically the same as the 31-32" tire @ 3.73, a 4.88 gear would be slightly more torquey.  To maintain driveability (mpg's) and useful trail torque, there is no other choice than a 5.13 gear with a 37" tire, and maintain that low 1st gear, and nice 30% overdrive that the 700r4 has, and not to mention the top speed on the highway all the way out to 117 mph (vs 82mph w/ TH400).  Now introduce the need to roast the 39" tires that was mentioned earlier, again, no choice but to utilize the very low 1st gear of a 700r4, and the tallest axle gear possible... no problem, now you need new axles, and just don't hope to go much faster than 60.

I might add that when you are operating at high altitude, you loose at least 3% HP for each 1000 ft above sea level in best case scenario. In practice it is higher at 5000 feet and above. Because of this you want to gear at least one step deeper than you would otherwise. This means if you think that a 4.88 might work, you really want a 5.13 or 5.38. BTW, a 5.38 is only about 10% deeper than a 4.88. The deeper gears will help make up for the power loss in thinner air.  All EFI does is help keep the loss from getting even worse due to mixture being off as it does not adjust to make it run the same as there is less air to work with so there is less power. Example, if you have a 300 HP motor, you are lucky if it can still make 250 at 5000 feet and at 10,000 feet you are not even making close to 200. Only forced induction like a turbo or super charger can help make up for thinner air.