Puzzled

[Authorizedcc]

So I have a 289 it used to have a 3 speed on the column and I would get 14.8 MPG. I swapped over to a AX15 and now I'm getting 10.5 MPG. who would of ever thought you would get worse gas mileage with an overdrive? the only other thing I have changed is the carb, might put the other one back on just to check if it could be the carb (both carbs same model)

 

[okie4570]

Carb change could definitely make a difference.

 

[Broncobowsher]

Carb could do it.
Also if you had the right gearing with 1:1 in the transmission you could be lugging the engine in OD, going into low vacuum power enrichment mode. Yes, you can have too much OD and hurt fuel economy.

 

[jckkys]

Exactly what are the 2 carburetors you're talking about?

 

[Authorizedcc]

Exactly what are the 2 carburetors you're talking about?

Holley 1850

 

[Authorizedcc]

Carb could do it.
Also if you had the right gearing with 1:1 in the transmission you could be lugging the engine in OD, going into low vacuum power enrichment mode. Yes, you can have too much OD and hurt fuel economy.

Running 411's with 32's

 

[SHX669]

How many miles/ tanks of gas have you driven to calculate new MPG ? Not uncommon to get some bad gas in one fill up.
Is this all highway miles or all in town or a mix? I get 3-4 MPG better highway than in town driving.
Are you using OD or 4th gear which would be the same as 3rd in your 3 speed.

 

[Authorizedcc]

How many miles/ tanks of gas have you driven to calculate new MPG ? Not uncommon to get some bad gas in one fill up.
Is this all highway miles or all in town or a mix? I get 3-4 MPG better highway than in town driving.
Are you using OD or 4th gear which would be the same as 3rd in your 3 speed.

Just finished a 800 mile trip with it on the interstate, mostly ran in OD at about 70 to 75 MPH

 

[DirtDonk]

Were you getting that 14.8mpg at the same 70-75mph speed? If not, then there's your problem right there!
If you were, then that was outstanding in a Bronco!

Engine rpm or not, trying to push a square Bronco through the wind at that speed is going to kill any fuel economy gains you might otherwise have had.
I could get 15.5 at 55mph, but only about 11 at anything above 65.
Same as your old combination. Stock trans, 4.11 gears and 32" (rated) tires. Never did measure their exact rolling height though.
Which tires do you have by the way?

Paul

 

[Authorizedcc]

Were you getting that 14.8mpg at the same 70-75mph speed? If not, then there's your problem right there!
If you were, then that was outstanding in a Bronco!

Engine rpm or not, trying to push a square Bronco through the wind at that speed is going to kill any fuel economy gains you might otherwise have had.
I could get 15.5 at 55mph, but only about 11 at anything above 65.
Same as your old combination. Stock trans, 4.11 gears and 32" (rated) tires. Never did measure their exact rolling height though.
Which tires do you have by the way?

Paul

No I only ran about 60 with the 3 speed.

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[Broncobowsher]

So you get 14.8 @60 W/O overdrive
Add overdrive and you are now getting 10.5 But are doing 75 MPH.
That sounds about right. Aero is a killer on an early Bronco.

 

[Authorizedcc]

OK I get the center block down the Hwy, but I would think if you dropped your RPMs you should get better fuel mileage. Next trip in October from Florida to Townsend TN, we'll check it again. Of course drop it down around 60mph.

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[half cab]

Never been quite able to get 10mpg outta my Bronco. Always hovered around 9.

 

[Rustytruck]

try driving at 55 that is the sweet spot.

 

[DirtDonk]

As Broncobowsher already mentioned, there is a point of diminishing returns with engine rpm.
Yes, lower rpm "should" equate to better fuel economy, and it can. But only sometimes.
It's pretty much an alignment of the planets thing. It's your engine, your chassis setup, your aerodynamics, and all that. If you lug your engine you're never going to get the fuel economy you would if you were in an rpm sweet spot. It's more about how much horsepower is needed to push the brick through the air at any given speed.

I've tested different speeds in every vehicle I've owned to see which speed consistently got the best mileage. Some it was as high as 65, some as low as 50. One it was actually 90! (more details on that sometime if you like)

If you keep your speed at 60 and under AND your engine is happy at that rpm AND there is no head wind AND there are few uphill climbs in a particular direction of the trip AND your Bronco is not lifted with larger tires, the slower speed and engine rpm might just make up for the alignment of the planets being slightly off.

In other words, just find the sweet spot and it can happen.
But don't bet much on it.%)

Paul

 

[Broncobowsher]

Don't forget that the economy car of that day was the Pinto. The target fuel economy number was 20 MPG. There was a program goal of 2-2-2, as in 20 MPG, 2,000 pounds, $2000. Anything about 12 I consider really good.

 

[Authorizedcc]

try driving at 55 that is the sweet spot.

Sweet spot was 60 before the swap. Lol the fuel mileage will never pay for the swap at this rate

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[Jdgephar]

I lose about 2-3 mpg between driving 60 and 70 mph.

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[m_m70]

got 15.7 last week on a 165 mile drive......mostly flat and all freeway.
RPMs at 2900 @ 65/68 MPH. all info below......
I get around 10 around town normal driving

 

[DirtDonk]

That's a great result for that speed m_m!
I wonder how much of that is the heads, how much the headers, how much the gearing, and how much all three working together?

Paul