gearing up
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Junior User
Joined: Sep 2004
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gearing up
I have a 1975 F750 with 5spd clark and single spd rear axle, 10.00-20 tires. At 55mph, your at 3k on the tach. I don't really need all that low gearing. What would be the cheapest way to gear me up. I have a 4spd out of a F600 that also had a 2spd rear axle under it but I got rid of the axle. I had driven it before parting it out and it was geared a little bit higher but I don't know if it was the trans or the 2spd axle. Up for suggestions. Might go with a diesel engine someday and then would really need geared up. But for now I will stick with the 391 in it.
Super Moderator
Joined: Apr 2004
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Your F600 relied on the 2 speed axle for the taller gearing.
You have a couple ways to go.
--two speed axle. This should be easy to price out, and you can tell by looking at it what you are up against for an installation. You will probably be able to swap out just the center section. It might be easier to swap the whole thing, depending on what you find.
--OD trans. Some Clarks are OD, you'll have to shop around.
--Auxilary trans behind the existing Clark. You'll have to shop around a bit, as you don't need the reduction, maybe you can find one that is direct/over:
Truck Parts Auction -: 3 speed Over Drive Spicer Auxiliary Transmission OD (Auction ID: 124510 end time Oct. 15, 2010 13:40:28 EST)
Sorry, nothing free, or particularly easy.
You have a couple ways to go.
--two speed axle. This should be easy to price out, and you can tell by looking at it what you are up against for an installation. You will probably be able to swap out just the center section. It might be easier to swap the whole thing, depending on what you find.
--OD trans. Some Clarks are OD, you'll have to shop around.
--Auxilary trans behind the existing Clark. You'll have to shop around a bit, as you don't need the reduction, maybe you can find one that is direct/over:
Truck Parts Auction -: 3 speed Over Drive Spicer Auxiliary Transmission OD (Auction ID: 124510 end time Oct. 15, 2010 13:40:28 EST)
Sorry, nothing free, or particularly easy.
Fleet Mechanic
Joined: Dec 2005
Posts: 1,603
Likes: 44
trans.
Oh there was one the other day on Ebay for like $350 maybe you can
find it I dont have the ebay #. You will be looking for a Clark "VO" which
is overdrive. I have 3 1953sF900s all with 391s with clark ODs all I can
say its wicked. 60mph gives me around 2450 rpms. They are goverened
at 3,000 rpms. Keep searching thats way better than messing around
with rear ends. They are out there one of mine came from an International and they were in some Dodges to.
find it I dont have the ebay #. You will be looking for a Clark "VO" which
is overdrive. I have 3 1953sF900s all with 391s with clark ODs all I can
say its wicked. 60mph gives me around 2450 rpms. They are goverened
at 3,000 rpms. Keep searching thats way better than messing around
with rear ends. They are out there one of mine came from an International and they were in some Dodges to.
Tuned
Joined: Feb 2006
Posts: 487
Likes: 3
From: Lampasas,TX
I'm in the same boat with you- my '58 F600 has a 292 V8 with a NP 5 speed (5th is 1:1) and a 2 speed Eaton rear axle (HI is 6.33 and LO is 8.81). I have 9.00x20 tires and at 60 mph in 5th and HI the engine is turning 3300 RPM. I'm also looking for a new rear axle as the only "higher" gear available for the Eaton is a 5.83 which will make very little difference. I'm looking for something in the 4.33 range and I was told by a salvage yard that Rockwell made such a rear axle for the F600 but it is very rare.
Freshman User
Joined: Jul 2003
Posts: 36
Likes: 0
I just was working on my 55 F600 and am in the same boat. I am going to put a diesel in it and convert it to a tow rig for a 5th wheel so I am probably going to just end up pulling the Eaton 2 speed out and stick a 10.25 Sterling duallie back there so I can get the gear ratio right. 55 mph at 3K. Somehow I don't think a Diesel will like that particular combination...
Freshman User
Joined: Aug 2010
Posts: 45
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From: Northern CA.
It was somewhat the same for pick-ups in the old days. Geared way to low to be anywhere close to efficient at hiway speeds. Biggest part of the problem was the gearing. Next was a cam that petered out at about 45 mph since the peak of the torque was about 1800 rpm. The combination made them miserable on the big road.
The OEM cams only had about a quarter inch lift and were junk! It wouldn't help in lowering the rpm's but a decent cam such as the RV series Comp Cams 260 would raise the torque curve substantially thus improving its hiway performance, economy, drivability, and capabilities.
A quick curve in the distributor advance and an upgraded high output coil with an electronic ignition can help as well.
As for gearing, I'ld look for a complete axle out of a unit with a diesel and an automatic transmission. Seems they have the best hi-way gearing. My 95 F700 with an Allison has a set of 4.10's. It would be a bunch easier and cheaper to swap the entire rear axle than a set of gears.
The OEM cams only had about a quarter inch lift and were junk! It wouldn't help in lowering the rpm's but a decent cam such as the RV series Comp Cams 260 would raise the torque curve substantially thus improving its hiway performance, economy, drivability, and capabilities.
A quick curve in the distributor advance and an upgraded high output coil with an electronic ignition can help as well.
As for gearing, I'ld look for a complete axle out of a unit with a diesel and an automatic transmission. Seems they have the best hi-way gearing. My 95 F700 with an Allison has a set of 4.10's. It would be a bunch easier and cheaper to swap the entire rear axle than a set of gears.
Freshman User
Joined: Jul 2008
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I just was working on my 55 F600 and am in the same boat. I am going to put a diesel in it and convert it to a tow rig for a 5th wheel so I am probably going to just end up pulling the Eaton 2 speed out and stick a 10.25 Sterling duallie back there so I can get the gear ratio right. 55 mph at 3K. Somehow I don't think a Diesel will like that particular combination...
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Senior User
Joined: Sep 2010
Posts: 352
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just typing as i think, but why not consider a overdrive unit like a gear vendors OD.
will it not mate up to your transmission?
it would give you 20 possible gear combinations with a 5 + 2. a bit excessive i admit.
will it not mate up to your transmission?
it would give you 20 possible gear combinations with a 5 + 2. a bit excessive i admit.
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