8 mpg this sucks
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Junior User
Joined: Aug 2003
Posts: 89
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8 mpg this sucks
Ok guys here is what I have '86 F150 4x4, C-6 auto with 3:50 gears and 33-12.50-15 tires. The engine is a fresh (500 miles) 351W with the following mods.
.040 overbore
PAW cam .448/.472 204/214
Performer RPM manifold
early 351 heads (D0 castings with the 1.84 vavles)
long tube headers
stock Ford Holley 4 barrel carb
I haven't tuned the carb any yet other then adjust the choke, but it have good response and I have checked the plugs and everything looks good. I didn't expect good mileage out of the truck but I figured I would get around 10-12 on the highway not 8 and that is running at a steady 60 mph. I havent put the tack in it yet but I'm guessing its running around 2200 rpm. The truck doesnt lug any even with the 33's and 3:50's even if I just moderately step on it it will lay rubber from a dead stop for a long way. So what seems to be the problem or am I just stuck with ****ty mileage?
.040 overbore
PAW cam .448/.472 204/214
Performer RPM manifold
early 351 heads (D0 castings with the 1.84 vavles)
long tube headers
stock Ford Holley 4 barrel carb
I haven't tuned the carb any yet other then adjust the choke, but it have good response and I have checked the plugs and everything looks good. I didn't expect good mileage out of the truck but I figured I would get around 10-12 on the highway not 8 and that is running at a steady 60 mph. I havent put the tack in it yet but I'm guessing its running around 2200 rpm. The truck doesnt lug any even with the 33's and 3:50's even if I just moderately step on it it will lay rubber from a dead stop for a long way. So what seems to be the problem or am I just stuck with ****ty mileage?
Postmaster
Joined: Mar 2003
Posts: 4,038
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From: Spokane WA
Originally Posted by ken75ranger
If you change your gearing up to 3.73-4.11 it will put the motor in it's power band better. 2200 @ 60 and that engine isn't in it's power band. Needs to be around 2500-2700.
Fleet Mechanic
Joined: May 2004
Posts: 1,436
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From: Montana
Yea with those 3.50 gears and the 33x12.50's you should be getting about 8 in town and doing really well on the highway. Mine is the same it gets 10 in town and it was getting about 20-21 on the highway at 70 mph. Thats what i got but that doesnt mean you will. I dont think the gears will net you any more mpg though.
Oh yea mine is fuel injected too....but you get the point anyways..you should be better off.
Oh yea mine is fuel injected too....but you get the point anyways..you should be better off.
Elder User
Joined: Jan 2004
Posts: 819
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From: Troy,NY
lower gears can actually improve mileage. At 2200 that engine is struggling to get up to a speed it's made for. Actually putting in the same or more fuel than at a slightly higher rpm.Get a tach. I'll bet your closer to 2000@ 60mph. After you get a tach in come back with the results. If you have a smaller set of tires try them out for a tank or 2 of gas and see what you get.
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Junior User
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I'd asume that smaller tires will net me better mileage because of the weight advantage, also you guys are saying that I'm not running in the cams power band, but it only have 204 degrees of duration on the intkake and 214 on the exhaust. I figured this cam came on at about 1800 rpm and peaked around 4500 or am I under estimating it?
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Elder User
Joined: Jan 2004
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From: Troy,NY
It's not the weight of the tire that is significant. By going to your getting the effect of a lower total gear ratio by putting smaller tires on. Your RPM's for any given speed will be higher. You really need a tach to start off right and see where you are now before you change anything. Without it your going by ear and best guess. If you keep your foot off the skinny pedal it makes about a 2 mpg difference for me. 
Last edited by ken75ranger; Dec 28, 2004 at 05:35 PM.
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Elder User
Joined: Jan 2004
Posts: 552
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From: Northern, Va
Gear's dude the engine is luging around . I would say 4.10 gear's If your not sure about gears use smaller tires 235x15 & try it use one of your bud's tires but don't crack down on the gas you will not see any diff. at all.
Don
Don
Fleet Mechanic
Joined: May 2004
Posts: 1,436
Likes: 0
From: Montana
Yea mine is overdrive forgot that.....All you need to do is know what your original gear ration is nadtire height and choose a gear that will bring you to that original ratio. nowthat you have bigger tires you need to find the gear that will put you back into your OE rpm range and you will be in there here's a calculator to help.
http://www.ring-pinion.com/content/calculators/RPM.asp
Bigger gears are going to put your cruise rpms up around 25-2600 and thats just going to suck more gas plain and simple. a slightly bigger than stock tire WILL net you better mileage....but too big and you will waste gas of the line to get up to speed but once your up to speed it wont be a problem. If your getting bad mileage in town you will require a slightly taller gear. but if your cruising on the highway at 2200rpm and your getting crappy MPG its not the gears. My truck runs at 1800 rpm at 65 and 70 mph with the overdrive, It weighs 6500 lbs and i get around 20mpg if lugging was the issue I would be way out of powerband, but i do get terrible MPG on the street roads.
http://www.ring-pinion.com/content/calculators/RPM.asp
Bigger gears are going to put your cruise rpms up around 25-2600 and thats just going to suck more gas plain and simple. a slightly bigger than stock tire WILL net you better mileage....but too big and you will waste gas of the line to get up to speed but once your up to speed it wont be a problem. If your getting bad mileage in town you will require a slightly taller gear. but if your cruising on the highway at 2200rpm and your getting crappy MPG its not the gears. My truck runs at 1800 rpm at 65 and 70 mph with the overdrive, It weighs 6500 lbs and i get around 20mpg if lugging was the issue I would be way out of powerband, but i do get terrible MPG on the street roads.
Last edited by EgoMan; Dec 29, 2004 at 02:42 PM.
Junior User
Joined: Apr 2002
Posts: 97
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From: Arcadia US
Get rid of the Holley, get an Edelbrock, I have an 86 4x4 with a 78 windsor, bored over .040, comp. extreme energy cam, tube headers, ect.. I get about 15-16 on the road about 12-13 in town, I have 265 75 r16's which are about 33" tall with 3:50 gears and my milage always averages about 15 no matter what I do, towing my car trailer with a load it'll drop to about 11-12 on the road, but never under 11 no matter what.
Junior User
Joined: Apr 2002
Posts: 97
Likes: 0
From: Arcadia US
Yeah you could tune the Holley and every day thereafter to make it run right, they're great for the dragstrip, but suck on the street, I tuned my Carter AFB once and it has ran excellent everyday thereafter.
Fleet Mechanic
Joined: May 2004
Posts: 1,436
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From: Montana
well its all personal preferance i pulled it out of the box bolted iton and have never touched it. If you do need to tune it its ashard as reading the instructions. Its not like holley's are the missing link a carb is a carb tune it drive it no sence in wasting another 250-300 dollars. If your having trouble tuningit justjump down in the FE forum and they will hook you up almost all of them run holleys. Scouder runs one on his at 6000 feet with no probs. They know there holley's.