Truck MPG/towing question.

OK, this has come about from the current rig. Love the truck but with age (we've had it almost 10 years now, since new) and changes in our usage have brought up some weaknesses. The main one is overall towing power.

Now, the truck has the power to pull 6k all day. Just be patient. The 4.8L is a great little motor and it LOVES to sing. The problem is, it makes no power down low. No torque that is. Here in lies the issue. We bought the truck for the occasional hauling/towing junk locally which it is great for, and for long distance cruising unloaded to Tucson to visit her folks with the pooches in tow. That is does fantastic as well. Lots of room, long wheelbase and smooth ride ad low 20's on the highway all day long. With the 3.23 gear, it's turning 1800rpm at 75mph.

Throw a trailer onto it, and that 1800rpm in OD isn't enough. Drop down to 3rd and you're now at 2500rpm at 70mph and it has plenty of power there. So this leads into my question:

Would a small engine turning more rpms be better for mpg than a larger engine turning lower rpms? The exact scenario I am considering is bumping the rear gears to either a 3.42 (1900rpm@70) or 3.73 (tick over 2000@70) from the 3.23 for better power delivery of the 4.8. Likely get it tuned as well to maximize the gain along with it's Magnaflow catback and K&N FIPK.

The other option is leaving the rear end alone (other than a posi) and swapping the shortblock for the 6.0L. Easy enough swap. Makes almost 100ft/lbs more torque in the 1500-2500rpm range. BUT, unloaded that is more engine to feed, so the lower rpms would help that as well.

So... which option would likely be the better move? This is still a "family" vacation hauler and "racecar" hauler and do it all platform. Gas isn't going to stay cheap forever so looking for the best long term solution to our needs. The truck itself is literally perfect for us. Plus, bein a GM, it's like legos for grownups. Whatever decision I come to, this coming winter will likely be teardown time so I have plenty of time to over think this.

Gratuitous pic for the last two people that haven't seen our truck:

That is basically the truck we have at work. It's an extended cab, not a crew cab, but the ~500lb of permanenently mounted equipment in the bed makes up for that.

IMO the little 4.8 and the gearing are a perfect match, if you drive with the mentality that heavy loading in OD is a sin against the 4L60 transmission. Any more low end is wasted because you really shouldn't be using OD under high power towing situation and at lower speeds it is still plenty of engine to do the job.

But that is, again, just my opinion. If towing with it, I'll only use OD if going downhill or on a flat where there isn't much throttle needed. As soon as I need to load the engine down, out of overdrive it goes and then I lean on it, since I don't want to be the one to break the truck.

Swapping to a 4L80 with the 6-liter would let you tow in OD, of course, but I'm not certain that fuel economy would improve since power required is power required and the engines' efficiency curves are pretty broad. The added torque of the 6 liter should be canceled out by the taller 1-2 gears of the 4L80 (2.48/1.48 like a TH400, versus 3.0/1.?) so you probably won't notice much of a difference at lower speeds.

Couldn't you add a medium size single turbo? It's only been done a billion times to the 4.8. It could increase power where you need it if sized right and keep the general efficiency of the engine if you didn't drive like an a-hole. This is the general idea behind the Ecoboost Fords, yes?
Should be cheaper than a 6.0 and yield better mileage. At that point I think the 4L60 is your biggest issue.

I went from a 4.6 F150 to a 6.8 F350. My mileage while towing our gooseneck horse trailer hasn't changed. My old F150 was really working to pull 6500lbs. The F350 does it without breaking a sweat. Unladen the F350 still gets lousy mileage and rides like a coal cart. The 6.0 is a really nice engine and should give you the best of both worlds paired to a half ton chassis.

In reply to mazdeuce:

Ask Ben about his suggestions to me for turbos! If it don't come with them, don't add them and expect stock reliability. That is the key here. Usable power with maximum reliability. I know I know. The 4L65 is the worst trans know to man blah blah blah. I'm at 90k now with pan drops and fluid filter change every 30k. External cooler was installed at time of purchase. I can rely on this stock for another 50k before rebuild. And when that day comes we are spending the money for a Monster. Again this truck is with us until it rusts in two

It is not the worst trans known to man, it's not a Ford

Not saying in the least that it is a bad trans. It's merely a relatively light duty trans that had certain engineering compromises done 30 years ago when it was designed to squeak an overdrive in to the TH350 for 200hp passenger cars. GM has done an admirable job of improving and updating it as more demands were placed on it, but it does need a little bit of coddling. Take care of it and it will take care of you, so to speak.

With that in mind, if you do take your truck somewhere for a tune, tell the guy doing the tune to NOT disable any torque management. Some people do this as a first step for any tune, and sometimes as an ONLY step. GM put that there to be kind to the drivetrain. Yes, it makes the engine feel a little sluggish sometimes, that is the computer's doofus-protection algorithm keeping the drivetrain happy.

I hear you on the concept of keeping it until it can no longer be salvaged.

Everytime I mention that we're keeping this long long looooooong term I get this befuddled look from people. What they don't understand is they LITERALLY don't make trucks like this. And by that I mean this size, these options and a rear seat that folds completely flat while still allowing you to store all your straps, tie downs, bungees and ball/reciever under the folded flat seat.

Can we buy a new truck? Absolutely. Is there a new truck we WOULD buy? Ah hell nah.

i have driven a similarly equipped 5.3 extended cab with 3.42 gears. it was sluggish off the line and the rpm were kinda low for highway OD towing.

my avalanche on the other hand, has 4.10 gears. it makes all the difference in the world. i still fear towing heavy stuff in OD and don't do it except downhill or on straight grade where very little throttle is needed to keep speed, but with my 3k work trailer i put it in OD and forget it.

i understand the 5.3 has a slightly lower in the range torque curve than the 4.8 due to the stroke length difference, but the results would be similar.

unloaded i get 17.5mpg. 3k cargo trailer(brick through the wind) i get 14. 6K bigger dual axle racecar trailer i get 11.5 in D mostly.

I was always told not to tow in OD.. so take that for what it is worth

That's a helluva mpg hit. Worst towing to date was 12.5 with the 1800lb trailer with a stock C4 coming up from Northern Arkansas. Best towing was first my Nats averaged 14 round trip with the same trailer and a 2000lb Civic. Ethanol percentages went up for my second year and we averaged 13. Unloaded trailer is 15-16 and current unloaded with 4 large adults(~1100 lbs in people alone) and luggage was 19.7 round trip to Florida... then again our speeds may have been a "little" higher than the 70mph we had been running. I know the wife spent almost 3 hours in the 80-85mph range.

If that were my truck and I were hellbent on keeping it, the first thing I would look into is reprogramming the computer. Places like Summit,etc. sell units where you can pick from different power levels. http://www.summitracing.com/parts/bly-40417

Would a twin screw supercharger be an option? Increases torque in the low end, keeps your gear ratios as-is to avoid the MPG hit when not towing. Might be a bit of a penalty in mpg when unloaded, but probably not much different than the 6.0 would be. Sounds cool too

Eh, just allow more time to get where you are going.

My KJ pulled 4K in overdrive with out a problem. 12/15 mpg. I always towed at the speed limit.

swap in 358 or 373 and leave the rest be. Randy's ring an pinion should have a kit for you or find one already set up. Randy's

If you could find Brake Specific Fuel Consumption data for your motor and any motor you are considering you could determine what your impact on highway MPG would be for different final drive/gear combos. HP to move the truck at a speed is pretty constant so it would just be comparing the 6.0l @ 1800 rpm making 150hp (for example) to the 4.8l @ 2500rpm. If your wanting to program and unloaded highway MPG is a concern you could always run different programs with a leaner one at highway sweet spot in OD and then run a more rich tune when you are towing.

In most cases, even if you had the torque to pull in OD at 1800-ish rpm, you wouldn't really be saving much gas.

In the Jeep, it'll pull 3k lbs on flat ground and mild hills no problem in 4th (1800 rpm at 60), but it's in power enrichment almost constantly and the instant mpg readout shows about the same as it does if you drop to 3rd (2600 rpm at 60). And you've got a lot more power available before it needs to drop a gear once you're in 3rd, so it's much more pleasant to drive.

In your case, the gearing change or the bigger motor will hurt your un-loaded mpg more than it helps you when you're towing, provided you're not short on flat-out power with the 4.8.

I am still not certain we are optimal gearing. It does downshift for me when unloaded and I've thought about the 3.42 with an eaton unit in there for a couple of years. That would be he smallest bump in rpm and I think would work the engine less overall. An extra 180rpm doesn't sound like much but it does move the torque curve a bit.

My 5.4 liter F250SD with a short bed and crew cab would tow my 24 foot enclosed trailer with a Miata and too much crap inside all day long at 9 mpg. I could do 80 with it, and it was a solid as a rock. The problem I encountered was unloaded without the trailer it got 9 mpg. Nothing I did would convince it that it no longer was towing a 6000 lb parachute.

I think with the 3.42 would be a good idea, like your saying not enough rpm bump to effect unloaded mpg enough (if any) and should help towing and normal around town driving tremendously. But I have to be another parrot here, stop towing in od. EXPECIALLY with those trans...

car39 wrote: My 5.4 liter F250SD with a short bed and crew cab would tow my 24 foot enclosed trailer with a Miata and too much crap inside all day long at 9 mpg. I could do 80 with it, and it was a solid as a rock. The problem I encountered was unloaded without the trailer it got 9 mpg. Nothing I did would convince it that it no longer was towing a 6000 lb parachute.

sounds like the one Isuzu truck I used to drive. Big Turbo Straight Six, six speed, 26 foot long box.. got 11mpg fully loaded.. also got 11mpg fully empty

chiodos wrote: I think with the 3.42 would be a good idea, like your saying not enough rpm bump to effect unloaded mpg enough (if any) and should help towing and normal around town driving tremendously. But I have to be another parrot here, stop towing in od. EXPECIALLY with those trans...

Y'all can say whaty\ you want. it's towed to Lincol twice, Chicago twice, Arkansas, and the last big trip to NAshville in OD. HAndled it like a champ. Then again, it gets regular fluid changes and has an external trans cooler as well as the factory in-radiator one.

It's to my understanding that the 6.0, or at least the older ones, is a proper work truck engine and consequently sucks down fuel way faster than a 4.8/5.3.

nocones wrote: If you could find Brake Specific Fuel Consumption data for your motor and any motor you are considering you could determine what your impact on highway MPG would be for different final drive/gear combos. HP to move the truck at a speed is pretty constant so it would just be comparing the 6.0l @ 1800 rpm making 150hp (for example) to the 4.8l @ 2500rpm.

This is the correct answer, and I'll hazard a guess GM picked 3.23 because it fit the BSFC map best with an unloaded truck in OD doing 70mph or so, that's why it gets low 20s unloaded.

I'd just leave it in 3rd when hauling (which probably isn't far off from ideal if it feels happy there). Perhaps do some A/B mpg testing on the next long towing trip, 3rd vs 4th.

Tow in OD, those that say don't are stuck in the 80's.

Get a blackbear custom tune http://www.blackbearperformance.com/ . Bar none, this will make the biggest difference out of anything suggested. They won't remove all torque management, but they will remove some as it is overkill.

Do the rear end last. My 5.3 with 4.10's and (265/70R17) tires would tow 6k lbs of car and trailer on flat/slightly hilly terrain without unlocking in OD at 65mph. I managed a one-off best of 15usmpg towing 400kms with cruise on before I sold it.

HiTempguy wrote: Tow in OD, those that say don't are stuck in the 80's.

A 4.8 in a 6000lb truck towing a trailer at sub-2000rpm is going to kick down on the slightest grade anyway, may as well shift for yourself instead of having the trans do it under load.

I think I returned 17mpg the last time I borrowed the truck, which hauled a too-large trailer with a 3000lb VW on top, driving mostly 70-80mph and in 3rd gear except for going downhill. The time I borrowed the truck before that was 16mpg, with no trailer and going through relatively flat Michigan instead of Ohio. That was also before we replaced the catalysts and the spark plugs.