I have a 1,700 lb camper on a 88 F250 with a 351W and C6 transmission. I wondered what the best method of handling the mountain passes in Montana. Is it easier on the transmission to put it in 2nd gear for the upgrade, or just leave it in D and let the transmission select the gear. When it is in D, the truck constantly shifts (downshift, upshift) etc. for the duration of the hills and it seems like all that shifting under load must cause some excess wear and tear. Any suggestions for the best method to handle the hills?
Thanks, DR

 

I'd put it in 2nd and let it sing till the summit. I have a 351w and e4od, that's my method when loaded down here in WY. If left to its own devices, my truck will hunt for gears at times too.

What axle gearing and tire size are you running?

 

Agree with manually shifting. It's what was (and probably still is) recommended by the manufacturer in those conditions.
The general rule-of-thumb is that if it's only downshifting and upshifting now and then, you can do whatever you want. Leaving it in Drive won't harm anything. But if it's shifting back and forth every few seconds, or even every minute or so, it is recommended you pull it down a gear.
Whether that's from 3rd to 2nd on a C6 or OD-4th down into 3rd, the concept is the same. Just keep the trans from constantly shifting.

It's only slightly for reducing annoying behavior too. They really are trying to reduce the heat generated inside the trans during shifting, and unloading the engine at least a little bit to keep it from lugging.

Paul

 

Thanks for the advice. I have standard size truck tires on 16" rims and I think the gear ratio is 3.73. I'm going to try leaving it in 2nd gear on the big hills - otherwise it does a lot of downshifting. Any idea of the speed I can expect out of 2nd on the hills? DR

 

Not sure. But obviously you'd be stuck driving in the right lane at a fairly reduced road speed, or the engine will be winding up something fierce.
I would not expect you to go much over 55 in second in fact.
If your truck has a tachometer you can literally push it as far as the tach says it can go. But I would not do that for very long. Be nice if you could keep it below 3500 rpm if it's going to go on for very long, but can run it 'till it runs out of breath if you have to. I don't like pushing things that far with a big truck and a heavy load though. Not a problem for me with a car or my Bronco. But a 3/4t truck with a camper? Maybe keep it around 3500 or less, and whatever that speed ends up being.
I spent most of 2 hours pushing my daily driver at the 5500 rpm redline once. Would not want to do that very often, but it did not seem to hurt anything. But a longer stroke 351 trying to yank the better part of 7500lbs over a hill? Slowing down is the better part of valor...

I wonder if we can use one of the many speed/gear/rpm calculators in this instance. We can find out the ratio of the second gear on the C6 (1.46:1 if I'm not mistaken), add a little rpm for torque-converter slippage, and figure out the engine rpm that way. Just not sure if it's that simple.
For example, in Drive at 55mph, given a tire with an (assumed for this example) effective "rolling diameter" of 30 inches, your engine speed would be approx. 2700-2800 rpm. Drive being a 1:1 ratio.
In second gear though, if we know the ratio is 1.46 does that increase the engine speed by 56%? I don't think so, but don't know how to do the calculations appropriate for this application. But if so, you'd be pushing 4000-4200 rpm @ that 55mph with that combo.
Not the end of the world, but higher than you'd like to be for very long I would think.

Maybe try it as a test on some flat ground near home with little traffic. Run it up to 55 in second and let us know what the RPM is.
Then check your rolling diameter and let's throw that into the calculations. For that, get the rolling radius of the rear tire (full weight on it, full air pressure that you normally run, measured from the center of the hub to the ground, then double that for diameter). Sorry if you knew that already. Figured I'd throw it in right up front though.

Cool test I think. Never done it in all these years even though I've certainly spent my share of time in second gear going up hills!

Thanks

Paul

 

Paul,

I just had the camper out for a test drive on a flat stretch of road. I don't have a tach but it seems that 45 mph is about all it wants to do in 2nd gear - it can go faster but its a strain - so I guess I'll put on the hazard lights, stay in the right lane on the hills and nurse it up and over. Thank for your input. DR

 

If you can find a Gear Vendor OD for a decent price, you could probably pull it in 2nd OD without issues. The problem with our trucks is that they just don't have enough gears for true load hauling.

 

mountainkowboy,

I'm not sure I understand your post. I have a 3 speed C6 - didn't think that OD was possible. Is there an aftermarket OD that can be added, or are you suggesting a transmission swap?

Thanks, DR

 

The Gear Vendors stuff is a separate box attached to the back of your transmission, or transfer case. Separate shifter and shorter rear driveshaft and gives you a nice overdrive in 2wd.
Not an inexpensive way to go, but been selling them for probably 40 years now and have a good following.

Paul

 

Sounds like you have the right attitude!
But I would definitely try to get a temporary tach connected to let you know just what your rpm is at those speeds. I can attest to the fact that every vehicle's combination of tires and exhaust sound and cabin insulation (or lack of it) can make a huge difference in perceived engine speed.

And it's just nice to know this stuff. If only to confirm how accurate your senses are!

Paul

 

There are companies like Doug Nash and Gear Vendors that make overdrive units that bolt onto the back of a c6. So for all intents and purposes, you COULD have a 6 speed. So you would have 1,2,3 and or. Or 1st, 1stOD, 2nd, 2ndOD, 3rd, and OD.
But new units aren't cheap, and even used ones can be pricey.
So, what he was saying above was, if you got an OD unit you could try 2ndOD. It would be somewhere between 2nd and 3rd, and maybe high enough ratio to pull the hill and keep rpms lower than regular 2nd.
And depending on how much traveling you do, it may pay in the long run to have OD on the flats at 60-65 mph.

 

^^^ What they said...

 

If you're hauling a camper, the engine will need to work and you will hear it. Without a tach, just like Paul noted, I'd be lead to believe 2500rpm was "straining" the motor, when in fact it sings at 4000rpm and makes good power. I've been to 80mph in second gear, but that's an E4OD with a lockup converter. 32" tires, 3.55 gears. RPM gets up there, about 4600, not something I do often. Peak power feels lower anyway, more like 3800rpm or so. Point being, 2nd gear shouldn't be limiting you to 45mph. I would get a tach, see where you're at. I'm not sure what typical C6 converter slippage is.

 

For a gear ratio, rpm, and tire size calculator use this Gear Ratio Calculator

Or you can do the math yourself RPM=MPH x rear gear ratio x transmission gear ratio x 336 divided by tire diameter in inches. How ever since the C6 is a non lockup transmission the final rpms could be 100-200 rpm higher depending on load and converter stall speed.

 

Thanks for all the replies, lot of new info for me. After some additional research, wonder if the following might help. I have a 3 speed C6 with a (39) axle code which is believe is a 3.55 gear ratio and I have stock 235/85 tires on 16" rims. If I understand the tire/gear ratio relationship if I go down to a 245/75 on the 16" rims the truck will behave as if I had a 3.73 gear ratio. Not a great difference, but maybe just enough to handle the hills. Is my math correct? Anyone tried this and what kind of results. Gas mileage is already poor, so that's not my main concern.
Thanks, DR