EcoModder Forum Calculating Combined Rating

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Coroplast Engineer

Join Date: Nov 2011
Location: suburban Chicago
Posts: 80

AV - '04 Chevy Aveo sedan - not LS
90 day: 34.28 mpg (US)

Jolteon - '00 Honda Insight MT
Team Honda
90 day: 77.25 mpg (US)
Thanks: 27
Thanked 25 Times in 11 Posts
Calculating Combined Rating

I just wanted to make a general announcement because I've seen this mistake being made a lot lately. Calculating the combined fuel economy rating for your car is not as simple as (.55*city rating)+(.45*highway rating). The EPA has a special formula for calculating fuel economy at Fuel Economy - On-road Vehicles and Engines | Cars and Light Trucks | US - EPA
Here is the formula for anyone who doesn't want to click on the link.

FEcomb = 1 / (( .55 / city FE) + (.45 / hwy FE))

For my car at 23/31 (city/hwy) the combined rating is
1 / (( .55 / 23 ) + ( .45 / 31 )) = 26, which is the correct combined rating.

I have also taken the liberty of calculating the actual rating that I should be getting using different percentages for city and highway (75% and 25%, respectively).

1 / (( .75 / 23 ) + ( .25 / 31 )) = 24.6

My new combined rating is therefore adjusted to 24.6 mpg. This helps me keep track of the money, gas and CO2 that I have actually saved.

Hope this helps

__________________
Quote:
 Originally Posted by Tango Charlie You know how when you buy a car, and then you start noticing how many others there are on the road? Yeah, that doesn't happen with a 1st gen Insight.

 The Following 3 Users Say Thank You to slowbro For This Useful Post: capnbass91 (04-29-2012), D.O.G. (06-11-2012), landsailor (04-29-2012)
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 04-29-2012, 03:07 AM #2 (permalink) Master EcoModder     Join Date: Aug 2011 Location: Warren, MI Posts: 2,456 Boo Radley - '65 Ford F100 90 day: 13.28 mpg (US) Thanks: 782 Thanked 665 Times in 408 Posts Also, they just give you the figures: Fuel Economy of 1996 Eagle Summit Wagon Fuel Economy of 1988 Alfa Romeo Milano Fuel Economy of 1991 Sterling 827 __________________ He gave me a dollar. A blood-soaked dollar. I cannot get the spot out but it's okay; It still works in the store
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