'84 K10 getting 8.9mpg....is this normal?

[USSkoval]

Originally Posted by dman1982

i just drove my 84 from jacksonville nc to south of the boarder sc, and i was getting 10 to 11 on the high way and i get a little better when im in the city, kinda wierd...running 35's with a six inch lift

Sounds like you need steeper gears. You have a 700r4? If your gears are too tall it will lug it down on the highway too much, causing a drop in MPG.

[rayfinseats]

I'm building an 83 K10 and milage is my main concern. I have knocked 800 pounds of weight off of it. 350/th350 2.73 gears, np208, 31x11x15 tires. edelbrock 600 carb. I have a performer intake and long tube headers I will be adding. We'll see...

[mdb]

For comparison, my dad's 84 F250 4x4 with a 460, around 3.55s and C6 gets just above 11mpg on the highway. This is driving at 55-60mph (maybe it stays the same if you go 75, we never checked).


Edit: Of course if you did check it at 75mph it would probably drop way below 10mpg. We haven't done enough city driving to ever calculate fuel mileage for that.

[Dan1969]

6 freeway...

Yeah, read it and weep...

[dman1982]

Originally Posted by USSkoval

Sounds like you need steeper gears. You have a 700r4? If your gears are too tall it will lug it down on the highway too much, causing a drop in MPG.

ive got a tired 350 with the th350 and 3.08 gears, got the 600 eldebrock carb and longtube headers and 35 inch tires.....i have a set of axles sitting in my garage that have 3.73 gears in them....think that will help?

[USSkoval]

Originally Posted by dman1982

ive got a tired 350 with the th350 and 3.08 gears, got the 600 eldebrock carb and longtube headers and 35 inch tires.....i have a set of axles sitting in my garage that have 3.73 gears in them....think that will help?

Lol, yeah you NEED the 3.73 gears. I've done the whole 35's/3.08 thing... it sucks

[dman1982]

Originally Posted by USSkoval

Lol, yeah you NEED the 3.73 gears. I've done the whole 35's/3.08 thing... it sucks

ya man....horrible!!!!!! take off is slow and your top speed is about 75!!!! freaking sucks...im hoping to do it here in the next month or so...got to goto school here shortly in cali....so kinda limited on time if not its going to take me months to get to it

[trevor73402]

I appreciate all the reply fellas. I will start with the basic tune up items first, but the truck runs as smooth as butter. I'll have the timing checked, but I can't imagine it being off. Throttle response is very smooth all thru the RPM range. This truck is fully restored including a fresh rebuild. It idles perfectly.

Now I will certainly check out the GPS thing and monitor it that way to see if my odometer could be off.
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[KMK454]

33k miles, 305 V8, TH350 three speed, 2WD 3.07 rear gears, stock manifolds, no cats, turbo mufflers. Truck runs nicely, has plenty of power, and starts up with ease. About the only thing it has issues with is every so often on shutdown it might diesel, but it's very rare.

Calculated off of the ODO and gas usage: 10-12 MPG, with or without AC.

Is high? Low? Right? I really don't know... the ODO could be slightly off for calculation, as could the fuel measurement since I can't top-off the tank unless I want to spill down the paint.

[tucsonjwt]

Drive like grandma on her way to church on Sunday - very slow when accelerating from a stop, let off the gas way before you come to a stoplight.
One quick/rough way to check your speedometer accuracy is to note your odometer reading as you pass a mile marker on the freeway - drive to the next mile marker - did your odometer turn over exactly one mile?
The most accurate way I found to check mpg is to pull off the freeway and fill up to pump shutoff. Go directly back on the freeway and set the cruise at the speed limit. Drive about 10 miles, then turn around and go back to the same gas station. Return to the same pump and let the pump shutoff the same way. Divide miles traveled by gallons used and you have accurate highway mpg. The last time I did this I got 13 mpg, which I think is accurate for my 83 big block 4:10 axle, with gear vendor OD.

[KMK454]

Originally Posted by tucsonjwt

One quick/rough way to check your speedometer accuracy is to note your odometer reading as you pass a mile marker on the freeway - drive to the next mile marker - did your odometer turn over exactly one mile?

Funny you mention this... they just installed one of those radar trailers on the drive to work to scare speeders... I went by at 40MPH according to the speedometer. Everybody else was going the same speed as me, but the posted speed limit was 45MPH. Sure enough as I passed the radar, it read "45 MPH," so it looks like my speedo is 5 MPH slow... which probably affects my odometer, which probably affects the validity of those 33k original miles

[mdb]

Funny you mention this... they just installed one of those radar trailers on the drive to work to scare speeders... I went by at 40MPH according to the speedometer. Everybody else was going the same speed as me, but the posted speed limit was 45MPH. Sure enough as I passed the radar, it read "45 MPH," so it looks like my speedo is 5 MPH slow... which probably affects my odometer, which probably affects the validity of those 33k original miles

haha Ok so your speedometer/odometer is reading %12.5 lower then how fast/far you're actually going. So really you're getting %12.5 better fuel mileage... and your truck has an extra 4k miles or so.

This is assuming your speedometer is actually off that much through the whole range... do a quick test and tell me what the radar says when you go past at 80mph

PS- my math could be completely wrong...

Edit: having thought about it long and hard, it might be off by %11. But all I've managed to do is confuse myself so it's best if you just ignore me.

[Palf70Step]

Also remember ethanol will drop the mpg by about 10% to gas without it. 9ish is a bit low, but not uncommon for a 4x4 of this era truck. The 31s are throwing off your mileage a fair amount also. a 2+ inch taller tire that stock is gonna change your true speedo reading a good bit

[USSkoval]

Originally Posted by KMK454

33k miles, 305 V8, TH350 three speed, 2WD 3.07 rear gears, stock manifolds, no cats, turbo mufflers. Truck runs nicely, has plenty of power, and starts up with ease. About the only thing it has issues with is every so often on shutdown it might diesel, but it's very rare.

Calculated off of the ODO and gas usage: 10-12 MPG, with or without AC.

Is high? Low? Right? I really don't know... the ODO could be slightly off for calculation, as could the fuel measurement since I can't top-off the tank unless I want to spill down the paint.

I have the exact same combo, except add in headers and 100,000 miles. I have been getting 14-16 MPG for a while, but it might be 1 or 2 mpg higher now. I fixed a fuel leak and fixed a fuel line that was WAY to close to the header (I think it was boiling the fuel). Also, I noticed I get slightly better mileage going 75 on the highway, rather than 70.

[f.monroe]

I read the thread and I think you should run a mileage test on the truck using drive only. Keep it in drive in town and on the highway. Do not use OD. See what that gets you .

Then try a test using only drive in town and only use OD on the highway when the truck will be at speed for a while.


I had a 94 Z71 that would get better mileage keeping it out of OD. I could literally get it up to 70 in drive , keep my foot as still as possible and shift into OD, and watch the speedo drop 5 mph. The trucks do not have enough power and torque to pull the load at such a low RPM. I think 3.73 is about as high of a gear I would run with an OD. 3.42's if you are using 15 inch tires under 28".
Frank

[trevor73402]

Thanks for the info Frank.

I dont drive the truck far enough for it to actually hurt me financially. I live 10 min from work. I just noticed that it "seemed" to drink the gas. It's only going to get worse when I put 33's or 35's on it, but eventually it will get a LS swap and be fuel injected and the old quadrajet will go on the shelf.
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