MPG opinion on custom 1988 3/4 ton suburban "Cliffy"

[MuddiGGEr25]

Ok I am thinking something is off, we went on a 200ish mile round trip today to sell off Cliffy's old axles and tires. Buyer backed out on the tires after I was about halfway there so if anyone is looking for a set of 30X9.50R15 studable M/T's for 6 lug let me know  :rotfl: 

Anyway to my point, i went 70-80MPH on the way there, minimal wind, weighing around 7,200lbs (1/4 tank; front and rear GM10bolt axles, and spring packs, 6 tires in the back (1 old spare 1 current), me and my fiance, plus easily 250lbs of tools, salt, pipe, etc.) after I calculated my MPG from the fuel used, we got a wholloping 6.9MPG on the first 100mile stretch, but total idle was about 1 hour including pit stop, not sure how much I used @ idle.

On the return trip we were less the 2 axles 1 tire, and all springs. we added possibly 50lbs in merchandise before we left (we dont get to the larger cities too often and this is why)  However we did have a significant cross/head wind on the return trip the entire way, i would guess around 20MPH sustained, as it would rock cliffy whenever we pulled over.



I kept my foot completely out of it on the way back, EXCEPT for about 10 rolling resistance tests from 65---->55mph, but i used the down hills for acceleration.
I managed a  :willynilly: 10.8MPG, not an exact calculation but i am confident +/- .5mpg due to gauge error. but i know good and dang well we never got over 12mpg on the return trip.


here's cliffy's important specs so far:

1988 V10 suburban chassis OEM equip was 6.2L N/A / 700R4

Now he has a rebuilt 5.7L TBI & TH350 both with 39K.
-6000RPM Redline
-2400RPM at 55MPH
-3300RPM @ 75MPH

-3700RPM peak TQ, I figured this out, when towing an RV on a deckover trailer weighing over 10,000lbs up a very very steep hill, well over a mile long, 3rd gear just couldn't keep me at 55MPH so I dropped to 2nd, throttle maybe 40-45% and although I couldn't go over 55MPH/3700RPM, there was more than enough TQ there to maintain it with ease. And cliffy was over 8,000lbs with full tank, almost every single tool I own and air compressor too.

He runs 200F period, towing, not towing, a little lower if hes idling in a drive thru, but even GCVW 18,000+ for 280 miles, he didnt flinch

Replaced both axles & springs a few weeks ago with 8 lug from a 1984 K20 with the HD option (C6P i think)


150A alternator upgrade 1 wire, was used but i know it keeps up with 6 x 55W bulbs plus running lights, 200W amp and 200W radio, and aux battery.

Tires are now 235/85R16 LT LRE 10ply 31.7" tall I think.

My speedo IS faster approx 10mph vs actual at any given speed, but i used my gps to check my real speed periodically.

I dont have the forum in front of me to convert the test data, but it was a consistent 9 seconds almost every single time from 65mph-55mph.







I am wondering what some of you guys with similar trucks/suburbans are managing.


I will add a pic tomorrow once the sun is back out, the springs in the front are the 3 pack for the HD 3/4 option with the 6.2L N/A and i know my 5.7L is much lighter, so my front sits noticeably higher than the rear even with both of us up front.


The rear springs are NOT sagging in any way. I intend to get a 2" or 4" block kit for the rear in the next few weeks to get the rear up above the front a little.

Tires were at 80% of max pressure (the ride is stiff enough without having rock hard tires too  :rotfl: )



I used to get 8MPG with the old 3.42 effective ratio I had with the 30's, but he didnt care if i was going 55 or 80, it never really varied.



My main question I am trying to figure out is, should i be able to expect 12-16mpg once the back is up a little compared to the front? I hear stories of many getting 15-17mpg on a regular basis, stock, going 55mph.


Also it may be a related issue, if I am at a dead stop, i.e. staging line, red light, stop sign, and racing someone, I get much much better response and times if i go about 1/2-3/4 throttle, but if I stomp it right to WOT, it seems so sluggish. and brake stall can only get me up to 1500RPM, but I cannot break my tires loose, even in 2WD.  Flash stall is close to that also.  Knock counts increase rapidly every time I rev it. starter and fuel pump have been changed, but haven't gotten pressure reading on used pump yet.


For reference, we owned a 1977 G30 RV loaded maybe 9,000lbs, I could steadily get 15-16mph with that, and even 14-15 fully loaded AND pulling a 25ft camper about 6,500lbs. going 55MPH, 6.6L sbc, TH400, 4.10 FF 14Bolt, 29.5" tires.


I was also thinking about 35" off road tires might help it would bring my effective ratio to 3.71, and i can switch back to the 235's for heavy hauling and regain 4.10.


If I forgot anything that is important let me know.

[K20Detroit]

On my '79 4x4 3/4 ton with a carbed 350, either th350 or th400, and 235/85/16 tires, I doubt I saw over 7-8MPG.
The engine was rebuilt, carb was kept in tune, I never went much over 60.
Axle, I believe was 4.10.
My truck was level.

Leveling yours may help. Adding a deflector, rear and front, may help.
My solution to poor fuel mileage was buying a 6.2 diesel, 19-20MPG all day.

[MuddiGGEr25]

After reading hundreds of forum pages last night about the ecm, BLM INT, closed loop, open loop parameters. Some 454 pcm were programmed to never enter closed loop etc. So i reviewed my data log video fron about.a month ago and CTS said 208 nut the pcm has. Never entered closed loop. My O2 readings never exceed 531mv. Never. So i am suspecting its either faulty or too down stream. And it wont let the pcm enter closed loop at which point it starts adjusting A/F ratios on the fly. So i think my high knock counts are real and i am on fact running lean. Since i can explain away all other symptoms if this is true.

[crx88ia]

Is the speedo right in the G30? How did you know the milage for the mpg?

My point is if your actual speed is wrong your mpg is going to be wrong since you don't know exactly how far you went.

[MuddiGGEr25]

Yes the speedo gear was compensated for in the g30 when the motorhome was assembled in 1978. The speedo was always correct and perfectly matched the gps readings. I am not usong cliffy's speedo sonce i know it is wrong. I used gps for this 200+mile trip and the 500+ miles towing the RV and gallons are accurate to within 1/2 gallon

[fitz]

If you want decent mpg's, put a 6.2 back in it.

[MuddiGGEr25]

If you want decent mpg's, put a 6.2 back in it.

but then I cant play in the mud or tow decently  

like i said up a couple posts, I am 99% sure my mpg is going to drastically improve once I get the o2 sensor up and running, I looked today and the wires are ripped out of it, and the only connector that can reach close enough is disconnected. I am not sure why the PCM is reading 430mv-531mv if there is nothing actually there, and it switches back and forth lean and rich on the exhaust but thats impossible without a sensor connected -.-   Soooooooooooooooooooooo tomorrow im gonna borrow the 02 sockets from work and pull out whats left and then put in a heated narrowband o2 i pulled from a pontiac, and once its wired up 90% of my problems will be gone once the computer has enough time to learn the fuel trims.

I have been able to easily get 15-18mpg with a 5.7L tbi in my friends 1994 ext cab pickup, 19-21mpg out of a 1977 ford granada 302/C4 pushing close to 475ft-lbs 2.47 ford 9". 72mpg from a 4dr geo metro, 29mpg from ND to IL and back again (1800miles) in a 92 blazer with a 4.3l tbi 4spd, yes o/d helped but in all of our previous vehicles I have easily been able to add 5-7mpg over what was advertised, and usually above similar vehicles in the area. cars, trucks, 1 tons, 1 1/2tons, suv's, etc.


Until i noticed the o2 issue yesterday I just could not figure it out hench why i posted because i know i should be able to do better at 2400rpm.


I will post an update tomorrow or tuesday after i get a chance to swap o2 and wire in the new one

[roundhouse]

Is the speedo right in the G30? How did you know the milage for the mpg?

My point is if your actual speed is wrong your mpg is going to be wrong since you don't know exactly how far you went.

we had a 1996 P-30 with the TBI 454. And it never ever got over 5 mpg




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[MuddiGGEr25]

My uncle has a 1500 express with the 8.1 15 passenger and i tink he said he gets 11ish in a city over 150000ppl. And roughly 15-18mpg hwy 60-90mph from illinois to texas and back. I believe its all ebout increasing the engines effeciency at the rpm to achieve cruising speed before your wind and rolling resisitance start multiplying.i.e. my speed before wind drag starts being an issue is 50-55 so i want the lowest rpm's that can produce the minimum tq needed to maintain that given speed. And then make the engine as effecient as affordably possible at that rpm +/- 100rpm. That would theoretically keep my desired speed while using the least fuel possible at the lowest rpm with enough tq.    Thats what i did with 95% of my vehicles. I reduce wind drag anywhere humanly possible, then raise the  rear 1-3 inches which usually nets me 1-3mpg, then do my best to get the rpms low as i can, then just increase effeciency in that band. It works everytime. Just on varying degrees.

[MuddiGGEr25]

Oh i had a 1985 chevy short bus on the p30 chassis 5.7 th350 and 3.73 gears. I got about 8 city and 10-13hwy dpending on wind

[Irish_Alley]

something else to keep in mind is the fuel quality. in my 04 impala the instant mpg display i would get 28 or so from one station, went to a cheap station and it dropped to around 24. then you also have winter blend fuel which will drop it even more. ive seen a drop in the cummins from 16 to 13 in my crew cab with the winter blend (at least i hope it all cause of the winter blend)

[MuddiGGEr25]

Just to close out this thread, after I got cliffy sitting with a slight rake, added 4 offroad lights to the visor, and found a good way to mount all 3 flags, i got a consistent 8MPG highway, and 4-8mpg (probably had something to do with my foot weighing 100lbs) in town, idling was using about 1 gal/hr and he is now officially an off road use only toy.