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Doubler with 3.73's instead of 205 and 4.88's?

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BlitzK5

1/2 ton status
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Jul 29, 2005
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Location
Sherwood Park, Alberta, Canada
My current setup needs new gears front and back aswell as new driveshafts. I installed a 14bolt FF with 3.73 gears and converted my 10 bolt to 8 lug. It has 2.73 gears. I also have a 454/TH350 and 208 setup. With the 38" TSL's is just installed I dont think I should get the shafts made up until I upgrade my transfer case because Im afraid it won't survive. I need to regear the front no matter what because of the miss match. I can get the set of 3.73's from the 10bolt 3/4 ton I got the 14bolt from and switch them to make them the same. I was thinking what If instead of getting new gears like 4.56 or 4.88's I keep the 3.73's and upgrade the 208 to a 203/205 doubler setup then buy shafts instead of geeting a 205, lower gears and new shafts. This way I will get better gas millage on the highway with the TH350(No overdrive) and with the 4.1 ratio I will get a better crawl ratio then 4.88's and a 205. 2.52X2X4.88 = 24.6, 2.52x4x3.73 = 37.6. Seems like a much better setup and advantage to me. I also want to upgrade to a D60 when I find one, so I dont feeling like wasting money on new gears and the install costs on the 10 bolt. Also this would be the last pair of shafts I ever buy unless I brake one of course. I priced out a doubler at northwest fabs and it was $1700 canadian. This would be around the same price to get new gears installed. When I get a D60 I will go to lower gears but for now I think this will work out good. Do you guys think this would be a good idea, and if not why? Has anyone else thought about this setup aswell or something similiar?
 
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What do you primarily use the vehicle for, are you trying to hit a 'same as stock' powerband on the highway? One large positive aspect regarding dual cases is the ability to gear a vehicle for the road (If that is an intended use some of its life), and still be able to have deep gears offroad.

3.73 still sounds a bit low for 38" tires, highway or not. That said when I had some time behind the wheel of a K5 with 38's and 4.10's, the motor was a sad little 307 small block, so I'm sure the 454 is going to pack a lot more punch. Apples to Oranges.

I would recommend using one of the availible RPM/Speed calculators availible online, and get a feel for what RPM/powerband is going to workbest for your specific application.

You could consider the dual cases, and getting your rear driveshaft in, then gearing the front when you get the D60. You wouldn't be able to run a front driveshaft until you geared to match, but it would get rid of the NP208, and you wouldn't be putting hard earned $ into a diff your going to swap out. This is all dependant on how long you think you can go before you find a 60...I know we all go a little stirr crazy without 4lo :D

-Kyle
 
I'm going to run 4.10's and at least 39's when I eventually get my junk back together. Not happy with the 3000rpm highway cruising I had with 33s.

I'll get the extra gearing I need with dual trans or dual cases. Money better spent IMO.
 
Thanks for the advise. Yes I did the calculator thing and found out that with 3.73 gears and a transmission ratio of 1 I will be at the same RPM as a 700-R4 overdrive and 5.13. So its kinda like Im running the TH350 as a 700-R4 when in overdrive of course. My gas millage will be much better on the highway, and with the 454 I will have more of the power needed to pass people (make up for not having lower gears). And beside I could always run the tranny in 2nd or 1st to get the higher rpms to get it higher up in the power band. But im sure thats not to good for the tranny.

Engine 454: Horsepower 230 @3800, Torque 375 @ 3200
 
Having high gears (3.73) with big tires (38") will smoke your TH350. It makes your tranny work way harder and it will not last long. You could do this if it had a manual (though I would still recommend 4.10's). And your 454 isn't doing wonders for a stock TH350 either. That's the only problem I see with your setup, switch to a manual and you will be set. :D
 
38377k5 said:
Having high gears (3.73) with big tires (38") will smoke your TH350. It makes your tranny work way harder and it will not last long. You could do this if it had a manual (though I would still recommend 4.10's). And your 454 isn't doing wonders for a stock TH350 either. That's the only problem I see with your setup, switch to a manual and you will be set. :D
Click to expand...

HEAT is what will kill a tranny.... if you keep the transmission cool, you can make the combo work.

I've been running 38.5s and 4.10s for a few years now (with a LARGE tranny cooler) and a BBC. It's no speed demon in the acceleration department, but the tranny temps are usually around 140 degrees....nice and cool, and no problems whatsoever.


Based on the fact that you're planning to both regear and run a Doubler, all you're really doing is deciding to to the doubler upgrade FIRST. You can certainly assess the results, and if it's a total dog you still have the ability to drop in the deeper gears.... :usaflag:
 
I think your overall logic makes sense. If you don't mind the truck being a little doggy during initial acceleration on the road, then the doubler definitely helps out more in low range gearing off-road then an axle gear change to 5.13, plus you get to keep decent highway cruising ability. I have a friend who has 4.10's and has ran both 38's and 42's on a trailrig......he drives it to and from the trails for long distances on the highway and the rpm's are kinda' nice. A lot of stop and go would be tough on the trans with the high gears, but most of the driving on his rig is constant 45-65 mph cruising to heat buildup really isn't an issue.

With that said, if this if your first trailrig I wouldn't spend all of that time and money right off the bat to do a doubler just to get rid of the 208 case. The 208 should hold up okay until you really start beating on it hard. I would be more concerned about the 10-bolt front axle and would recommend using the money you would spend on the doubler to put towards a D60 front.
 
I'm running almost the exact same setup as you. 454, TH350, NP208 3:73 gears and 38" tires. This is my DD part time. I really like it. Part of my decision for 38's was to drop my rpm a little going down the road for better economy. I run about 2100 at 60 mph. My tires actually measure 36" even though they are new 38" TSL's mounted on a 10" wide wheel. I have NO problems taking off from a start, in fact I hardly notice the difference from the 33" tall tires I had before. The 454 is stock except for headers and a Holley carb. I am running a good sized trans cooler, but I dont' have a trans temp guage yet.
 
Well then maybe I will wait and see before I do anything drastic. My 38" Radials meassure around 36" to and my 454 is stock other then a edlebrock carb. Have you calculated what your getting for milage? How's the offroad performance and what kind do you do? What type of tranny cooler and headers you running and did you have any clearance issues with the headers?
 
with 38.5 Boggers (37 real), I can say that even with a manual trans and big block with 3.73's I found it very necessary to upgrade to 4.56's The bogging up to highway speed with traffic behind me isn't cool. I could live with it, but it's just not fun, and I know my motor is stressing. My carrier should be here next week. Now, if you wanted to twin stick it up to highway speed that might be interesting :eek1:
 
For mileage I don't know yet with the new tires. I need to check it, I'll know in a few days. I was at 11.4 with my 33's. Offroading is nothing tough so far since I mostly just drive it to work. It's not the best commuter vechicle, but I just like it way too much not to drive it. I don't remember what brand tranny cooler I got, but it was fairly large.

I put hooker headers on 1 3/4" primary's. What the exact part number was I don't remeber but they fit pretty good. I posted back about 2 months ago with the exact part number. I did have two small problems. The first was on the drivers side I had to put the plugs in first because I couldn't get my socket on #1 plug. There wasn't quite enough room. I probably could have dented the pipe and made the clearance easy enough but I chose not to. The second small problem was I was going to run a mini starter since I already had a spare one sitting around, but it just hit one of the header tubes. Odly enough a stock starter fit just fine so I just used it.

I don't have any problem getting up to speed. I hardly even notice that I changed tires as far as acceleration goes, but that's exactly why I put a 454 in. I wanted the extra torque to get the thing moving.
 
I found the part number for those headers. I got them from summit. It was hok-2463hkr.
 
Biggest thing I would worry about is up stream stress due to the large tires and abnormally hight axle gears. I personally wouldn't want to run it like that. BBC or not, perf will suck IMO.

For comparision (and to expose my bias), my K5 is just a tad low for the street, but great off-road. Only very small complaint is at 70+ on the highway, but still easily runs 80+, just turning more rpms than I would like ideally. The drive train consists of a mild built 350 / 700R4 / 241 / D60 / C14FF / Detroits / 4.56 gears. BTW, 700R4 and 208/241 (often called the "poor boy's doubler") both have considerably lower lows than th350/205 setups. Currently running 35" BFG ATs. RPMs are just a tad high on the street even with the OD, but I think that combo would work great with you swapping taller tires and loosing the OD (241 is comparable to the 208 in gearing). Problem is, even though the OD/tire size sorta cancels out, you completely loose the decent low range. Which given the tires size and a 10 bolt might not be a bad thing. Swapping in a doubler will get your low range back where most people running a 208/241 are without the doubler. Better carry spare shafts! :D
 
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