Mercedes truck overdrive gearbox options.

Sitec

Adventurer
#1
Hi all. So, the shed build is well under way, the body has been sourced, a late Actros muffler is now 'in stock', and I'm all but ready to start my build. I want to go through my truck before I do though. I have a Mercedes 1222A which has a straight 6 speed gearbox which stands alone and has a prop shaft which connects to the transfer box. It's one of the later ones so comes with the slightly larger OM441 V6 engine. In it's current state (even with the larger 22.5" tyres it has a top speed of 95 kmh (engine RPM at 2000rpm just short of the red zone...). My question is... How do I gear it up? Gearbox or transfer box? I dont think playing with axle ratios is an option as the front axle is a different design to the rear.. That said, I can get hold of a Hub Reduction front axle if this is an option. Anyone ever had anything to do with gearing up Mk's and SK's? Thanks in advance.
 
#2
As far as I know, Mercedes only offer a single gear set in each gearbox, ie. I do not think it is possible to swap gears out in your box using factory parts. You'd have to change the whole box.

Do you know what the 6th gear ratio is? The overdrive in the G85-6 on the newer axor trucks is 0.73 which I think is about as low as you'll get. All of the 9-speed boxes and up have s straight 1:1 top gear, which means they have longer diff ratios to allow decent top speed. I'm not sure how straight forward it would be mating the newer Axor gearbox to your engine though.

Mercedes do fair range of diff ratios, and I suspect that this would be the easiest way to go, although not cheap if you use new parts from the dealer. You'll have to work out what ratios you can use front and back to be matched, I do not think you can change the ratio of the planetary gears in the hubs
 

Sitec

Adventurer
#3
Hi Nick.
I believe 6th gear is direct on my box... (Or have been told that). I know the gearing in the Hub Reduction ends can't be changed, but do have access to rear ring and pinions of different ratio's from a local wrecker for the HL7 axle. He's also got the heavier Hub Reduction front axle so I could go that way. Was hoping there was an 'easy swap' gearbox option. One of the older 1617's I drove for a while in Africa had a really user friendly 8 speed (4 over 4) gearbox, which was coupled to a hub reduction rear axle. It was only a 4x2 truck, but it used to fly! 120kmh if pushed! I'm also told the 8 speed is 1to1 in top gear, and also won't handle the torque of the larger V6 (especially when I put the turbo, pump and injectors of my donor 1734 on it!). Some locally suggested I fit the Road Ranger from that truck in mine... It would have been a nightmare on sand tho, and prob not Wifetec friendly!
 
#4
What size 22.5s does it have?
Going from 385/65R22.5 (or 11R22.5) to 445/65R22.5 (or 13R22.5s) will drop rpms about 9-10% at a given speed.
 
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#5
Hi, have a 1124 AF 4x4 and have been looking at this. Similar problem in that loads of power but revs getting high above about 85kph. Found this website where they changed the diff ratios http://7globetrotters.de/ausbau - you have to look round the site a bit to find it but they show what they did. I priced it up but so far looks about £5k in the UK to do it. Not done it yet but still thinking about it.
 

Sitec

Adventurer
#6
Hi Charlieaarons. I managed to find a set of 8 Goodyear Omnitrac 315/80 R22.5's on Mercedes original steels for very sensible money last year. They're a lot taller than the 10.00 R20's that were on it. I'm loathed to go down the large Super Single option, and also priced/searched for 13R 22.5's before I found the 315's, and they are rare here in Oz, and expensive if found.

Hi Toby3. I'll have a look at the link. Thanks. Either way it's sounding like diffs is the only really viable option. There is an Allison auto on offer.. Not sure if it's an overdrive box tho!

Thanks guys.
 
#7
I had a quick look online and the bellhousing pattern on the OM906 and OM441 seems the same. You might be able to bolt up the G85-6 if it's available to you. The hydraulic gear shift on that gearbox makes placement of the shifter flexible, but bleeding after you've removed hoses is a huge PITA.
 
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