Lifting instead of lowering Bronco II

[ngrimm]

This boxy thing will never get great mileage anyway but is there a point where lifting a vehicle no longer decreases mileage if it has a full belly pan? One variable is that it has 4.10 gears with overdrive so I was thinking about replacing the 27" tires with 30" Supposedly they will clear without any lift. I wouldn't mind going even larger with a lift. I'm not getting into the tipping issues just the mpg. Any thoughts on this?

[bryn]

i would say if you were lifting, via suspension or body mounts it would be a negative, but as you are only going with taller tires. i say go for it. you may even get more air flow under the bronco because of the added space. the the effect on your final drive ratio will definitely be improved.

[ngrimm]

I do have something of a benchmark using the MpGuino gauge. The highway I do the testing on is slightly uphill so at 55 mph, 2200 rpm, using the cruise control I average 24 mpg and the other direction it gets 27 average. 3800 lbs with me in it so any hill really brings down the average. I also want to make a removable top so I am thinking about angling it down toward the rear to see if that helps.

[euromodder]

Bigger tyres will mean bigger frontal area and more drag.
That's going to take away part of the effect of going for taller gearing through taller tyres.

Calculate where your engine rpm will end up.
Then consider wether your engine likes that rpm range or not.
My car won't take a longer gearing without remapping the ECU for more torque at low rpm - as has been done from the factory on the later models.

Bigger tyres are also going to weigh more, and use more energy to get them rolling.
If you do long motorway stretches @ constant speed, fine as far as the tyres go - but the aero effect is going to hurt more in that environment.
If you do much driving around town, the higher mass has to be accelerated often, and they'll likely hurt.

[basjoos]

The main reason that lowering a vehicle improves mileage is that, in conjunction with installing a front airdam, it diverts the air around the draggy underside and it exposes less of the wheels to the oncoming airflow, thus slightly reducing the frontal area. If you raise a vehicle that has a smooth underpanel installed, then the only drag increasing effect is exposing more the wheels to the oncoming airflow and increasing the frontal area. Raising a vehicle reduces the drag of the air passing around the vehicle since the air is displaced less as it splits around the vehicle. Instead of air being able to pass only around the top and sides of the vehicle, the air can pass around the top, sides,and bottom of the vehicle, so it is displaced less as it splits in front of and then recombines behind the vehicle. If you look at a lot of the ultra high efficiency solar challenger type vehicles, they raise the vehicle's body very high above the road and use skinny wheels with fairings to reduce the drag increase from the tall wheels. They, in essence, turn the vehicle's body into a low flying aircraft fuselage supported on tall, thin wheels. To get the same effect on your truck, you would need to keep the wheels skinny (think of the skinny tires on the mail delivery trucks of the past) and install fairings on front and behind the wheels to divert the air flow around them.

On my vehicle, it do this to some extent as I have a smooth underpanel and have enclosed my wheels in a pontoon that diverts the air around the wheels for the length of the car. My vehicle's nose has a low stagnation point so most of the wind goes over and around the sides of the vehicle, but I have arranged for a good portion of the wind to go under my vehicle where it helps to fill in the area under the boattail at the back. It also allows possums and other small animals to pass untouched underneath my vehicle, which wouldn't be the case with a lowered, airdam equipped vehicle.

[ngrimm]

Thanks to all of you for your replies. I was thinking about tapering the belly pan from just under the front bumper under the suspension and transfer case thus eliminating the air dam. The gas tank is behind the rear axle and tapers up. I was also thinking of removing the side skirts and angling filler panels from the rocker panels down to the belly pan since that part is still uncovered.