Thoughts on gear ratio

tmsbandman · 07-02-2013, 02:59 PM

Last week I towed our TT (1992 300BH, 30 ft) from Atlanta to Destin with my 2004 F150 SCrew with tow package. It seemed to do fine, but towing is a new thing to me. I became concerned when i realized that most trucks at the campground were 3/4 or 1 ton, and that the few guys with 1/2 tons were mostly towing smaller TTs.

So-pertinent info: TT is about 6,000 lbs modestly loaded. For the sake of discussion lets say TT weight is 7000. TV weight with the whole fam + gas would be about 6500 (was only 5700 on this trip). I have weighed on CAT scale and am within GVWR and GAWR and TT is within it's GVWR. My concern is that I don't want to kill my truck.

The gears are 3.55. Ford says I can tow 8500 lbs, and my GCWR is something like 14500. The tow rating on the same model with 3.73 gears is 1000 lbs higher. Would I make things easier on the truck if I change to 3.73 gears? To complicate matters- I have 285/60/20 tires. Changing to 3.73 might just get me back to original numbers. Thoughts? Anybody on her done similar? Unfortunately, an F250 is not in my future.

DocBrown · 07-02-2013, 03:25 PM

The short answer to this is, "it depends". Do you plan on long trips and towing through the Rocky Mountains on a regular basis? Or are your trips going to be more like the shorter one you just took (I don't know how the grades are in that part of GA and FL)? If its the later, your probably just fine. If you want to do the longer trips and 7% grades, you may want a "bigger" TV. We towed our old 3500# GVRW hybrid with our Trailblazer to Yellowstone and back. It performed beautifully. About 50 MPH up the grades in 2nd gear at 3500 RPM. The truck has a 4900# towing capacity and a 10000# GCVWR. We had a lot of margin with that trailer. Now we have a 4750# GVWR trailer. It's pushing the capacity of the Trailblazer but in WI, its tows just fine. But no way would I take this combo through the mountains. No worries, we decided a couple of years ago that we weren't towing long distance anymore. Why get a new truck? I have no payment!

You are well within your tow rating and it sounds like you have a handle on the numbers game. You say it tows fine also, so what's the worry? Don't let a few 3/4 and 1 ton PUs in the campgrounds convince you that you don't have enough tow vehicle. Keep in mind that many of these folks are full timers or traveling long distances more than the rest of us. Also, some on RV.net will make very convincing arguments that you shouldn't be towing a pop up camper with anything smaller than a 3/4 ton truck. Changing gears might help some, but its expensive and may not gain you all that much. My Trailblazer "only" has 3:42 gears. I've been told that I can't tow anything but a pop up with it. Show how much some people really know.

Another thing to keep in mind is that some people need to tow their rig at 75 mph and want as much power as they can get to do it. Others are fine putzing along at 55. Most of us are in between there. I run at a bit over 60. Its easy on the truck, saves a bit of gas, and gives me a wide margin to react to things going on in front of me.

troutslayer · 07-02-2013, 06:49 PM

I didn't see anything about a tranny cooler (speed reader

). If you don't have one, that may be a good idea.

tmsbandman · 07-02-2013, 08:04 PM

I do, and I monitored the trans fluid temp along the drive. I used OD some of the time, but if the trans fluid started approaching 200 I took OD off and it cooled back to 185-190. I found an "effective gear ratio" calculator online and plugged in my tire diameter (33.5), the stock diameter (30.1) and my 3.55 gear ratio. It seems that my effective gear ratio with the 285/60/20s is more like 3.19. I like the 20 inch rims, but it sure does screw up gearing. If I were to make a change, think I'd go with the 4.1 gears. That would give me a 3.68 ratio with the oversized wheels/tires. Probably won't do anything, though, unless we decide we want to go to more mountainous areas. The grades are not very steep in our area. Can't say I wouldn't enjoy the extra pep of lower gears, though :-)

33 RLDS · 07-03-2013, 07:16 AM

My Avalanche with 4.10 gears is rated for 12,000 lbs and with 3.73 gears only 10,000 lbs. I think it helps and the small decrease in MPG is worth it to me..

DocBrown · 07-03-2013, 07:49 AM

Trans fluid only approaching 200? That's not even taxing it at all. You can get the trans up to 220 for short periods without damaging it. Its the prolonged periods of high temps that cause the issues. Also, never tow in OD. The torque convertor will never stay locked and just generate a lot of heat. You might look into some transmission upgrades, a shift kit at a minimum. I don't know what else is upgradeable on a Ford trans. I haven't installed yet, but I have a shift kit, new accumulator pistons, and a Corvette servo. Everything I've read says these upgrade really tighten up the shifting and make it much better for towing. All the parts were only about $100.

tmsbandman · 07-03-2013, 11:49 AM

Thanks for the info. I was not sure what normal operating range is. So-I should never tow in OD, even if it's flat and will stay in 4th?

I've been looking into changing from my 3.55 to 4.10. It's (barely) within my DIY skills, but I'd have to take the pinion and bearing to a machine shop. My only concern is getting it shimmed correctly. It's a heck of a lot of labor to have to tear into a 2nd time. But that's a discussion for the F150 forum :-) Sounds like my truck would be better off with a 4.10 rear end. Looks like it would tow better, have more "get up and go" AND get better city MPG. Obviously the hwy mileage would suffer, but most of my driving is around town. But...if the rule is not to tow with OD at all, the 4.10 would put my cruising RPM around 2500@65MPH. It's currently about 1800 in OD, and about 2000 without.

DocBrown · 07-03-2013, 12:11 PM

Your experience might differ, but I've towed with several different trucks and I've never been able to reliably keep it in OD for any length of time. Every travel trailer and truck forum I've been on no one will say OD is good to tow with. My dad toasted a transmission in a GM full size van because he towed in OD. The few dollars you save in gas isn't worth it.

I wish you the best on that 4:10 conversion.

tmsbandman · 07-03-2013, 05:15 PM

Yeah. Not happening in the forseeable future, anyway. I'm not sure I have the guts to tackle it, and it's an 8 labor hr job I can't afford. Thanks for the advice. Might not be worth it if I can't use OD. Engine would tach pretty high in 3rd with 4.10.

LiftedAWDAstro · 07-04-2013, 10:27 AM

Is your truck 4x4? If so you have to do the front diff too. Also, you'll want to re-program the ECM to compensate. If you are happy with the combo and it pulls fine, I would not worry about it. Maybe start saving a few $$ a week until you have enough to have a shop do the gear swap. I agree with going to 4.10's at a minimum. My Tundra comes with 4.30's and 32" tall tires and can run in 5th gear which is the 1st OD gear. If you get the right gears you can run OD as long as the torque converter locks in and the RPM's are at least 2k you will pump enough tranny fluid and keep it cool. You just don't want the TC unlocking and locking all the time.

tmsbandman · 07-04-2013, 03:02 PM

Thanks. 4x2, which makes life easier. I have a programmer on my truck that I can use to tweak the lock point if necessary. Watched the video again today of how to change it out. Feeling more confident about a DIY project. Just need to leave PLENTY of time. I'd just be really ticked if I changed it and got a whine :-(

Flyrotor · 07-04-2013, 06:46 PM

Personally I would not mess with changing out gears, rather put that money in savings for future down payment on larger truck built to handle the load properly

tmsbandman · 07-07-2013, 10:59 PM

Well, after talking to a couple of mechanic friends I've decided not to pursue a gear swap. It seems that the benefit is not great enough to warrant the expense. No need to dig into a perfectly good 3.55 diff:-)

I am thinking about something like the Firestone Ride Rite air bags to help out my WDH. It would be nice to not have as much tension on the bars. Experiences with the Ride Rite or similar?

kris_12302 · 07-12-2013, 02:02 PM

I had a 07 silverado 1500 with a 5.3. The truck came with 3.73 gears and I installed 4.10 and I went from 13.5 average mpg to 15mpg! and it def towed better!! I truck seemed to lag through each shift with the 3.73s with the 4.10s it felt better and went through each gear much better and the motor was not lugging as much if that makes sense, plus I could tow 1000lbs more with the 4.10s

tmsbandman · 07-12-2013, 11:30 PM

Good to hear. I've pretty much talked myself out of it for the time being, but maybe sometime down the road. Did you do it yourself, or pay a shop? How much did it wind up costing you?

kris_12302 · 07-13-2013, 08:07 AM

I paid a shop to do it, my truck was a 4x4 and it was around 1,300 for both diffs, all new bearings and synthetic fluid, I also bought a hypertech programmer to fix the speedometer plus I wanted one anyway. I towed my new whitehawk 27dsrl with it twice and it towed pretty well but the tranny temps were between 170 and 200 so I ordered a secondary tranny cooler to go with the factory one, I took my truck in to the dealer for service and left with a 2011 2500hd with a duramax lol ( wife was thrilled) but once I towed with the new truck I was amazed how well the 2500 handled the load and that diesel is amazing! I don't even know the camper is behind me anymore

G Bush · 07-13-2013, 12:17 PM

I don't know what axle is in your truck but if you do it yourself the use of setup bearings will really help save you a lot of effort.

Buy an extra set of bearings and gently grind the ID until they press on by hand. With the right bit it takes about 5 minutes per bearing.

Do your shim setup with these then press on the unaltered bearings when you have a good pattern. Avoids risking damage to a new bearing. Bearings are very precise one to the next so the pattern won't change.

Paulx213 · 07-13-2013, 01:11 PM

Quote:

Originally Posted by tmsbandman

Yeah. Not happening in the forseeable future, anyway. I'm not sure I have the guts to tackle it, and it's an 8 labor hr job I can't afford. Thanks for the advice. Might not be worth it if I can't use OD. Engine would tach pretty high in 3rd with 4.10.

The reason you shouldn't use overdrive has nothing to do with overdrive itself. The problem is it makes the gearing to tall. If you installed 4.1 gears that would shorten the gearing to the point where you could use all your gears.... and more gears is better.

I have an Expedition with a 5.4 and 3.73 gears. At 60 mph it's turning over around 1800 rpm which is great when running empty but too low when towing my trailer. If I leave the OD off it's around 2500 which is great for torque and tows with ease. Downside is it's now just a 3 speed auto. 4.1 gears would bump the rpm to around 2100 at 60 in OD which means I could use it on the hwy.

My truck is a 4wd though which doubles the cost of changing gears and my trailer is lighter so for me it's not worth the effort. In your case it might be.

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