Streamlining Light Truck Fuel Economy

On October 2, 2004 I drove from Las Cruces, NM to Odessa, Texas to meet Philip Knox who was driving his aero test 1994 T-100 PU from Sanger, TX to meet me. The Toyota T-100 is a stock PU with a 2.7 liter four cylinder engine and a five speed manual transmission. The truck is rated at 25 mpg by the manufacturer.

Phil has had one previous baseline test when he drove to Breckenridge, TX and averaged 24.107 mpg on a round trip to and from Sanger, Texas. He had a half bed cover on the truck for this test. We will use this as a baseline because the truck has 200 K miles on it and is not in new condition.

For this trip to Odessa from Sanger and back ( 786 miles) Phil was testing a full bed cover with a cab high gap filler tapering to the tailgate. See attached pictures in following e-mails. On the first leg of the journey Phil traveled 426 miles on 15.527 gallons of gasoline averaging 27.436 mpg. On the leg back Phil traveled 360 miles on 13.077 gallons of gas averaging 27.529 mpg's. The average for the 786 mile trip using 28.604 gallons of gas was 27.478 mpg. This amounts to a 12.77 % improvement over a half bed cover stock truck.

Using a figure of .44 for the CD of the stock truck and using the standard of 2X the percentage of milage improvement to recalculate the CD we have 100% minus 24.56 % = .7546 as a multiplier. A coefficent of drag of .44 x .7722 = .332 CD. We might as well round off to .33 CD which is a significant improvement from .44 CD of the stock truck.

Rear wheel skirts were the only other improvement to the T-100. Conditions for the trip were far from idea with a quartering wind from the north for the first leg of the journey. On the return trip on October 3, 2004 it rained for 260 miles of the journey back to Sanger from Odessa. Phil recorded the higher milage on the return trip. Speed was kept to between 65 mph and 70 mph. Temperature ranged from 63 degrees F, to 64 degrees F, and the humidity ranged from 55% to 75 %. Barometer was at 30.33 inches.

On a side note I averaged 25.83 mpg in my brand new 2004 Toyota Highlander with a 2.4 liter 4 cylinder engine matched to a 4 speed automatic transmission with overdrive. The truck used 14.119 gallons of gas to travel 364.7 miles. Stock the vehicle is rated to have a CD of .34. The speeds I drove on the return trip were from 70 mph to 80 mph. I think this vehicle would do better if average speeds were kept down to below the 75 mph speed limit. Care must be taken to avoid downshifting on hill by cause of the cruise control. I was constantly changing my speed because I was operating the truck during the break in period which is recommended in the owners manual. Not bad for a stock SUV that weighs 3,500 LBS.

Phil beleives that although weight is definitely a factor in milage performance it is not that critical for highway mileage. It shows up more in city stop and go driving. Air resistance is a form of vehicle mass in that two vehicles with the same weight but one with a lower CD (all things being equal) will get different mileage figures. Aerodynamics is the critical component on vehicle efficiency, with alot of room for improvement in light trucks.

Paneling the underside of the T-100 and reducing the parasitic losses from the radiator would probably give this truck a 30 mile per gallon capability. This would mean a drag reduction of from .44 to .29 would net a 16.7 % improvement in highway mileage. Vehicle utility would not be significantly effected but the truck would definitely look exotic. The tested bed cover with tapers to the tailgate really needs compound curvers to be even more effective. This model bed cover is for testing a proof of concept only.

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