Belly pan

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Under belly pan

The underside of your car is one of its most aerodynamically dirty areas. Most vehicles will benefit by fitting a smooth undertray - something manufacturers are doing more and more where they want to squeeze the most efficiency from any particular model (eg. Jetta diesel vs. gas; Camry hybrid vs. non-hybrid).

Sheet aluminum or corrugated plastic will do the trick. Just make sure that you don't put any flammable materials near hot exhaust pipes.

Instructions for mod


Underside covers can be made out of many different materials, Some suggestions are coroplast (corogated plastic) or metal sheet. When choosing a material make sure it is strong enough to survive wet weather and being hit by small objects picked up by the tires.
The material used around the exhaust should be metal, coroplast will melt and deform. Metal can be used above the coroplast to ensure it does not touch the hot exhaust. The picture on the right shows an example of a frame that is used to mount coroplast to, it keeps the coroplast off the hot exhaust and keeps it generally flat. If a full frame is not used it is recommended to use either a metal frame or metal sheet just above the hot exhaust.

To install the sheet material drill and tap or pop rivet into suitable fixing points on the car. When screwing into the underside of the car check that sharp screw ends dont poke in through the floor of the cabin.

Note: Pop rivets work well when fixing to thin sheet metal areas like the floor of the cabin.


Cut out a suitable shape for the sheet material ensuring the wheels have suitable turn radius and that if a material other than metal is used that it is clear of the hot exhaust.

Screw the cut sheets onto the underside of the car, where sheets overlap arrange the sheets so that sheets at the front of the car are ontop of sheets behind. This makes sure air flows from one sheet to the other smoothly.

Access holes can be cut in the underbelly material for access to oil and radiator spigots. This will be very useful when changing oil.

Note: If you have many modifications planned for the car the underbelly under the engine bay may be the last mod you want to do as it reduces access and therefore makes many other modifications harder.

User experiences

Please enter your user name and any relevant data in the table

User data
User Name Car Make, Model, Year Cost of Mod Time to Perform Mod MPG Before Mod MPG After Mod MPG improvement Instruction Link
MetroMPG Pontiac, Firefly, 1998 56.55 57.78 2.2% A-B-A Tests Metro undertray belly pan
Saand Mazda, 626, 1991 Coroplast $20 5 Hours
Weather Spotter Toyota, Matrix, 2006 Coroplast $15 1 Hour Front half of car only. I did this mod before I had a scangauge but I gained some costing distance. I will call the gain 1-2% MPG (no hard data on this one yet).
jedi_sol 93 Del Sol Si 41.76mpg 42.84mpg 2.58%
puddleglum 07 Rondo EX Based on highway trip mileage numbers,I'm consistently seeing at least 3-5% (1-1.5mpg) improvement at 65-70mph.
cons 04 Ranger $10 1 Hours 30 30.5 4.76% (1.5 MPG) tank to tank data, easier for mpg improvement on the truck
Echo-Francis Toyota Echo 2005 few Screws 3 Hours Not tested. Echo-Francis Frontal Belly Pan
Daschicken Honda, Accord V6, 2006 Coroplast $40 20 Hours 40 42 5% Trip to trip data Rarity- Accord V6 6MT Hypermile/Build Thread

Problems / Consequences of mod

- Underbelly material can cause extra drag if damaged.
- Underbelly can be easily damaged during normal driving.
- Underbelly can rattle in wind if not properly mounted or supported.
- Underbelly can collect water.
- Warmer engine bay, reduced air flow through the radiator.
- Inconvenient access to oil pan and filter for oil changes.


Forum thread links

Aerocivic - how to drop your Cd from 0.34 to 0.17

Autospeed article: adding a chin undertray to a 1st gen Prius

Daox's 2003 Matrix Build Thread

Belly pan thrash 2008 (Saturn SL)

Metro undertray / belly pan

under belly pan for 02 Q45

Belly pan how-to

Aerodynamic Belly Pan on Toyota Echo

New belly pan on a07 Rondo EX

Underbelly Tray

97 civic

Front Belly Pan on a Toyota Echo

Rear Belly Pan on a Toyota Echo

Rarity- Accord V6 6MT Hypermile/Build Thread

External links wind tunnel testing

Flow Field Features and Aerodynamic Drag of Passenger Car <<< Page 23 underbody roughness