Start a thread in the tech section, someone will have some suggestions for you.

 

it sounds like you're experiencing torque converter drain back.. and it will create precisely what you're describing.. stall conditions, and slippage..

the easiest fix, if that is the case, and something you will benefit from even if it isn't: install a remote trans oil filter.. intercept your return to transmission line, install a filter system (dorale makes a good one for $40), and route it to return to your transmission.. it serves as an anti-drainback device as well as filtering your oil...

try this to confirm your issue, though: start your engine, put it in something other than park for about fifteen seconds.. preferably neutral or drive.. THEN, drive away.. the pump doesn't engage on these 'missions until it is in gear, when it pumps, the first order of business is filling the torque converter..

 

I am trying to understand why changing rims from 20" to 17" with the same size tires would increase fuel mileage. Isn't the circumference of the tires the same? The wheel rolls the same number of times in a given distance. Is it due to the weight of the 20" rims?

 

yup.. weight...

rotational weight is roughly x4... 100# wheel/tires feel like 400# to the truck.. it takes extra effort to get them rolling, and keep them rolling..

I have recent first hand experience to share with this...

I just dropped 20" cepek FCii 35x12.5 wrapped around motometal 951's.. they were 175# each.. running through 4.56:1 gears, I sat a hair below 2k RPM at 60mph.. this is a mechanical thing that can't be altered.. you can take someones gear ratios, and their engine speed (rpm), and calculate how fast they are moving, or any variation in between using two of the three variables (engine speed/gear ratios/forward speed).. it's a mechanical function plain and simple and a precise math.. except:

engine load as measured most easily by vacuum signal from the intake manifold..

I loaded stage 8 spoke five 17x9 rims, and wrapped them with 285/75/17 dura grapplers, which is a highway tire.. lessor rolling resistance, and lessor weight..

running the cepeksx20's, and at 60mph/just shy of 2kRPM's, and on flat surface on the same stretch of highway, I would pull about 14 inches of vacuum signal..

running the durasx17, and at 60mph/just shy of 2kRPM's, and on the SAME flat surface in similar environmental conditions, I pull 16~18 inches of vacuum signal.. the engine is working a lot less to maintain that speed..

it's due to lessor weight, and lessor rolling resistance..

it plays a huge role.. or, roll.. huh..

edited to add: the duragrappler 285/75/17's are the precise height of the advertised 35"x12.5" Cepek FCII Radials, and around 34.25"... given the FC's were worn the eff out, and the dura's have new treads..

 

Leave it in the driveway and ride a bike. I love Dodge Trucks(screw FIAT and their Ram brand), but their motto should be, "I am therefore I drink gas". Hell, I got 20-22 mpg in mostly highway driving in my 78 D200 318 with a 3 spd Torqueflight and Dana 90 rear end. Since 94(2nd Gen) Dodge threw away the concept of mileage in favor of durability and power. You wanna save gas in a Dodge? Leave it in the driveway.

 

there's also the width increasing the wind resistance... most 20's will be a little wider in section than what most run on 17's.

 

which is amazing thing for those of us around since at least the sixties, and watching it happen, no?

I had a '65 buick wildcat... 455, 400turbo trans.. quadrajet.. monster of a CBB engine, and everything on it for 'getting there' in a hurry.. on the highway, I managed almost 25mpg consistently..

the tree huggers involving themselves with paper figured concepts has cost economy hugely with all their mathematical figurin' and theories.. in practice, there are two things on an engine that cost economy for every item that increases it, it seems.. and they are generally there in the name of 'environment'..

not to say they are all bad.. unleaded gas has made a huge positive impact.. electronically monitored fuel trim and ignition has too.. cats? cost.. designing a big displaced engine and then limiting its ability to produce power due to breathing restrictions? costs.. electronically controlling acceleration through means of electronic delivery of fuel or torque management on transmissions? costs..

what tickles me the most: boosted engines as a means to save fuel.. ???

every 14.7 inches of vacuum represents an atmosphere.. so, if you are running a 300cid engine naturally aspirated, and it is volumetricaly perfect, you are sucking 300ci of air through each complete cycle.. if you boost it w/ 14.7" of boost, you are running the equiv of 600cid engine..

which burns more gas? 600cid of engine naturally aspirated or 300x2 boosted? if it is running at a stoich ratio, they burn the dang same!!!!! too dang funny to me..

before you guys descend upon me, yes, I understand a 150ci displaced engine under full boost would only burn the same fuel as a 300cid engine while under 14.7" of boost, and would run half the fuel (in theory) as the same 300cid engine if boost wasn't engaged, which lends credit to having that option only when needed, which is the point.. but it still doesn't explain why makers/legislators won't allow 100% volumetric efficiency from engines due to environmental concerns.. crazy..

 

re-reading what I wrote.. something occurred to me worth sharing..

I have a honda st1300 sport cruiser bike.. obviously, it's displaced around 1300cc.. it weighs just shy of 700#... I get 45MPG on it average.. maybe 38~40MPG when I'm running it hard, and have never seen better than 48mpg's in any circumstance though it claims 55~60mpg's are possible..

that dang wildcat was a boat.. it must have weighed 5k#, and ran equiv of about 6 of those 1300cc engines.. which means it got hella better gas mileage than even the honda motorcycle, all things considered equal.. all things considered equal weight wise, it got about four times better mileage.. go figure.. shameful.

 

My brother's 440 '67 Polara convertible got 18-20 consistently. By 1973, a Monaco coupe with a 400 and half the horsepower got 10-12 thanks to decompression rules and smog crap. In the last 2 years, though, they have started to finally(after 40 years) get the efficiency with the power again. How many billions of gallons of gas have been wasted in the interim?

 

I totally agree about the breathing restrictions on engines, makes no flippin sense to me.

Perfect stoichiometric ratios are only used during cruising and idle. You can get better mileage with a turbo as long as you stay out of the skinny pedel. When you are under acceleration and load you are running rich, and with a turbo you can introduce more air which means you can use even more fuel which can bring you boost in excess of 20psi, depending on the engine. But at cruising speeds your only running ~4-7psi which only makes the engine more efficiant. You are using the energy lost through the exhaust to pump more air and create a positive pressure on the intake stroke of the combustion cycle, sence you have a positive pressure pushing down on your piston your engine no longer has to "suck" the air in which frees up a few ponies and lbs of torque. Its all about effiency and using as much energy from every drop of fuel that you can.