Roof rack and MPG

Messages
694
Location
Saskatchewan, Canada
I currently have 4 Pontiac Montanas in my taxi fleet, two 01's and two 03's. They are identical in terms of drivetrain, wheel base, useage, maintenance, and most optional items. The big difference I've noticed is that one of the 01's consistantly gets 2-3mpg better fuel mileage than the others. The only thing I can attribute that to is it's the only one without a roof rack. My question is, is it possible for a roof rack to make that big of a difference in mileage(~13%)? Has anyone tested this out? Or is this van just a factory freak? Because for 13% better fuel mileage, I'll tear all the racks off, seal the holes with silicone and toss them in the trash...lol! [Big Grin]
 
Messages
4,478
Location
Central Arkansastan
This may sound strange, but I have been told if you remove the crossbars only, that your mileage will go back up, and it is the crossbars themselves that kill the mileage..
 
Messages
956
Location
Eastern burbs, MN
I'm going to try this out on my '01 Windstar (remove the crossbars). I'm going on a 300 mile trip this weekend for my daughters state softball tournament and I'll see if this van gets any better mileage. I'll give a report Sunday afternoon.
 

olympic

Thread starter
Messages
694
Location
Saskatchewan, Canada
Yeah, please do! One of my vans has an "average MPG" function in its information center, it should have a nice average built up from the past few months. It drives the same route with the same driver every week so maybe I'll remove the cross bars, reset the computer and see what happens. It's accurate to 0.1MPG so it'll show if there was any improvement at all.
 
Messages
2,230
Location
Southern Ontario
quote:
Originally posted by olympic: One of my vans has an "average MPG" function in its information center,
My 2000 Montana has no 'average MPG' feature [Frown] . Which year is this vehicle Olympic?
 
Messages
4,646
Location
The Garden State
quote:
Originally posted by Tim H.: This may sound strange, but I have been told if you remove the crossbars only, that your mileage will go back up, and it is the crossbars themselves that kill the mileage..
Interesting you say that. My wife has a 2005 Eddie Bauer Explorer with the 4.6L V-8 without the cross bars on the roof rack and she gets the same mpg as her 2002 XLT Explorer with the 4.0L V-6 that had the cross bars on the roof rack. You'd think the V-8 would get worse mpg than the V-6 in the same vehicle. Plus her 2005 has more options and is heavier than the 2002 [I dont know] . Whimsey
 
Messages
1,855
Location
Australia
I have a Land Rover Defender 130 Crew Cab, which literally has the aerodynamics of a brick. The difference between a pair of Rhino Roof bars with a 6' step ladder up there, and a clean roof is a repeatable 10+% increase in fuel consumption. Stick the 8' steps up there and it increases again.
 
Messages
7,077
Location
Ontario, Canada
Take off those crossbars. Do your vehicles have holes in the front where fog lights would go? If no fog lights, make sure the holes in the front end are sealed up with a smooth cover, or they will also increase drag. If they do have fog lights, you could make some clear plastic covers to smoothen the airflow over the fog light holes, which are normally recessed. In fall, cover half the radiator opening, and you will reduce drag as well. When temperatures are below 0C, completely cover the radiators. I have an indoor/outdoor thermometer in my car, with the outdoor part under the hood, to make sure I don't overheat the stuff there with my experiments. The front end of the vehicle is where high pressure air builds up while driving. This should be made to pass around the rest of the minivan as smoothly as possible, and not be allowed to leak into places where the pressure will be lost. You need to use some of the air for cooling, but as much as possible should be sent over the hood. Another idea: the front wheels do most of the braking, so the front wheels need ventilation to get rid of the heat. You can improve fuel economy even more by smoothly sealing up the holes in the hubcaps on the rear wheels. This way, they won't be pumping ventilation air that's not needed.
 

olympic

Thread starter
Messages
694
Location
Saskatchewan, Canada
If I have time tomorrow I'll pull the rack off my 03 Montana with the MPG computer and track the results in a month or so. My vans do have foglights in the bumper but the holes are really odd shaped so covering them up with lexan would be near impossible. Hopefully the weather around here holds out until the end of October so I can get a fair comparison.
 
Messages
956
Location
Eastern burbs, MN
I got 20.24mpg over 324 miles this weekend with the crossbars off and the van loaded with 2 kids, 1 wife, softball gear and overnight bags. Almost all the of the miles were highway at 70-75mph. Usually with the van loaded like that going at those speeds I get about 19-20mpg. Not a real scientific experiment, but maybe a 0.5mpg improvement. I'll leave them off and track the mileage until I need to mount the cargo box.
 
Messages
350
Location
Indianapolis, IN
Ditto on removing the xbars. The rails won't effect MPG much at all, but the xbars act like air hooks. Many years ago, I had a diesel Rabbit. Got 49 mpg [Big Grin] . With bike rack = 43. 1 or 2 fork-mounted bikes = 38 [freaknout] .
 
Messages
3,094
Location
Metro Detroit
My new minivan ('05 Buick Terraza) comes with the side rails, but no crossbars. You can order the crossbars as accessories from the dealer (which I did), but they advise you to only install them when needed as they affect gas mileage. So this is just my roundabout way of saying that I agree you should just remove the crossbars.
 
Messages
7,077
Location
Ontario, Canada
Cooler weather will increase drag because the air is heavier, so you might not notice an increase, but it will be helping. If each crossbar on average creates one kg of drag force, and you go 10,000 km, you can calculate how much energy is eaten up by them. 2 kg x 9.8 = 19.2 N of force 10,000 km = 10 million metres force x distance = 19.2 x 10,000,000 = 192,000,000 joules of energy. There's 3.6 million joules in one kW-hr, so that works out to 53 kW hrs of energy. A gallon of gas has about 40 kw-hrs of energy, and the engine can extract maybe 6 out of that, so the crossbars will burn about 8.8 gallons of gas every 10,000 km. If you drive those crossbars 400,000 km, that would be over 350 gallons of gas.
 
Messages
2,604
Location
MN
I have a 98 Montana with a roof rack. I still get about 25 mpg on long trips. Around town and with my everyday driving I get about 20mpg according to the built in computer.
 
Messages
2,230
Location
Southern Ontario
quote:
Originally posted by tmorris1: I have a 98 Montana with a roof rack. Around town and with my everyday driving I get about 20mpg according to the built in computer.
tmorris1 and Olympic. I have a 2000 Montana. Does mine have a fuel economy 'calculator' built in!? [I dont know] Is it an option on these vehicles?
 
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