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HDT gear ratios

3K views 48 replies 21 participants last post by  isbj60 
#1 ·
Current Pete has a 400 big cam, 15 speed, 40 inch tires a 3.54 gear.

It sucks. Overdrive is only really good over 70, and it doesn't have much ass behind it.

We are keeping the motor, but changing the rear and the tires. Tires will be 42s.

New rear will have an air locker and a new, non oil soaked suspension, but that suspension will depend on a bunch of things.

A couple things to consider:

Big cam makes 1,200 ft/lbs at 1350 and torque curve crosses the hp curve at 1550 rpm, but it really prefers 1700-2100 rpm. Redline is 2400, but under load, probably 2250-2300. It's a big industrial motor, you can run it hard all day with no problems.

Realistic top speed is 80. I did push it to 85-90 for three hours across Kansas once shooting the gap between thunder storms, but it's a rare thing.

Splits on the super 10 are all 35%, so it not really worried about the rest of the gears as long as I get the top 2 right.

Current 15 has a .75 over like the new super 10.

"Factory"

Direct is 57 mph at 1700 and 71 mph at 2100

Feels best around 65, or 1900 rpm

Overdrive is 76 mph at 1700 rpm and 94 at 2100

Feels best around 78, or 1740. Really that's about as hard as I like to push it speed wise for a long amount of time. It still feels sluggish at those rpm.

3.73s with bug rubber puts us at the same numbers as 3.54s on small rubber.

3.90s on new tires will put us at

Direct: 54-67 and 65 mph at 2000 rpm
Overdrive: 67-90 and 75 mph will be 1750 rpm

I think that's about as low as I want to go. If you get into 4.10s it shifts it down even more

Direct: 52-64 and 65 will be 2130 rpms
Overdrive: 64-85 and 75 mph will be 1850 rpms


If I were heavy hauling, I would go for 4.10s but we usually gross 45-55k which is nothing for this chassis.

Old school truckers will tell you to gear it to run 100, but the 3.54s will run 99 and it's just too tall. 3.90s will run a little over 90 flat out, which is more than fine.

My only worry with 3.90s is the 65 mph range, which is run quite frequently. It will be either close to max rpm or in the bucket at 1520 rpms. That speed has to come up sometime, and I guess 65 is a good enough speed for it to happen.


I would love a .89 or .85 overdrive, but we already have the super10 in the shop and I think it's better over all.
 
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#3 ·
I've been hanging around 80 in the desert and gotten my doors blown off by a cattle hauler that was easily going over 100:flipoff2:

I mostly agree with your comment, except it's more about what rpm you are in in the gear you are in, not the overall top speed.
 
#4 ·
80MPH at 80,000lbs is about all you want to do. BTDT. Steer axle blow out at that speed can be interesting. BTDT too. But, your 45-55K gross could use a little more top end in speed. You would be 'loafing along' at 80 with that weight. I would certainly be satisfied with 90MPH.
If you want a 'dollar bill' truck (100+MPH) go twin stick with 3.33s. For all you do/haul I would go RTO12513 with your 3.54s or 3.31s and 1100X22.5s(42" rubber").
 
#5 · (Edited)
Gear it for 100mph since youll be cruising at that speed all the time, makes sense right :shaking:

Semi passes me at 100mph... im gonna follow him and cut his feed lines to the brake cans into 1" chunks and leave them on the step below the drivers door in a pile.

If you want a good usable truck go with the 4.10, if you wanna go full retard listen to the post above
 
#7 ·
I would suggest gearing it so the engine is right where it is happiest in direct on the trans at the speed you normally cruise. You will have the least amount of drivetrain losses this way, with the option to either increase cruise speed or decrease rpms and stay at the same speed with the overdrive. The latter for bobtailing mostly.
 
#8 ·
Staying out of this one, since I gave you the Super 10 and want to see it used.

Me, I am 3.31 gears, 42" tires and .65 OD.
 
#11 ·
Why in the FUCK would take out a 15 and put a super 10 in?
VanderHaag can't GIVE used super 10's away.
Both have same overdrive, so learn to skip shift. Actually hauling as light as you do, not much need to split shift.
Gives you way more options, but then, your toy, build it your way.
:shaking:
 
#14 ·
Why in the FUCK would take out a 15 and put a super 10 in?
VanderHaag can't GIVE used super 10's away.
Both have same overdrive, so learn to skip shift. Actually hauling as light as you do, not much need to split shift.
Gives you way more options, but then, your toy, build it your way.

Why are you so high on a 15speed? On the highway its a 10speed anyway? The extra gears are in low range, and have little use to his application if you read his notes. Sure you could make an argument for a 13, but your 15speed comment is out to lunch.

I am thinking 3.90's. Those 855's don't mind the revs, they seem to like running a bit faster than the 3406b's from that era. 2000rpm won't hurt an 855 when you need to run there.
 
#17 · (Edited)
So 75 mph and 1,900 rpm is what it sounds like you want. I'd aim for them in direct. Direct is quieter and one less shift to make (ok, flip of the switch, but still) and running in it on the flats means you can use OD for downhill runs and bobtailing. There's a lot to be said for getting a run at grades without running out of rpm. Momentum is free horsepower. 3.08:1 gets direct @ 1,900 to 77 mph and OD (.75) to 103 mph. That r&p is probably not in the cards but 3.31:1 gets direct @ 1,900 to 72 mph and OD @ 1,900 to 96 mph. That's where I'd go.

OR just keep the 3.54s and go to 42" tires?
 
#18 · (Edited)
3.73's, 42's, and maybe a smidge more power. I assume that engine has a lot more to get. Let's you keep up with traffic (75-80) in top gear at an RPM it likes.

With a super 10, that would probably let you do most mild hills without physically shifting, just splitting the top gear.

At least that was how I drove a 425 hp 13 speed that was similarly geared.
 
#19 ·
"Gear it to turn a dollar bill" or go 100 mph is not the point of running 100, it's gearing it to turn nice and slow at 70. It's more a distinction between highway trucks (with 3.xx+ gears) and off road trucks/dump trucks (with 4.xx and lower) gears. New trucks are geared to run even faster, they gear them to run in direct and they have some really high rear gears (2.54 and maybe I saw a 2.31 coming out) but of course the speed limiter prevents that.

And, honestly, it's about right for the old gears under normal circumstances.

We are always somewhere between bobtailing and loaded, even "empty" we usually have one jeep in the back and are pushing 25k, "heavy" we are only 50k... It has only been moved a couple times truly "empty". It's quicker, but I just don't see gearing for that.

We are running 315/80r22.5s, so about the size of a 12r22 or 11r24. That's not going to change.



We were always considering bumping the power "a little", but we had a bunch of weird and frustrating overheating problems up to and including replacing the water pump in a truck stop parking lot. Now that we found a plug in the bypass I'm thinking it will run more consistent and that might be an option again.

Running it mostly in over and a button shift for most hills is absolutely where we want to be.
 
#21 ·
Time to do the fuel line to the front of the block.

You have more fuel in the pump... the turbo will support it.

Ask me how I know.
 

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#25 ·
One thing to remember is that those old 855's at 400hp are not a new 500hp ISX or Detroit. The torque curves are not quite as low, and they really really come alive above 1500rpm.
Sask nailed it, these old mechanical diesels are a different animal from modern big displacement diesels. Our 855 couldn't pull a waxed string out of a cats ass at 1300 rpm.

Biggest problem is there is absolutely no dynamic timing in an old 855. You are set where you are, wherever the cam hits the injector is where you inject diesel. That's why they are so sloppy at idle and they smoke like a bitch at low temps.

Even when you change the turbo and "add more fuel" you don't change the injection timing. You get a little bit of timing boost when the boost reference ups the fuel rail pressure with lower rpm boost, but it's minimal.

It just likes getting up in the rpms.
 
#30 · (Edited)
tappet housing gasket thickness is how they set timing on the 400

the 444 was STC injectors which seems to me are just a hydraulic link that cn increase lash

the 475 was MVT which used eccentric shafts for the tappets that were moved by an air pot, this moved the roller tappets on the cam

at best you get two timing settings from the factory high or low

tappet housings

 
#32 ·
I think some r missing the point. This truck has plenty of power and the power band is fine. What we r trying to do is make the truck easier to drive. Less shifts is one way to do that and we dont really need the extra low of the 18. We got a good deal on the super10 so its off getting the rear syncro and air valve replaced and it looks to be the same length as the current 15 in the truck so no driveline mods r needed.

We will probably run the extra fuel line to the front of the block for the 50 extra hp now that the cooling issue has been solved, but thats about the extent of the power upgrades. This motor is very reliable at its current hp and that something we have no desire to mess with. Its not an under powered mdt, its a 14l hdt that was designed to haul 80k.

We just want to figure out where we should be with the current round of upgrades.

Btw, some current numbers i got from todays trip home from wheeling. This is based on our tach-o-graph so the numbers r as close as i can get.

75mph=1700rpm OD
70mph=1650rpm OD
65mph=1900rpm 1:1
70mph=2000rpm 1:1
75mph=2150rpm 1:1
 
#33 · (Edited)
I think some r missing the point. This truck has plenty of power and the power band is fine. What we r trying to do is make the truck easier to drive. Less shifts is one way to do that and we dont really need the extra low of the 18.
But it is along a different path to the same destination (of an easier to drive truck)
A wider powerband means less shifting, imagine if you could make it tolerable to drag down below 1300 rpm, possibly with just more pre-boost fuel to light up the turbo earlier.
Try it before you go regearing, set up the afc screw so there's a tolerable puff of smoke before boost comes on.
Costs you nothing, and it's an adjustment that can't hurt a thing without you actively trying to.
 
#34 ·
I agree that there are different paths to get out ultimate solution, but the truck puffs smoke and hazes under hard acceleration already. Also the truck hits about 20psi of boost and i have never seen higher then 800f egt preturbo, so i can handle more fuel. I just dont want to push the coolant system as it seems to be the weak link.
 
#38 · (Edited)
i would try to gear it for about 67mph in 9th. cuz i bet if you average out all your highway driving, thats probably the most common speed. leaves you with one more gear for anything faster. so when youre running daytime with all the traffic, its happy in 9th. when youre haulin bawls on empty roads overnight, go whatever speed its happy at in 10th.
 
#39 ·
Personally if I was worried about making the truck easier to drive and stay in the power. I'd go with 3.90s and a 13. That's my current setup behind a 87 big cam 400 and it does pretty good. But I'm also on 11r24.5s and have the dual fuel line kit and a little bit of pump work.
The 15 speed you have now depending on model is a damn good trans but is kinda useless for what you're doing. I don't think you're really going to gain much by going to a super 10 either as far as gear splits go. And anybody that says you need an 18 is just wanting you to throw money away. You're grossing 50k for christs sake. In the semi world that ain't shit.
And for gods sake don't listen to these idiots with new emissions trucks or older cats on gearing. The 855s like to turn rpm to make power, unlike a cat or old mack motor. 2.xx gears would make that truck an absolute dog.
 
#41 ·
11r24.5s are about the height of our 315s. A 13 would be great but the super10 was the right price and I think it will be fine for our needs. There is no way we need to split the basement with a 18, but it would be cool for the sake of being cool :flipoff2:

I agree, if we were hauling rocks or logs at max gvwr through rough roads the 15 would be tits. But we aren't.

100% agree on needing to spit the 855. It just is what it is. They have nothing down low. Not a bad motor, just no balls below 1600 rpm.
 
#40 ·
Those old Mack engines had incredible torque rise. I remember first running one around the city/gravel pits, and the old 350 Mack felt like it had a fair bit more snort that the Fords with 425hp 3406B's. The Macks would make the turbo whistle at 1000rpm, and start pulling hard from there. It wasn't until I started running on the highway that I felt the Cat Power. Those Macks were pretty much out of steam by 1700rpm. I don't think they had big cubes either, 673 if i recall correctly.
 
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