Truck MPG/towing question.

jstand wrote:

Bobzilla wrote: This truck has a 1-piece driveshaft, which means it has a 96mph top speed limiter to keep it from exploding. That means, a brick on the throttle, this thing hits the top speed at 2300rpms. 96mph at 2300 rpms.

But, does that also mean if you re-gear to a lower gear ratio ( higher numerically) that your max safe speed also decreases ( or should decrease) to maintain the same limit for driveshaft RPM?

Assuming the max safe speed is actually limited by driveshaft rpm, yes, it does.

jstand wrote:

Bobzilla wrote: This truck has a 1-piece driveshaft, which means it has a 96mph top speed limiter to keep it from exploding. That means, a brick on the throttle, this thing hits the top speed at 2300rpms. 96mph at 2300 rpms.

But, does that also mean if you re-gear to a lower gear ratio ( higher numerically) that your max safe speed also decreases ( or should decrease) to maintain the same limit for driveshaft RPM?

Yes it does.

There is a "fix" for this driveshaft issue as well. You can buy a really nice Denny's or something similar, OR you can convert to the 2-pc. Still.... No going to be running a 2.5T 20' long behemoth to 100mph.

Not 100 mph, but it does have the potential to drop down enough for concern with a gearing change.

Assuming you start with 3.23:1 the limits is 96 mph, it drops to the following:

3.42:1 and the limit is 91 mph

3.73:1 and the limit drops to 83 mph.

91 is still a bit fast, but 83 is getting into the range where it may be reached under some conditions (not towing).

The 3.42 is what I'm still considering with a posi unit.

So today was 120 miles of towing. Half unloaded the other half with a 3500lb mustang. Unloaded it was happy inOD cruising at 75mph. Loaded, it was happy as all get out in 3rd at 2500 rpms and 70mph. Any faster and the chassis was unhappy over large bumps with the 5300lb load.

Huh. Maybe I should just put an LSD in and leave it the hell alone.

So I finally got around to filling the truck again. averaged 14mpg round trip. 55 miles with empty trailer in OD, 65 miles with 3500lb mustang on the trailer in 3rd. So I was likely getting 15 empty and 13 towing 5300lbs. Not great.... but it could be a helluva lot worse.

So it looks like I'm just going to tow the beast in 3rd and call it a day leaving it alone.

I'd agree with that decision. Considering the mpg you're getting now, you're probably not going to gain anywhere near enough mpg with the trailer back there to offset the re-gearing costs.

Agreed. 13mpg towing 4500 ish lbs is pretty dang impressive.

I wasn't really worried about mileage towing, more about wear and comfort towing the heavier loads that re-gearing would generate. But since this thing appears to be 100% happy at 2500rpms at 70mph with 5k+ behind it who am I to argue?

Still needs a real diff though.

OK, so an update on this here.... Towed the Forte out to Lincoln. stopped at one of the truck scales at the truck stops, truck/trailer/car/junk came in at 11,100lbs. This is the dash on the way home, and is exactly where it seemed happiest:

So, while out there I noticed the rear end whines a bit now. 35-75mph under load it is noisy. Let off the gas, and it goes away. The instant you put it under load the whine is back. Now, the fluid is fresh. I changed it at 30k with Royal Purple 80-90 Synth. Changed it again before the trip at 88k. Trans fluid has been drained and filled every 30k miles.

First question is this: Does this noise mean time for a rearend rebuild? Everything in the diff looked good. Gear surfaces showed no signs of wear, there was no extra metal in there (obviously being a diff there was some metal flake to the fluid) that I could tell. No leaks and the diff was full (checked it there before coming home).

If so... do I still consider bumping the rear ratio? Friend of mine tows with his 5.3, 4L65E and 3.42's ad is about 200 rpms higher at 80 (2200 compared to my 2000 with 3.23). I don't know that 200 rpms at 70 would be all that much help, so would I jump to 3.73's? Or just keep the 3.23's, add an lsd and when towing this much weight just keep it in 3rd like this? It seems to be happy there.

Interesting data I picked up with the scanner.... Load in OD at 70mph at ~level road was 65-70% load, and 40% throttle and 1740rpms. In 3rd, it was showing 2510rpms, but 48-52% and ~30% throttle. so 3rd the engine is spinning faster, but working less.

Get the Royal Purple out of there. The gear oil is NFG. Put in something decent and the noise may go away. Every time I've had to use that stuff in a rearend, gear clatter increased dramatically and the rearend got whinier. Given that it seems to flow like water, this is understandable.

IIRC your truck does call for synthetic, but also IIRC it's supposed to be 85W140. If not, put that in there anyway. GM 1/2 ton rearends are kind of overstressed and anything you can do to make them happier is Good.

It's spec'd for 75W90. I ran the RP for 60k and no noise.

Oh, well in that case, yes it is probably the bearings starting to eat themselves. It's a 10-bolt thing. Before GM did synthetic as factory fill, they would need rebuilding every 80-90k.

The good news is, if you catch it early, you can get away with just replacing the bearings, R&P are fine. Use Timken bearings and all of the old shims in their original locations and you shouldn't need to play musical spacers.

Bad news is, it sounds like you want to replace the gears anyway so this is moot

In reply to ProDarwin: This goes along with what I was taught in the Air Force when we had piston engines and propellers.

High manifold pressure (large throttle opening) and low rpm. This was discovered in WWII on long flights.

This theory was explained to me why Overdrives were popular in the 50's and 60's.

It is specced for 75w90 when towing? A lot of trucks/SUVs call for 75w140 for severe service/towing, and if it doesn't it probably should.

HiTempguy wrote: You are wrong. RPM kills mpg, end of discussion.

Well, looks like I've killed this one off. Considering I achieved nearly identical economy in a higher gear with a heavier load.

Next up..... will a 6.0L in a half ton with the right gears get the same economy? lol

Bumping this thread since I searched for MPGs and 6.0 motors.

I towed a 5,000ish lb open trailer this weekend down to Bowling Green with my 2004 Suburban 2500 6.0 and achieved 9.8 MPG at 75 MPH.

My rig has the deeper gears and the 4L80e allowed for OD towing at right around 2,500 rpm running 75 MPH. 

Penalties include: Suburbans are heavy, trailer is old Uhaul unit, 4 dudes and gear and luggage in the burb.  So while it wasn't great mileage by any means, it is a big old 4x4 beast pulling a decently large load at speed so I figure it's about as good as I'll get.

I wonder what you would've gotten at 55 or 65 mph

In reply to yupididit :

Probably a good bit better.  It just grinds at me to drive that slowly. 

But for the purposes of science I may do the next event in a month at 65 and see.

I know this in an ancient thread, but I'll add my 2 cents for anybody searching in the future.

I have a 2001 1500HD with the 6.0 4L80e combo. 3.73 gears. Compared to the 4.8 and 5.3 engines (I've owned both) the 6.0 is a gas guzzler. Day to day I average around 14-14.5 mpg with a 30/70 mix of city/highway driving with a very light foot. Highest I ever got was 16 mpg in the 32,000 I've got data for. Adding a car and trailer behind it I drop to around 11-12 mpg keeping it around 70 mph. 

I must say the added torque over the smaller engines is nice. At 6,000 ft elevation it still downshifts on the slightest hill with a trailer behind it, but as soon as I drop to sea level it'll pull a tank uphill in OD. 

Man, all these newfangled trucks. I went a different route.

This is the Period Correct Tow Pig (PCTP), also known as Johnny Bravo. Photographed here, in Amarillo, TX:

This vehicle is the answer to "Do I need an $XX,XXX truck to pull my Civic/Miata/Mustang/Buick on an open trailer?"

No. You don't.

This vehicle is a 1990 F250, long bed, standard cab, 2wd with the handling and towing package, which came with 4:10 gears and the E4OD heavy-duty transmission. It has the 300 cubic in inline six.  Fuel injected, it's the pinnacle of development on the Big Six. 235 horsepower, 320-ish torques. I bought it for $2000 with 101K miles on it. About $1500 in maintenance later (all new brakes including calipers, lines, and wheel cylinders, some exhaust work, A/C repair, transmission front main seal and engine rear main while we were in there, modern stereo, and a rear camera system), it was on the road.

We towed that Buick from Louisville, KY, to Las Vegas, NV for the 2018 SEMA show and the Optima Invitational. On top of the car and trailer, which loads out somewhere between 5500 and 5800 pounds, there was at least 800lbs of gear in the bed. Then we towed it all back. It pulls at 70mph comfortably in the flat in OD. Hit a hill and you kick down to third. On a big hill, it'll drag down to 55mph and kick down to second. You will slow no further. Sure, the engine sounds like it's screaming, but it's not. The Big Six is the honey badger of engines. It just doesn't give a f***.

There's a giant hill between Kingman and Flagstaff. 8% grade for something like ten or fifteen miles. We hit the bottom of the hill at 70, getting passed by dudes in duelies doing 80-90 pulling their campers.

Halfway up that hill, we passed them.

Above 6000 feet, you give up on OD, but third works fine.

When I bought the truck, it was consuming about a quart of oil every 1000 miles or so. By the time we got to Amarillo, it had ceased consuming oil and hasn't burned a drop since. And that was 10,000 miles ago, almost all towing around the southeast and midwest. I guess the rings finally wore in. The last Blackstone report on the oil showed metals well below universal averages.

We averaged 13mpg on the Vegas trip. With the dual 17 gallon tanks, we go 350 miles between fuel stops with a 100 mile buffer still in the tank.

Would I tow an enclosed trailer with this rig? No. Everything has a limit. But for pulling an open trailer at ~6000lbs, you don't need as much as many would lead you to believe.

In reply to WillG80 :

This goes back to the same problem I have stated numberous times before, the 6.0 was ONLY offered in 2500 and larger trucks with a 4L80 and 3.73 or taller gears. My original proposal was the 6.0L engine in the lighter truck with the 3.23 or 3.08 gears. A 6.0 at 1800rpms makes significantly more torque than the 4.8 or even the 5.3 and the slower engine speed and load would theoretically improve economy. 

Maybe someday I'll find out. If this 4.8 ever pops I'll snag a 6.0 to replace it. 

Huh. I had no idea they put overdrives behind the 300. Every one I ever saw had a C6.

In reply to bobzilla :

I may have an answer to this sooner rather than later. I'm currently gathering bits and pieces in preparation for an LSx swap into my '95 C1500 extended cab. It's got 3.42 gears and I just picked up a 4L80e last week, which will sit behind either a 5.3 or 6.0 that I still need to source. I'm not seeing much of a price difference between LQ4s and LM7s locally, roughly $800 vs $600 on average, so my inclination is to go with the extra displacement and torque of the 6.0. The downside of course is the 6.0's reputation for thirstiness, however I have yet to find a single roughly relevent data point to which I can compare, and the truck isn't a daily anyway so I'm less concerned about MPG. That being said, I'm not ruling either one out yet if the right deal comes along.