Mileage versus rpm's

[kikkegek]

guys,

I have spent a lot time (and some money) optimizing my 350 to get the best mileage possible and still have a driveable vehicle.

see this topic on that:
http://67-72chevytrucks.com/vboard/s...d.php?t=429908

the thing I noticed during this proces is that whatever improvement I get my mileage for 100% highway (long stretches) and our regular 50/50% city/highway daily driving always get the same mileage.

We always get around 8.2-8.7 mpg....

Now whenever I read a mpg topic people always seem to have a 10-30% better highway mileage then during their 50/50 driving.

My setup is:
- Suburban K20
- TH400 with 4.1 end gears
- 350 small block (original from 1986)
- all the smog stuff has come of
- no more gasoline carb, just the throttle plate
- propane setup: IMPCO 425 mixer on the throttle plate
- IMPCO E-type evaporator
- electric fan for cooling
- MSD 6AL connected to HEI dizzy with light springs for fast and flat curve (for propane)

what ever we do, highway mileage doesnt seem to improve compared to the 50/50 driving. I installed a tach and I noticed we do 3,000rpm when driving 90 km/h (which is slow on Dutch highways).

could the 3,000rpm be the cause? or are there guys over here that also do 3,000rpm's and still get better mileage on the highway?

please let me know

[gchemist]

Ouch. Ideally, stock blocks like 1800-2200 RPMs as the cruise RPMs. Cam design is important.

[INSIDIOUS '86]

You need to ditch the th400 trans and go with a overdrive. That will bring highway mileage up a good amount. This is going to bring the best mileage gain. With the first gear ratio of the 700r4 being so high you can prolly go with 3.73 gears and gain a little more mileage.
Otherwise lower your rig for better aerodynamics
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[kikkegek]

@gchemist: thanks. I was afraid of that. It isnt really 3,000...usually its about 2,800rpm...but that is till high I guess...

@ Insidious: I know that will improve mileage and that will absolutely be the next step. But TH700R4 or 4L80E are a lot harder to get over here in the Netherlands. That is why I am trying to optimize my current setup and researching why our highway cruising isnt any better...

So I guess the rpm's during cruise are killin me...

[INSIDIOUS '86]

Yah Rpms are killing you. You can try lowering to gain some aerodynamics or even go so far as covering the bottom of your rig in flat sheet aluminum to make the bottom totally smooth.
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[Stepper86]

Plenty of 700r4 boxes around here in Sweden, I have one in my truck and I see them advertised on regular basis. I have trucks going to Denmark all most every week, so I could get it down there for you for nothing.

[kikkegek]

Originally Posted by Stepper86

Plenty of 700r4 boxes around here in Sweden, I have one in my truck and I see them advertised on regular basis. I have trucks going to Denmark all most every week, so I could get it down there for you for nothing.

pm sent. thanks bud. Maybe you can help me.

[Goldfinger]

Unfortunately about propane is that it's not as energy efficient and it will use more to keep you going. so in theory you should get less mpg. to get the engine at higher rpms it's sucking down more fuel. so the higher you are the more fuel you should burn. 700r4 will being them down do you really need 4.10 gears? a taller gear would help as well.

[Stepper86]

Originally Posted by kikkegek

pm sent. thanks bud. Maybe you can help me.

Didn´t get any PM, try again.

[Dooley]

I had a 1983 K20 Suburban. Reading your threads make me appreciate it more now than when I owned it. I had the 350 engine/700 transmission,($650.00 option),/3.73 gears and 12-13 mpg was no problem. My wife got even better mileage than that as I have a heavy foot. I know that 3.73/4.10 were the only gears available for the 8600gvw Burbs that year.

What size tires are you running? With 4.10's you should have at least 255/85-16's on it if they are available. Simplest upgrade improvement.
The next upgrade I would get an overdrive transmission.

We upgraded a K5 Blazer to a 700 transmission and it gets 22-25 on the open highways with it's 6.2 diesel.

[wilkin250r]

90km/hr is right about 55mph. 2800 rpms is rather high for 55mph, is there any special reason for the 4.10 gearing? Do you tow often?

I would say a combination of both a gearing change and an overdrive transmission would benefit you tremendously, and there's absolutely nothing wrong with doing them separately.

Of the two, the gearing change sounds like it would be the easiest and cheapest to start with. I would say you could easily switch to a 3.73 ratio and still tow just fine, if you're only towing light loads or not towing often, you could even switch to a 3.42 ratio. If you almost never tow anything, you could go all the way to 3.08, which would drop your highway RPMs to around 2000 and possibly eliminate the need for an overdrive tranny at all (the drawback is that it would feel gutless)

[Corts60]

I agree with all above. You need to keep the engine RPM's in the peak torque range about 1800-2200 during highway cruising if you want to see better mileage.

[Dunenutt]

If you don't want to get rid of the TH400, heres an option. I am going to put one in mine, as soon as I do some other things to it.

http://www.gearvendors.com/4x4gm3s.html

[kikkegek]

Originally Posted by Dunenutt

If you don't want to get rid of the TH400, heres an option. I am going to put one in mine, as soon as I do some other things to it.

http://www.gearvendors.com/4x4gm3s.html

thanks for the info:
so help me out here....I replace the transfer case by one of these baby's and instantly I go from 3 to 6 gears and my end gear goes from 4.1 down to 3.2?

If that is a plug-n-play installation and I can keep the rest of the car...then WOW!

just the 3000 bucks is a lot....
So do I get this right?

[wilkin250r]

Originally Posted by kikkegek

thanks for the info:
so help me out here....I replace the transfer case by one of these baby's and instantly I go from 3 to 6 gears and my end gear goes from 4.1 down to 3.2?

If that is a plug-n-play installation and I can keep the rest of the car...then WOW!

just the 3000 bucks is a lot....
So do I get this right?

Close, you'll also need to change the rear driveshaft. And while that is definitely an option, it's also probably about twice the cost of regearing your axles.

[Dunenutt]

Originally Posted by wilkin250r

Close, you'll also need to change the rear driveshaft. And while that is definitely an option, it's also probably about twice the cost of regearing your axles.

True, but i want the low gears. In town stop and go,and off road,low is better. Kick in the od on the highway,and cruise. Plus the advantage of splitting gears.


To the op,its plug and play. Call them, they will talk your ear off, lol.
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[kikkegek]

Originally Posted by Dunenutt

True, but i want the low gears. In town stop and go,and off road,low is better. Kick in the od on the highway,and cruise. Plus the advantage of splitting gears.


To the op,its plug and play. Call them, they will talk your ear off, lol.
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okay....but $3,000,-? is that correct?

[tucsonjwt]

4:10 rear axle and 4 WD and 3/4 ton will kill gas mileage. I just took my 83 C20 4:10 SM 465 granny 4 speed on a 110 mile round trip highway ride and got 12 mpg. If I had engaged cruise and gear vendor (I did not because both were not working properly) I could have picked up 1-2 mpg. In the city I get anywhere between 8-11, depending if I have it loaded down with 3,000 lbs of junk on the way to the dump or not.
Having said all that, I think you should get 8-9 mpg city, maybe 10-11 highway with everything operating properly. I have no tachometer but I know that 4:10 is revving the engine way more than I would like.
A very light foot in the city will help. Anticipate stop lights ahead of time and take your foot off the gas - coast to a stop. Accelerate very slowly.

[Corts60]

Originally Posted by kikkegek

okay....but $3,000,-? is that correct?

yep. They're expensive. They are a very well built unit though. Some people say they are clunky and weird, but really, if you know how to shift them they are not. I have one in my truck behind a T400 and 3:73 gears. On the highway with a GV, my final drive ratio is 2:91. The combo of that ratio and having a diesel, I get 22 MPG consistently. It is really fun to bang through all 6 gears off the line (it takes some practice being able to shift everything smoothly).

[kikkegek]

ok, checked it out, but $3,000 is not even taken into account the labor to put it in. Thats a too expensive upgrade for me. I'll never pay back that investment in mileage. I probabyl go down the road of the 4-gear 700R4 or something.

thanks everybody

[cayoterun]

Originally Posted by Goldfinger

Unfortunately about propane is that it's not as energy efficient and it will use more to keep you going. so in theory you should get less mpg. to get the engine at higher rpms it's sucking down more fuel. so the higher you are the more fuel you should burn. 700r4 will being them down do you really need 4.10 gears? a taller gear would help as well.

Goldfinger is right, Where we live, Propane was used alot on farm tractors, trucks, and some cars, but propane is a colder fuel than gas. Rule of thumb here was propane would get 10% less mpg and 10% loss of power. A gain in power could be had raising the compression by using higher compression pistons, and milling the heads. That was only done during overhaul. Propane cost per gallon has risen so much in the last 25yrs, it no longer offers any savings over gasoline. I don't know what the cost comparison is on the late model factory equipt motors.
The Impco conversions are really good, simple, trouble free, and was the perferred choice.

How does the purchase price of propane compare to gasoline in your country? Just curious?
Cayoterun

I like propane for fuel, but no longer use it for that reason.

[kikkegek]

over here propane is less then half price of gasoline, so I dont really care about lossing 10% power and mileage..hahaha...

I was just curious why mi highway and city driving are equal in mileage. I guess my rpm's are killing mee..

[tucsonjwt]

Gear vendor advertises 20% improvement in fuel economy. I went from 9 city to 11 city mpg when I started using mine. I don't know if GV alone would account for 22 mpg - that is like 100% improvement in fuel economy.

[BigBlocksRule]

If you have a way to get one from London to your location, I can get a 4L80E that far, I shop a bunch of blocks, etc. to a builder in London. I'm sure he wouldn't mind piggy backing a tranny in the next shipment...

[kikkegek]

Originally Posted by BigBlocksRule

If you have a way to get one from London to your location, I can get a 4L80E that far, I shop a bunch of blocks, etc. to a builder in London. I'm sure he wouldn't mind piggy backing a tranny in the next shipment...

a 4L80E would be supersweet...I just dont have a clue what else is involved to install it in our truck? I'll pm you...cause I think you do know this...