Two potential fuel-saving ideas


…to get a thumbs up/thumbs down on.

1. Would a 4–>1 exhaust, tuned for highway cruise RPMs, pay for itself? To throw some numbers out there, let’s use my '98 Ford Contour, driven 14k mi/yr (500 gal), returning at least 15% of purchase price per year.

2. Vortex generators (basically thin metal tabs on roof) could theoretically improve aerodynamics by allowing air to stay attatched to the down-curving rear window, producing a smaller area of “dead air” behind the car. Any practical results?


Honestly, I doubt it. You will probably get better results from the usual boring stuff; proper tire inflation, no extra weight in the car, minimize AC and open windows, maintain air filter, maintain engine service, drive a little slower, avoid rapid acceleration and rapid braking, etc.


It is doubtful that anyone here will have good numbers on that stuff. It is all very specific to the vehicle and more important the driver. How you drive makes a big difference in how these things work.

In any case I would not expect much from either, and in fact you might loose mileage.

The three fixes that has the most potential are:

  • The right foot adjustment. Your driving style has the most potential. Slow down drive easy and save.

  • The Owner’s manual. Make sure all the maintenance, including tyre pressure etc. is done

  • The right foot adjustment. Your driving style has the most potential. Slow down drive easy and save.


I have an Explorer with a carrier that fits in the trailer receiver. This gets loaded with boxes that sort of taper down. I get about 1 mpg better with that on a long trip than when I take long trips without it. I doubt vortex generators would do it.


Don’t you already hav a 4 into 1 exhaust? How does your idea differ from an exhaust manifold attached to a single tail pipe? If you mean reducing back pressure, that would provide a modest increase in power. A judicious driver might not lean on the gas quite as much, but the small increase in power might not be perceptible. I’m not sure that there is much of a weight savings as there would be in the cafe-style motorcycles.

  1. I seriously doubt it.

  2. It’s long been established (via wind tunnel testing) and known that the cleaner the cleaner the path is in the rear between the airflows between the top and the bottom of the vehicle, the lower the drag coefficient. Boattails are the best option. Vortex generators, while an interesting thought adventure, would not help.


No, the Contour’s stock, so I imagine (never looked under the heat sheild, though) it’s got a big, cast-iron exhaust manifold that dumps all the exhaust gases together right out of the engine. A “tuned” exhaust is designed with a critical length such that, at a given RPM, an exhaust pulse travels down a header, gets reflected back at the 4:1 collector, and arrives back at the exhaust port just as the new exhaust opens up, creating a standing “low pressure” wave at the exhaust port. The length of the header determines the critical RPM; race cars usually have them tuned to peak HP rpm, but an economy-minded driver could have it tuned to whatever RPM he spends the most time at. In addition to more power, they allow for better exhaust gas scavenging, so perhaps there are longevity benefits, too. General aviation owners find the fuel savings usually substantial, but then stock GA exhaust systems usually stink.

As for the vortex generators, the inspiration was my old '95 Nissan 200SX. One morning, I drove off with a thin coating of dry snow on the rear window, thinking the speed would blow it off. Not on the center of the window. Then I cracked the sunroof open at the rear and blew the snow off, most likely with the edges of the sunroof producing vortices that re-energised the boundary layer and inhibited flow seperation at the rear window.

I’m definitely trying the VGs, at it allows me to tape numerous tufts of yarn to the rear window and do some ‘xperimentin’!


I seriously doubt most cars today have ANYTHING made from cast-iron.
here is a picture of an exhaust manifold:

here is a picture of a 4-1 header: