So with one family member looking/finding/buying an old work truck
recently and my boss bouncing ideas off me for a tow rig/modding his
old C1500 I've been giving this alot of thought lately. And note
please this has all been theoretical at this point.
Having owned an 86 Iroc, and an 88 K2500 diesel I just have a hard
time believing that you run into a mileage wall at 20mpg on a gas
motor in a truck. The Camaro had a TPI 305 700r4 and 2:73 gears, and
it would run 80 mph at just over 2000 rpm and get 27-28mpg doing it.
The K2500 was a 6.2 diesel, granny 4spd truck that was ragged out and
as long as I kept it around 60-65 indicated (oversized tires, so it was closer to 70-75) it would
return mid 20's mileage as well.
If one wanted to build a gas engined truck to get mid 20's or better
how would you go about doing it? My thoughts so far: Started out
with a 350ci engine TPI setup from f-body/corvette ran via MS, (stock
sized runners to keep intake velocity up) 700r4 trans and numerically
lower rear gears. I know there is a math equation out there to figure
out the effective gear ratio of a 2:73 ring gear and 245/50-16 tires
(stock on the Iroc, 26inches if I recall correctly) but that effective
gearing would be the target for R&P+tires.
Thinking on it some more I got to looking at diesels, they run lower
rpms, with more torque to get the ability to pull and get mileage.
Hence the TPI setup, 5k rpm should be the highest this thing would
ever see. So a 383ci stroker setup with TPI and cammed for 1500-4500
maybe 5k would be the hot setup right? The 6.2 was 372ci IIRC, my
biggest complaint with that truck was it's lack of overdrive.
Now would you want to run an auto OD trans behind it or a manual
trans? T-5 would have to be world class to handle that kind of torque
with a load behind it or have a sympathetic driver that was easy on
it. Or go nuts and just jump to the T-56 for the double OD. I'm sure
there are truck OD manual transmissions out there, but I don't know
the make/model of them. I know the 6spd's in the f450's that a
previous employer used would probably be a decent setup as well, just
pricey.
What's the hive mind say on it????
The truck tranny would be a NV3500 for overdrive. Someone at work says that the TPI 350 was the best truck engine GM ever made, too bad they didn't put it in a truck.
What hurts truck MPGs is the weight and having aero slightly better than a barn. You can get better mileage with some tricks, ecomodder.com has some good ideas like a sloped bed topper, air dams in front, etc...
Quick search when I got in front of a high speed connection got me this: the 6 speed in the f450 is a zf trans, and not as expensive as I would have thought. Apparently it was used in gm hd trucks as well behind the duramax and big block engines, and it has a LSX bolt pattern for the bell housing.
http://m.performancetrucks.net/forums/showthread.php?t=404105&styleid=4
Which means it should bolt to any small block right?
Mid 20's truck able to tow something of about 4-5k pounds... average or highway?
That answer determines my answer. 
Highway would be fine, as I live in the sticks and most of my driving is at highway speeds anyway.
81cpcamaro wrote:
What hurts truck MPGs is the weight and having aero slightly better than a barn. You can get better mileage with some tricks, ecomodder.com has some good ideas like a sloped bed topper, air dams in front, etc...
I can understand that theory, but that k2500 with a diesel and cast iron 4 speed still turned mid 20's. That can't be all that is there.
Starting with a diesel will put you quite a bit ahead in the mpg game. I mentioned the aero stuff as every bit will help with a gasser, just imagine what a diesel will get with the aero mods.
well, I know for a FACT that you don't run into a "wall" at 20mpg with a truck. I've owned a FEW fullsize trucks that saw 24+ on the highway. If I can still find non-E10 gas I can still get mid 24's out of our 06 CrewCab. Stock canned tune. I would imagine with a good ECU tune where mileage was the focus, a TC solenoid that actually locked up and stayed locked up and a decent trans build it would easily see mid to high 20's at 70mph.
My goal (before I got sidetracked) was 25mpg @ 80mph.
Now... if one were to start with a 2wd truck with the 6.5TD and a 4L80, I could see that thing getting mid to high 20's with the right rear end and some tweaks.
Dad had an 84 'burb with the 6.2L and 700R4. It had been lifted with 1-ton axles under both ends, had 4.10 gears front and rear. Even with the OD of the 700R in it, it still topped out at 80mph. But with all that, it was still a 20mph highway beast. In all honesty... I think it pretty much got 18-20 ALL the time.
Do want to get 25 hi-way while towing or running empty?
Building a 383 would be fun, but not 'cheap'. I don't think you'd get the mileage you want, but yes on bigger torque. I like the TPI idea though. I think 25 mpg could be done.
NV3500 should be a cheap and easy swap, maybe change to a 3.08 gear.
Gearheadotaku wrote:
Do want to get 25 hi-way while towing or running empty?
Building a 383 would be fun, but not 'cheap'. I don't think you'd get the mileage you want, but yes on bigger torque. I like the TPI idea though. I think 25 mpg could be done.
NV3500 should be a cheap and easy swap, maybe change to a 3.08 gear.
Ideally both, reality empty. 
Remember this is honestly a theoretical discussion at this time. maybe more later this year. But just brainstorming at this point. And I agree a diesel starting point would put me well ahead of the game, availabilty around here, even in the sticks is limited to about 1 or 2 stations in a 10 mile radius.
Full size truck is what I'm thinking of. At least extended cab, Small trucks are easy, SWMBO has an S-15 that pulls down 26 around here and closer to 28 on the highway.
Honestly with the right tune, there should be little difference in the 383 and 350 in fuel consumption. I'm more intersted in it's ability to provide twist. You can build a long stroke 5.3 motor but by going with the bigger bores of the 383 I think in theory it would be able to breath better. I like the idea of 3:08 gearing.
So mid 20's highway....Hmmm...
I'll buck the trend with this one, take the later model Dakota's(97-03), Colorado's(04-up), and Ranger's(this can float from 93 or 98-up), cut down the "free flow" frontal area and lower mildly, like a 2/3 or 3/3 in the case of the Ford. Replace amenic powerplant with Hemi/LS/5.0 of your choosing. Bolt on trans appropriate for the torque, proper converter too. Box the rear frame in. Bolt on class 3 hitch and tow 5k easily.
If you have to try a fullsize, same kind of rules apply, lower, 2wd, cut down the visible "free air" area, and motor swap to something more modern. If it is available, 6spds in the trans, although 4 can be made to work too. I know if I wanted to, I could get my wife's Av into the 23-24mpg range without spending a ton of money. All the mods I have thought of is about changing the airflow around and through it.
As much as I like diesels, I can't say 4BT. 800# kills the idea for me. Plus everything else you need for a diesel swap like finding said 4BT for GRM money.
JMO.
Spinout007 wrote:
Small trucks are easy, SWMBO has an S-15 that pulls down 26 around here and closer to 28 on the highway.
Sure, but can it tow 5k#'s??? No. So it fails the tow challenge.
I dunno what kind of mileage it pulled down but I pulled a 7k lb trailer with an 88 2.8l 2wd 5spd S-10. Now I gave myself plenty of time to stop, and kept it under 45mph on backroads. All went well till I had to attempt a bit of panic braking. 
I got it shut down though.
I think your biggest limiting factor is aerodynamics. Trucks are shaped like bricks. While operating at a steady state, say a constant speed and flat ground, the only "force" that your engine has to generate to maintain speed is to counteract friction in the drivetrain and tires and wind resistance. Lowering rolling resistance via "gas saver" tires and higher pressures will affect the former. The only way to affect the latter is aero modifications.
Well, the truck you want is already out there for sale. 2005-06 Silverado/Sierra, 2wd, Crew Cab, 3.23 rear end, 4.8L LR4. stay with the stock 245 width tires, it'll tow 5-6k lbs really well, sit at 80mph on the interstate at 2k rpms and knock down 24mpg.
If that's what you're looking for, the answer was easy.
Seriously? wow, can we do better though? 
Spinout007 wrote:
I dunno what kind of mileage it pulled down but I pulled a 7k lb trailer with an 88 2.8l 2wd 5spd S-10. Now I gave myself plenty of time to stop, and kept it under 45mph on backroads. All went well till I had to attempt a bit of panic braking. I got it shut down though.
If it was like my 96 Ranger pulling my Mustang on a dolly, it didn't break 20. I got 19 on that trip. I can't say for sure on the Colorado, but both the Dakota and Rangers can parts bin themselves bigger brakes in front and upgrade to discs in back.
Bobzilla wrote:
Well, the truck you want is already out there for sale. 2005-06 Silverado/Sierra, 2wd, Crew Cab, 3.23 rear end, 4.8L LR4. stay with the stock 245 width tires, it'll tow 5-6k lbs really well, sit at 80mph on the interstate at 2k rpms and knock down 24mpg.
If that's what you're looking for, the answer was easy.
3.42's and 265/70R17's will easily cut that by 2, if not, 3mpg. 
The Av turns 23-2400 at 70....
yamaha
SuperDork
2/1/13 3:00 p.m.
Wait, I thought the point of a gas truck was to consume multiple dinosaurs per mile
Spinout007 wrote:
Seriously? wow, can we do better though?
Yep... a good tune, headers and smaller section width tires plus a 3-4" drop front/rear would probably get that close to 30mpg @ 70mph.
yamaha wrote:
Wait, I thought the point of a gas truck was to consume multiple dinosaurs per mile
No, the point of my wife's truck is to sound glorious and look pretty while cruising the highway effortlessly.
Bobzilla wrote:
yamaha wrote:
Wait, I thought the point of a gas truck was to consume multiple dinosaurs per mile
No, the point of my wife's truck is to sound glorious and look pretty while cruising the highway effortlessly.
+1. Why else would she buy it.
Get a new truck. 
Yesterday I made a 150 mile run across DFW on a mix of highway & city streets and maintained 24.2 mpg in my full sized 2011, V8, F150. This was based upon the electronic mpg gauge in the truck.
In 2012 they got cylinder deactivation which would increase the gas mileage.
Regardless of the driving style or mix of city streets vs. highway I almost can't fall below 20 mpg.
And yes, it will tow.
Ranger50 wrote:
Bobzilla wrote:
yamaha wrote:
Wait, I thought the point of a gas truck was to consume multiple dinosaurs per mile
No, the point of my wife's truck is to sound glorious and look pretty while cruising the highway effortlessly.
+1. Why else would she buy it.
Because I told her to! It tows very nicely for my needs and is a great cross country beast. 3 summers ago we spent 7 days going 5600 miles, spending 4 days above 6k feet. It was AWESOME!
