MPG check

[TDS-MN]

Hey All,

Just refueled after a weekend trip, and was not that thrilled with my calculations.

Background, the truck is my '85 K-10, 3:08 gears, 700R4 ATw/ OD. 31x10.5R15 tires. stock 305 w/ quadrajet. Stock exhaust w/ custom cat-back duals.

Had to sit in traffic on the trip north. Calculated 10.55 MPG, including the highway miles, sitting in stop-and-go traffic for maybe 45 minutes, and misc driving around town. Highway speed was around 65 MPH, rarely over.

Checked what i got on the drive home, took a different route to avoid the traffic congestion and road work. Calculated 11.33 MPG. Travel speed was more between 60 and 65 tops this route. No stop-and-go congestion to be found on the simple 2-lane highway!

Both trips were with minimal weight, just myself, a cooler and a couple bags. Ran in OD for all of the travel.

I've recently replaced fuel pump due to failure. Also new cap, rotor, wires and plugs. Also reconnected the Electronic Spark Control module, and adjusted timing close to spec.

Granted that these trucks are far from aerodynamic, but does anyone think that I'm getting less MPG than I should running the setup i have? I've read on other forums of some people acheiving better mileage, and am curious how those of you on the forum here are doing with your trucks.

TomS.

[Keith Seymore]

31x10.5's are a pretty big tire to be hauling around.

I almost wonder with those big tires, a 3.08 axle, and 700R4 if you are running below the power band of the engine....

Did you notice your engine RPM at 65 mph?

K

[TDS-MN]

I can double-check that tach reading, don't recall off the top of my head. If I were to GUESS, I'd say just a little above 2000 in overdrive, but don't quote me on that yet.

You raise an interesting point, though. When encountering a slight rise in grade the truck really loses momentum, and needs to downshift. Of course, then it's pulling 3000+ RPM.

These tires are relatively new, but from my understanding, were also OE size on some of the Trucks/Blazers. Have previously run a 235/75R15, but thought that my MPG back then was pretty poor too. Of course, I had the ESC system bypassed due to faulty controller, and that hurt performance. But these symptoms are not excusively since the current tires were added.

[Keith Seymore]

Originally Posted by TDS-MN

These tires are relatively new, but from my understanding, were also OE size on some of the Trucks/Blazers. Have previously run a 235/75R15, but thought that my MPG back then was pretty poor too. Of course, I had the ESC system bypassed due to faulty controller, and that hurt performance. But these symptoms are not excusively since the current tires were added.

Yes, they were available OE, but may have forced a higher rear axle ratio like a 3.73:1.

K

[piecesparts]

Originally Posted by Keith Seymore

31x10.5's are a pretty big tire to be hauling around.

I almost wonder with those big tires, a 3.08 axle, and 700R4 if you are running below the power band of the engine....

Did you notice your engine RPM at 65 mph?

K

I agree, with the gear ratio and that tall of a tire with an OD tranny, you are killing your power band. This style of truck and engine design really likes the power band around the 2000 RPM and above area. You are basically idling down the road until you have to climb a hill and then the truck has to work twice as hard to get back to the power band and then do the work to move the truck. I had my son's truck running a 3.08 gear ratio and a 700R4 tranny with a lower profile tie and his MPG went to crap. We put in a 3.73 gear ratio and got his RPMs back up to the 2500 RPMs at 70 MPH and gained 3 MPG.

[TwoFiftyShifter]

My K20 with a TBI350 and 3.73 and NO O/D will get 15mpg highway..... You should be doing that. If you have hilly highways, do a trip in "3" and not OD and see if it helps.

[glock35ipsc]

Originally Posted by TDS-MN

3:08 gears, 700R4 ATw/ OD. 31x10.5R15 tires.

This combo, in O/D, would be turning 1500 rpm at 65 mph!!!

In 3rd (1:1) it should be turning just under 2250 rpm at 65 mph.

I agree with TwoFiftyShifter, I bet your mileage would greatly improve by not using overdrive.

With 3.73's you would be turning around 2100 rpm at 75 mph in O/D.

[HOTFUN]

The standard gear set on that year 4x4 10 series was 3.42 and most dealers around here ordered the 3.73's with that 305 and od. I didnt even remember that 3.08 as an option for the 4wd's. Another thing to question is the speedo gear correct since you increased tire size. You might be another 10% slower on the miles and speed after going up to 31" tires. You might try a GPS or mile markers and check your speedometer calibration. That can make another mile or two difference.

[2tons of fun]

haha, you said power band when refering to a 85 305!!!!!!!!!!!!


talk about boging down the motor, you might as well push 2.21 with stock tires.

i quite using o/d in mine just to make it drivable, it would bog down pulling up and on ramp.


being that it has a 208 and auto hubs you had better check the hub to make sure they are not fubard and locked.

[Keith Seymore]

Originally Posted by HOTFUN

... Another thing to question is the speedo gear correct since you increased tire size. You might be another 10% slower on the miles and speed after going up to 31" tires. You might try a GPS or mile markers and check your speedometer calibration. That can make another mile or two difference.

That's a good point, too.

K

[TDS-MN]

OK, I confirmed on the drive home last night, it pulls 1800RPM at 65MPH.

Maybe I'll try a week without using O/D, just D, and see how my numbers work out then. I have not really checked the odometer against the mile markers on the road, I will try and do that next chance I get. I'm almost 100% positive that the 3.08 was what I saw on the glove box door sticker. I know that my tire diameter went from 29.1" to 30.8", but it seems that the speedometer must be pretty close to right on, with some rather un-scientific testing I have done so far.

"haha, you said power band when refering to a 85 305!!!!!!!!!!!!"

That did make me laugh out loud. I know it's not much to get excited about, but I'd like to be sure I'm getting all I can out of it, at least.

I have manual hubs, and they are unlocked.

I appreciate all of your comments, everyone.

[glock35ipsc]

Originally Posted by TDS-MN

OK, I confirmed on the drive home last night, it pulls 1800RPM at 65MPH.

Originally Posted by glock35ipsc

This combo, in O/D, would be turning 1500 rpm at 65 mph!!!

In 3rd (1:1) it should be turning just under 2250 rpm at 65 mph.

OK, so I was a couple hundred rpm off!

Then you will probably be around 2500 at 65 in 3rd. That's a good spot to cruise.

[piecesparts]

The formula that I have makes the RPMs at 65 MPH is 1528 RPMs. This is with an OD tranny putting out at a .7 % vice a 1.0 output ratio.

If you are going 65 MPH at 1800 RPM, then something is different, than a 3.08 gear ratio. 1800 RPM would be a 3.73 gear ratio not a 3.08.

Take a stopwatch or a wristwatch and time what it takes for you to do a mile between several mile markers. It takes 60 seconds to do a mile at 60 MPH. (60 X 60 = 3600 seconds) If you do a mile in 55 seconds, then divide that into the 3600 and you get 65.45 MPH. So, 3600 divided by 50 seconds is equal to 72 MPH and so on.

[TDS-MN]

Originally Posted by piecesparts

The formula that I have makes the RPMs at 65 MPH is 1528 RPMs. This is with an OD tranny putting out at a .7 % vice a 1.0 output ratio.

If you are going 65 MPH at 1800 RPM, then something is different, than a 3.08 gear ratio. 1800 RPM would be a 3.73 gear ratio not a 3.08.

Take a stopwatch or a wristwatch and time what it takes for you to do a mile between several mile markers. It takes 60 seconds to do a mile at 60 MPH. (60 X 60 = 3600 seconds) If you do a mile in 55 seconds, then divide that into the 3600 and you get 65.45 MPH. So, 3600 divided by 50 seconds is equal to 72 MPH and so on.

Aw man, I hate math.
I will have to double-check the gearing ratio, was almost positive I had the 3.08 from what I read. I was hoping I had a little lower gear, but figured I'd make do with whatever it ended up being. From my research, either the 3.08 or the 3.73 were the options for '85.

So it sounds as if the consensus is that I should have the RPM's above 2000 for highway travel to maximize the engine's power curve (or lack there-of, LOL!).

It will be hard to train this old dog to shift it into D and not O/D. Struggled with it over the lunch hour, had to catch myself and go that extra click with the shifter.

[TDS-MN]

Alright, so I drove it home and back to work in Drive, not O/D.

Was turing right around 2500RPM at 65 MPH. I think I was right near 2200 at 55.

Your thoughts were about right, though, as the truck seemed to be more in it's power band there, a little more throttle, and it started to move much more instantly than it would when running in O/D. It is different hearing the engine turing those RPM's, I'll have to get used to that I guess. I will keep this experiment going to see how my numbers calculate after this tank of gas.

I checked my odometer against the couple of mile markers that I did pass. First mile was reading almost right on, but the second came about a 10th too soon, the odometer was a little bit behind. I will need to expand on this check a bit more, but need to find suitable road to do it on.

I was going to verify the ID tag on the glove box door as well, to see about ratios, and confirm I read it right before. Still need to do that.

EDIT = confirmed the 1GU4 option on the glove box door, 3.08 axle gear. not saying the the calculated numbers lie, butI aslo don't recall my uncle eveer having the gears changed, nor any reason for him to do so, since it always had the 235/75 metric tires on it