Help me figure out my fuel mileage, it's complicated...

[Smokeater11]

Smoke,
When the heck did they start teaching algebra in Polk County schools?
Must have been after I left.
;D
Aaron

Hey I had a lot of help from these guys. I didn't know how to figure it out myself hense this thread. ;D

Viper, I haven't filled it back up yet. I just dumped the 5 gallons I had in the can in it and went on to work. I'm gonna fill it back up tommorow night. If it takes 18 gallons plus a little then I'll know for sure it holds 23 gallons. If not then I'll just add 5 gallons to whatever it takes to fill it up. I'm spending the night at the station tonight so I won't be driving it more than a few miles on the 5 gallons I put in it this morning.

 

[Smokeater11]

Just for grins and giggles, what do you guys think I can do to help with the mileage? I've got a '86 5.0 base model motor with a 5.0 HO cam. Holley 600 cfm 4 barrel with #66 jets. Long tube headers and dynomax race bullet mufflers. 33's and 4:11 gears Np 435

I'm open to suggestions.....

 

[broncnaz]

Very hard to say as I hear the HO cam is not that great under 2200 rpm Peak torque is said to be around 3300 rpm. Currently at 65 mph you should be turning about 2700 A OD tranny would drop the RPM's to around 2200. Eitherway I still think a OD tranny would be your best bet. either a NV or ZF.
About the only other thing you could try is to swap to a smaller carb. I picked up a little mileage going from a 600 to a 525 CFm carb.

 

[Smokeater11]

I have thought that I might do better with a smaller carb. I have this 600 jetted down 6 sizes and have to run the idle mixture screws just 3/4 of a turn off of closed stumble to get it to idle where it doesn't choke me. Are the 525 and 470 Holley carbs still 4 barrel carbs?(just to sleepy to look it up tonight) I have the aluminum Edelbrock intake and really don't want to have to change it.

 

[broncnaz]

Yes the 470 is still a 4 barrel. The 525 I use is a barry grant road demon jr(basically a holley copy).
Sounds like something is amiss with your current carb the idle screws should be closer to 1 1/2 turns out. My first thought would be a loose or leaking power valve, or the metering block gasket is leaking.
Whats the P/N of your carb? most 600 cfm carbs come with about #64 jets. Have you checked your spark plugs to see how the jetting is? I usually ended up going with #62 jets but I am a little higher in elevation than you. So you may be jetted about right for near sea level.
Also whats your timing set at?

 

[Smokeater11]

Don't know the part number off hand but I did check the part number when I first got the carb to make sure it was a 600. When I got the carb(used & free) it had #72 jets in it and I was told it was untampered with and we also completely rebuilt it with the blue gasket set and all new internal components. I have heard of the Holley carbs getting small hairline crack inside of them and causing them to act up too. Haven't pulled the plugs lately but really do need to, last time I had them out they were a nice greyish brown color and dry. Timing is set at 13* right now but I have been playing with that as well lately. I have a sneaky suspision that my cheapo HEI dizzy has something amiss in it too because from time to time when I put the timing light on it the flashing is very sporatic at best and cuts on and off like it's not sending fire to the cylinder every time. Other times it works perfectly and I have tested the light itself on 3 other vehicles and it works without issue.

 

[broncnaz]

Your probably running a bit to much timing to get a clean idle. but then again if the dizzy is screwing up that will causes issues in both idle quality and overall mileage big time.
I've never heard of holleys getting cracks and I've been around them for a very long time. So thats a new one.

 

[Smokeater11]

I'm gonna look into a new dizzy or at least a new cap and rotor and see if that helps that issue and I'm also gonna look into one of the 470 carbs if I can find one cheap enough. Any other ideas you guys??

 

[Smokeater11]

Are you just assuming it used 23 gal of gas because that's the advertised size of the tank or did you fill it back up to see how much it really used?

After I dumped the 5 gallons in it Wed. morning I drove it 23 miles by the odometer which equals 24.84 miles corrected before filling it up tonight. That is roughly 2.5 gallons used of the 5 gallons. It took 18.5 gallons tonight to fill it up as full as I could get it. In fact it was dripping out where the vent hose attaches to the tank. So 18.5 gallons added to the 2.5 gallons that should have been in the tank is 21 gallons full. So if I take the 212 miles from before and divide that by 21 gallons it gets me to just a fuzz over 10 mpg. Still not good mileage but at least it's into double digits%). I reset the trip meter and I'm gonna do it again to see if a few changes I made to the carb helps any.

 

[ol gray mare]

Nonsequitur

The factory accuracy specs of the speedo and odometer are somewhere around +/-4% for each but if a speedo gear change is done to make one closer the other may be made worse.

I guess that explains the inconsistancy in mine. I changed the speedo gear. Now, I'm almost spot on with speed, according to the local Smokey's radar, but the odometer shows almost a tenth long between mile markers on the freeway.

 

[DirtDonk]

By emptying the only tank you have, you're just adding another level of factoring here Smokeater. At least I'm presuming you only have one tank, from the way it sounds. Otherwise you could drive to a station on the other tank and just fill the empty one up in one step.

With one tank, don't run it dry. Just run it down. Sounds like you can easily get to work and back, so fill it up, run to work, then fill it up again and see how much fuel you've used vs the known exact mileage. Voilá! Exact mileage with no fudge factor.

Unless your fill-up procedure is typical Bronco, and very inconsistent. If that's the case, then the best way is to run it dry, put an EXACT KNOWN amount of gas in it, then run it dry again. Compare the known fuel amount vs mileage traveled, and there's your mileage.

But try the fill-up method first, since running a pump dry isn't doing it any favors, and running a single tank dry is a big fat hassle.

Paul

 

[Smokeater11]

Yeah Paul, just the single tank. Now that I have an idea of what my mileage is I'm planning on running it to around 100 miles and then filling it back up. That should either confirm or adjust my mileage figures.

I've been trying to figure out my mileage so that I can use my odometer as a gas guage since my guage is FUBAR. I guess one way to solve the whole issue would be to spend the $50.00 and get a new guage.%) But what's the fun in that!! Plus I relearned some new math skills during this process;D

 

[DirtDonk]

Yeah, no fun at all fiddling with gas gauges. In the end though, you'll finally get 'round to it and find that it's just a bad connector at the Orange wire under the cluster! But what the hey, these new-found math skills just might come in handy somewhere else as well.

Just don't look in the check book register!

Paul

 

[Smokeater11]

Ooppss, I meant I need a new sender not guage. The guage is only a few months old but the sender was in the tank when I got it. I tested the sender but apparently I didn't do a very good job.

 

[markperry]

Borrow a GPS from a friend. Then you will also know how far off your speedo is. I'm horrible with math.

Alot of the new cell phones have a navigator function on them you could use that also.......That's how I figure mine!

 

[DirtDonk]

Ooppss, I meant I need a new sender not guage. The guage is only a few months old but the sender was in the tank when I got it. I tested the sender but apparently I didn't do a very good job.

How'd you test it? Ohm it out, or hook it up to the gauge and see what it read?
I think hooking it to the gauge is the best way, because, as has been said here (I think) and in other threads, the whole system has to work together. And what with 40+ years on the wires and other components, they ALL might be suspect.
Then, if they're all "close enough", you might just be able to adjust the sender to work within a range you can accept.

Wouldn't hurt then, to verify that your IVR is putting out the proscribed amount of pulsing power at the proper intervals, check the resistance in both the gauge feed and ground circuits. Then, if they all check out reasonably well, hook up the sender and see if it can be adjusted to fit the system.

I did that to my two stock sending units and they have worked just fine ever since. Probably can't fix every sender that way, but it can work sometimes at least.
I never expected my gauge to read perfectly at every point (it pretty much does though), I just wanted it to give me a reasonable facsimile of Full and Empty. I like it to read just a needle's width above full after re-filling, and two needle's worth or so below empty when running on fumes. That way, even at empty I have a couple of gallons left in the big tank, and about a half-gallon on the small tank, when the gauge hit the empty mark.
I still try to fill up at a quarter tank, but at least I know where I stand when I can't be that choosy.

Maybe give that a try before buying a new sender? Either way though, you're going to have to fiddle with the sending unit. Even a new one might have to be tweaked to make work with your gauge.

Paul

 

[Smokeater11]

I was driving it for a little while before I put a new aftermarket guage in and I would hook the ohm meter up to the wire I had run to the dash and it would read normally until it got to around half a tank then it would go haywire. The guage is the proper ohm range and once I installed it works properly until the needle reaches half tank then it begins to rapidly bounce to well below the empty mark as the gas sloshes around in the tank. Once the fuel reaches a certain level it just buries the needle to way below empty and doesn't move. All this leads me to believe that the sender has a short in it at around the halfway point in it's arch. The truck will run for another 50 miles or more after the needle quits working.

 

[Smokeater11]

The changes I made to the carb and my driving style have helped a little with my mileage. The last full tank ran out at 197 miles on the odometer whick is 212.76 miles corrected. This tank is now at 209.8 miles and hasn't run out yet. That's 226.58 miles corrected. It is very low and probably has less than a half a gallon left in it, the fuel filter on the frame rail is less than a third full. I'm gonna guess that it has only a few miles left in it, but this tank I got right at 11 mpg. Still have a few more changes to make to hopefully make it better.

 

[broncnaz]

Ever figure out that ignition issue? I think that will be the biggest mileage getter.

You dont really need to empty the tank and try and figure how much gas is still in the sytem just fill it back up and figure mileage what ever it takes to fill it back up is your true number.

 

[Smokeater11]

I'm gonna work on the ignition issue tommorow, check the plugs to see how they look and then I might change the jets in the carb if i can find the right ones. I'm running #66 jets and depending on what the plugs look like I might go down to #62s to see if that'll let me set my idle screws at a more normal position.