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Old 04-29-2012, 02:56 AM   #1 (permalink)
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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

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Old 04-29-2012, 03:07 AM   #2 (permalink)
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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

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Old 04-29-2012, 09:48 PM   #3 (permalink)
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I've had the same problem. My trips are about 75% urban/suburban and 25% freeway. I've made the same adjustment in the past. Always felt a little guilty. Now, I just use EPA.

All the best.

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