I saw this topic has been asked several times and nobody has solid experience with it so I thought I would post mine.
I took a trip last week to Drummond Island on the border of Michigan's Upper Peninsula and Canada. It's a 400 mile drive from my house in the Detroit area. Everyone said I was stupid for considering taking my Fiero instead of the F-150 so of course that means I had to try with the main benefit being overall fuel economy. I went alone and took camping gear so the car was fairly full. I used a Thule rack system which clamps down on the top with four rubber pads and hooks that grab under the sides of the roof panel. Then I attached Thule Kayak J-racks. The roof system costs about $4-500 but I got it for free from a very awesome friend and PFF member. The J-racks are about $100 but they go on sale for $85-90 sometimes. I found them for this price at Dunhams a couple years ago. I do not have T-tops or a sunroof on the car. It is a '87 2M4 base coupe with 5 speed. The car struggled up Michigan's rolling hills even, the aero drag is just too much. It slowed to almost 60 in top gear though I could have downshifted. If there was someone behind me I probably would have. I'm guessing the kayak weights between 40 and 50 pounds and the racks another 30 but it's just so big. The rack instructions say to use front and rear straps if traveling over 35 mph but in the case of the Fiero I would use them at all times. I've never had the roof panel off but I think it's glued on, right? The straps help pull the entire assembly back down onto the car so the load isn't constantly pulling up. I tightened them fairly snug on both ends. The main issue I have with this whole setup is paint damage. This is a $600 car and I repainted it myself for $280 so I can't say I care a whole lot about it's looks. But if you are picky about scratches, especially on black, I wouldn't try this. The marks you can see are actually left from the rack during winter. I had it up top for about 3 months during the winter to move cross country skis and snowboards. You can't really see the lighter marks offset from the big ones that are from the kayak trip. I already waxed these areas so this is as good as it's going to get. Black is a curse but I still like it the most.
I traveled between 65 and 70 about 90% of the trip. The car averaged 29.83 mpg over 805 miles. This saved a little over $50 if the truck would have gotten a generous 18 mpg. Probably not worth the setup time and cost to most people but I like driving my Fiero. No real power issues (other than normal duke slowness) until entering the highway. I pushed to 78 mph on some downhills and this setup stayed rock steady until you get into someone's dirty air. Not just semi trucks, even small cars could upset the boat from several hundred feet back. The nose would wiggle 1 - 2 inches and I was only thinking about this scratching the roof so I moved over or slowed up when encountering traffic. There were some curvy roads and I hit them fairly well, no issues or movement. The front strap is attached to the metal bumper by sliding the strap hook between the hood and bumper cover. The back strap curves under and attached to the metal subframe. I cut a clean towel and wrapped the straps where they contact the body to minimize any scratching. They did not leave marks at all. This worked perfectly for my kayak which is 12 feet. You might want to look at a different front strap location if yours is really long as it would pull forward on the plastic bumper cover. Anyways, enough blabbing, here are the pics.
[This message has been edited by Cheeze_87 (edited 06-30-2015).]