Hunting MPG
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From: Casa Grande
Hunting MPG
I have an 85 F150 300/6 with the 4od trans. 3.50 rear and 31X10.50 tires. My average mpg is about 14 at 60-65 mph. I was thinking the low fuel mileage was due to the setup but after reading other threads and running some of the gear calculators that what I have is about equal to 3.08's and 215/75r15 tires and factory was 215's and 2.47 gears. So why would my mileage be so low when everyone else seems to be getting 4-5 mpg better? Timing and carb are set using a vacuum gauge and it starts and runs great. I am running the factory TFI system but it seems to be running good.
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From: Virginia
Your tires are not totally equal to a 3.08. On paper they may be, but they have some problems;
-They are wider and have a wider footprint on the road. This causes more drag.
-They are heavier
-They raise the truck up higher into the air.
That being said, you are probably only losing 1-2mpg because of the larger tires, but that's going to be part of your problem.
Also, when is the last time the carb has been rebuilt? The new ethanol fuels are hard on carbs, and the newer rebuild kits have different rubber components in them to handle the alcohol in the fuel. I have never seen a carb over 10 years old that did not benefit from a rebuild, and if you let it sit for 6 months or more, the newer fuels pretty much ruin the carbs on vehicles that sit around.
-They are wider and have a wider footprint on the road. This causes more drag.
-They are heavier
-They raise the truck up higher into the air.
That being said, you are probably only losing 1-2mpg because of the larger tires, but that's going to be part of your problem.
Also, when is the last time the carb has been rebuilt? The new ethanol fuels are hard on carbs, and the newer rebuild kits have different rubber components in them to handle the alcohol in the fuel. I have never seen a carb over 10 years old that did not benefit from a rebuild, and if you let it sit for 6 months or more, the newer fuels pretty much ruin the carbs on vehicles that sit around.
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From: Northern California
Also these trucks were geared to get the best mileage at 55 mph. The national speed limit at the time. With the 10 mph increase in speed, and the larger tires, coupled with the ethanol fuels, that could account for the 5 mph difference right there.
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From: North Carolina
When you are at 60-65 mph are you in OD? If so, what is your RPM? Is around 1600 or is it higher? If higher, then your setup might not really exactly equate with a 3.08 rear and 215R75 tires in OD at that speed.
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From: Casa Grande
Thanks for the replies
Heavier, wider tires could be causing some of it I agree. And maybe the carb could use a rebuild, it's only about a year old. There's no sign of a leak when checked with cleaner or butane but, still might be the problem.
And yes the figures were for 55mph but those were 29mpg to. What I'm talking about is the mileage people on here are saying they are getting now with the 300/6 and 4od trans, around 20mpg.
Heavier, wider tires could be causing some of it I agree. And maybe the carb could use a rebuild, it's only about a year old. There's no sign of a leak when checked with cleaner or butane but, still might be the problem.
And yes the figures were for 55mph but those were 29mpg to. What I'm talking about is the mileage people on here are saying they are getting now with the 300/6 and 4od trans, around 20mpg.
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From: Casa Grande
Novak Conversions - Gearing & Gearing Math for Jeeps
Fleet Mechanic
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From: North Carolina
That's in OD at 60-65. I don't have a tach so can't tell for sure but using the gear calc it shows 3.08/215 combo at about 2080rpm and my setup at about 2070rpm at 65, that's figuring .78 overdrive.
Novak Conversions - Gearing & Gearing Math for Jeeps
Novak Conversions - Gearing & Gearing Math for Jeeps
But I am not 100% certain so I don't want to confuse the issue by claiming it is so.
I don't think most people here claim 20 mpg with their 300, and even if so you have to subtract 1-3 mpg from that claim due to exaggeration (human nature)! I'll bet most of us get around 15-18. How do you know what your average mpg is at such a specific speed (60-65)?
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From: Casa Grande
At 60 you are well below 2000, should be about 1670rpm. The 2000rpm was for 70mph. I used that when I ran the calc just because it was a nice round number.
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From: North Carolina
OK, that sounds right then. Good. Well, it could be just due to higher height, wider tires, and above average speed, as others have already pointed out. Little things add up. Once you have run out of gears and can only increase speed by increasing RPM, you are losing mpg. My car has a readout that can show what your actual instantaneous mpg is, it is frighteningly low when accelerating by increasing RPMs! In the moment of pulling away quickly from a stoplight I am only getting 3-6 mpg.
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From: Casa Grande
It seems like the little things are what's doing it. As far as the height of the truck with 31's, it's only about 1.5" taller. 215/75's are almost 28". But then again that's a little thing to.
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