Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - GM: Chevy Volt's Cd is 0.28, Prius is 0.30 (tested in multiple tunnels), EV1 is 0.21

DieselHybrid · 12-10-2009, 10:21 AM

Hi- great site!

I question the validity of the statement regarding the Volt's superior aerodynamics.

CdA= Cd * wetted area

Dubious claims of Cd aside... let's focus on frontal area.

Without taking into account side mirror area (btw GM's claim of "drag neutral" mirrors is ridiculous)- the Volt is supposedly 70.8 wide by 56.3 inches high while the Prius is 68.7 inches wide by 58.7 inches high. However, the Prius does not have as deep an airdam as that which is installed on the Volt.

Assuming an average ground clearance of 5.5 inches for the Prius and 3.0 inches for the Volt- frontal area would be 25.4ft^2 and 26.2ft^2 respectively. Advantage: Prius.

Yes, the airdam serves to direct high speed flow away from the vehicle's relatively "dirty" underbody. It's an easy/cheap fix that GM is exploiting to help earn impressive EPA hwy fuel economy ratings with their latest crossovers and SUV's.

However, deep airdams also increase frontal area. (Cd of an object is measured as the fraction of its drag when compared to the drag from an equivalent flat plate area.) Of course, with a larger equivalent flat plate area, GM should possess a lower Cd.

This would help explain GM's claimed low Cd of 0.28, vs. the Prius Cd of 0.30.

Careful shaping of a vehicle's underside or underside fairings can reduce drag better than an airdam- whose increase in frontal area may mask an overall potential drag reduction. Reference Aptera's lack of an airdam as an example.

An early morning coast-down test with each vehicle would help dispell all the aerodynamic conjecture.

Finally, in the absence of independent testing, I would not place much faith on any manufacturer's claims.

12-10-2009, 10:21 AM	#42 (permalink)
DieselHybrid EcoModding Lurker Join Date: Nov 2008 Location: Annapolis, MD Posts: 27 Mom's Taxi - '05 Toyota Sienna LE 8-passenger 90 day: 22.59 mpg (US) Diesel - '09 VW Jetta TDI Thanks: 0 Thanked 0 Times in 0 Posts	Hi- great site! I question the validity of the statement regarding the Volt's superior aerodynamics. CdA= Cd * wetted area Dubious claims of Cd aside... let's focus on frontal area. Without taking into account side mirror area (btw GM's claim of "drag neutral" mirrors is ridiculous)- the Volt is supposedly 70.8 wide by 56.3 inches high while the Prius is 68.7 inches wide by 58.7 inches high. However, the Prius does not have as deep an airdam as that which is installed on the Volt. Assuming an average ground clearance of 5.5 inches for the Prius and 3.0 inches for the Volt- frontal area would be 25.4ft^2 and 26.2ft^2 respectively. Advantage: Prius. Yes, the airdam serves to direct high speed flow away from the vehicle's relatively "dirty" underbody. It's an easy/cheap fix that GM is exploiting to help earn impressive EPA hwy fuel economy ratings with their latest crossovers and SUV's. However, deep airdams also increase frontal area. (Cd of an object is measured as the fraction of its drag when compared to the drag from an equivalent flat plate area.) Of course, with a larger equivalent flat plate area, GM should possess a lower Cd. This would help explain GM's claimed low Cd of 0.28, vs. the Prius Cd of 0.30. Careful shaping of a vehicle's underside or underside fairings can reduce drag better than an airdam- whose increase in frontal area may mask an overall potential drag reduction. Reference Aptera's lack of an airdam as an example. An early morning coast-down test with each vehicle would help dispell all the aerodynamic conjecture. Finally, in the absence of independent testing, I would not place much faith on any manufacturer's claims.