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Gas mileage. Is this normal? Please help.

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Old 10-15-2018, 12:35 PM
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Gas mileage. Is this normal? Please help.

I've never really payed much attention to mileage, but we just did a 600 mile run (met up with some youtubers, offroad, camping thing).
I calculated (assumed) I'd get like.. 12-15mpg on the freeway/whole trip. Since that's what I've seen guys talk about this motor online.
Most the time doing 70mph, I got like 7.5mpg. Ouch. So.. maybe it's normal? idk. If it's not normal, then maybe some ideas on what could be changed/done.
She runs like a perfect TOP. Idles nice and low and smooooth.

Specs: 79 4x4 longbox
motor: Re-man 300 (fresh) with 4K miles so far. 15/40 oil.
Carb: 1981 Carter YFA 7394S. 8509 5. CFM= 150-187
New 38gal gas tank. ONLY non-ethanol has been ran through her.
new fuel pump this month.
tranny: NP435 . Fresh gear oil this month (80/90 GL5 + some lucus)
tires: 33x10.50R15/C 114R BFGs K02 tires (new this month 40psi / alignment this month)
gears: 3:50
exhaust: EFI headers, Y pipe, turbo muffler.
Electrical: New battery, 3G ALT, autolite copper 45/.044gap (6mo old), new solenoid, new cap/rotor, new wires, new coil.
Dana44, fresh oil last month, new breather tube. New U-joints this year. New track arm, tie rods and drag link.
NP205 - checked oit - looks clean still.
Brakes: new calipers, rotors, pads, lines. Repacked inner/outer wheel hub bearings.
9in: New breather this year. Oil changed about 4yrs ago.
Cooling. It was pretty cold drive. The electric fan didn't even kick on with the new and large aluminum radiator. Running "right before the N". 180deg thermostat.
Not towing anything. Just camping gear in the bed and spare tire and my woman isn't a cow.
If you need more specs, ask.

Some things I did that maybe are retaliative to this?
- deleted charcoal can, evap system, all emissions stuff. (gas can still vent behind the cab)
- deleted the solenoid wire that goes to the coil. (I hear that's only for helping starting).
- put breather on valve cover and don't have a return to intake.
- EGR block off plate.
- I do get a vibration @ 60-65mph , not all the time, but most of the time - not sure why yet. Drive lines where balanced about 3yrs ago (yokes ujoints replaced then).
- The dizzy and points are probably really old, never even looked at. But... since she runs smooth, would I even consider looking at that?


I don't know. Need someone smarter than me.
 
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Old 10-15-2018, 03:57 PM
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Should be able to get a bit better than that. Maybe look at timing first. I can get about 16mpg at 60mph (street demon, comp cam, headers, intake, 35s, 3.55 gears, 4 inch lift, 4x4, bunch of junk on the truck).

Check timing first, especially check the vacuum advance works. Keep you're speed to 60mph or less. Make sure you have good tight clean grounds between body, battery and engine. Always check for vacuum leaks. The stock carb I don't think you can mess with the jets or anything....

If you removed anything emissions that tied into the ignition or carb that may mess things up.
 
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Old 10-15-2018, 04:52 PM
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Hot spark at plugs is important, plus ignition timing. initial, mechanical, and vacuum advance. Then good carburetor setup and tuning. Post a clear pic of a plug or two.
 
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Old 10-15-2018, 07:01 PM
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If you haven’t regeared the speedometer, the odometer will be way off. Unless you are using a gps for speed and mileage. That is the same tire/gear combo I have and if I hold the speed at 40, I will be going closer to 50 so your 70 would actually be over 80.
 
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Old 10-15-2018, 07:10 PM
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O wow. Hey Shakin, do you have a part # to the right gear I'd need? I did not re-gear. I'll kick on the gps on the way home tonight and see how far off I am.
I'll have a smart look at vacuum, timing, plugs etc.
 
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Old 10-15-2018, 09:10 PM
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Via GPS, I was about 3 to 4mph off through the whole speed band. So not too bad.
 
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Old 10-16-2018, 08:52 AM
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Sorry, I just checked mine this morning and it is about the same as yours. Shows how far off the factory speedometer is. 60 mph with 28.8" tires =2450 rpm. 2450 rpm with 33 inch tires =68.75 mph.
 
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Old 10-16-2018, 01:14 PM
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The most accurate way to re-calibrate your speedometer.
  1. Get up to speed. The faster the better (ie. 70mph on the interstate)
  2. Check the speed on a GPS and write the two down.
  3. Crawl underneath and pull out the current speedometer gear and count the teeth.
  4. Calculate your new gear and replace your current one.
The formula to calculate is:

New Gear Teeth = Old Gear Teeth * (Speedometer Reading / Actual Speed)

So, if you currently have a 17 tooth gear, and your speedometer read 70 while your GPS said 78 (which would be the scenario when going to larger tires).

New Gear Teeth = 17 * (70 / 78)
New Gear Teeth = 15.2

You'd need a 15 tooth to put your speedometer back where it needs to be.


If you still have the factory setting, and like ShakinBacon said, your speedometer is 60 vs 68, that'll change your gas mileage. You'll have traveled 13% further than you thought you did, which will also increase your gas mileage by 13%.
7.5 * 1.13 = 8.5mpg


That's still really bad.

I've never had gas mileage anywhere near there, even in stop and go traffic in the dead of winter.
With 32" tires, a 450cfm 4bbl, and an NP-435, I'd get 15 - 17mpg. My worst when my carb was way out of tune was around 11 - 12.

None of the mods you did, like block of the EGR would have that kind of affect on it. 3.50 gearing with 33" tires is also really good, especially with the NP-435 which is aggressively geared.

What do your spark plugs look like? Are they sooty black? It should be dumping fuel if your engine/carb is causing your fuel issues.
Otherwise, you might have something else going on. Some possibilities are sticking brakes. A rusted e-brake cable that has your rear drums partially engaged, or a sticking front caliper. Something that's causing parasitic drag and making your engine work extra hard to maintain speed.
 
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Old 10-16-2018, 06:07 PM
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Just took it in, here next door to work. Let smart people look at it will all my data printed off.
He just called and said my timing was 80 advanced which would really hurt mileage so he's going to work that out. Also said I don't have points in my dizzy as I thought I did.. which is cool, but I did want to upgrade.. o well. Also, we both agreed that she isn't getting hot enough. I put in a 180deg stat and the temp never even hit the N on the gauge. We think also just not great atomizing, of course the Efan never kicked on the whole trip until we were crawling rocks. so I'll be putting in the 192 stat back in. But while crawling rocks for hours, the gas gauge never really moved.
I DO smell gas when going up a hill.. so.. Once they are done I'll work on a few other bits.

Thanks AB. Yeah I just did all new brakes and E cable is all new too. not to say that couldn't still happen though. I was and probably will jack her up and spin the tires too.
 
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Old 10-16-2018, 08:45 PM
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Originally Posted by gfburke View Post
He just called and said my timing was 80 advanced which would really hurt mileage so he's going to work that out.
What does that mean? 80??
 
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Old 10-16-2018, 09:25 PM
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Well, 78. Should be about 10-15 advance.
Not sure how that happened.
 
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Old 10-17-2018, 07:19 AM
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Yes it's important that the mechanical advance moves freely and smoothly up and down the RPMs for best efficiency. Retarded ignition timing engine usually runs hot. And vacuum advance probably adds another 3 or 4 mpg on top under ideal conditions when working properly and also helps engine run cooler.
 
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Old 10-17-2018, 10:19 AM
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I don't think 78 advance is correct. I don't think your engine would run! If it did, you'd have so much pinging, you'd probably have blown the tops off of your pistons by now, especially out rock crawling.
However, if your timing is off, that could definitely affect you rmileage.
I remember I accidentally set my base timing to 22 instead of 12 and went out for a drive. That was extremely noticeable.
 
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Old 10-17-2018, 11:33 AM
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Right on. Yeah these guys are working her over. I'm just relaying what I was told lol.
But it makes sense that something like that is off. I have another temp switch for the fan ordered 200on/185off and I'll put in a 192 stat. (was 180 stat with 185on/175off switch).
I'm not expecting magic, just sometime more normal.
 
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Old 10-17-2018, 09:06 PM
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Ok, timing was 8 off. So they backed it down to 6 (from 14).
Said my plugs where pretty black. Air filter pretty dirty and yes, probably running too cold.

So tomrw at work when it's slow, I'll install new plugs, air filter, 195deg stat.
Also I'll jack up and turn tires to be sure they are free.
Then other than the worboling, pulsating + sometimes vibration @60-70mph, she should be running great.

Thanks guys.
 

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