fuel economy

Messages
385
Location
Montreal, canada
Hi guys, I always wonder how come my present car and my former car always get better mpg during 80mph compared to 65mph. Present car: Cadillac CTS 2005, 3.6l vvt 65mph/2100-2150rpm=28-30mpg 70-75mph/2300-2500rpm=24-26mpg 80mph/2600-2700rpm=29-34mpg I did some test on the same road, same day, same weather, cruise control was set, same outside temp and humidity(or almost). Former car: Buick Regal 2002, 3.8l 65mph/1750rpm=25mpg 70-75mph/2000rpm=28mpg 80mph/2100-2150rpm=32mpg BTW, the data I give you is from the DIC for the CTS. As for the Buick, it was the calculation from gas bills compared to miles I did. I am always curious why do I have to go faster to save fuel. Kinda weird to me and thats why I'm asking here!
 
Messages
4,422
Location
Guilford, CT
Engines all have their "sweet spot" where they operate at peak efficiency and get the best mileage. If they're running any faster or slower, they'll get worse mileage. So, your Cadillac apparently gets its best mileage when it's running at about 2600-2700 rpm. Try and keep it there as much as possible.
 
Messages
971
Location
Boston
Something in your data maybe askew? Wind resistance would make your results unlikely. In most gasoline engines of your size... 1800 rpms would likely use the least amount of fuel.
 
Messages
4,622
Location
Western Washington
It's hard to believe that an engine would use less fuel at 2700 RPM than at 2100 RPM. Makes me wonder about the accuracy of the DIC in your Caddy. I always get better mileage when I slow down.
 
Messages
4,844
Location
Saskatchewan
It's unusual that your engine efficiency increases enough at higher speed and load to overcome the additional aerodynamic losses, but it's certainly not impossible. However, I think it's more likely that either your fuel economy display was confused or the wind was inconsistent, and that your fuel economy records do not account for wind conditions.
 

Spykem4e

Thread starter
Messages
385
Location
Montreal, canada
thanks for the replies guys, at 1800rpm, my caddy is doing about 52mph and of course, I get around 30-32mpg. I checked all my receipts from previous business trip and I have the habit to write down the mileage done on the trip and approx the speed set on cruise. Except the wind condition: Trip A speed sets at 105kmh (montreal-rouynnornada): 621kms, I get 9.3L/100km (mostly uphills) RouynNoranda-Montreal: 625kms, I get 8.7L/100km (mostly downhills) Trip B speed sets at 120kmh (montreal-rouynnoranda): 621kms, I get 8.8L/100kms RouynNoranda-Montreal: 626kms, I get 8.1L/100kms When I checked all the same business trip with the bills/kms, I get about the same results except during winter I get about 10.0 and 9.5 to come back. And of course, if I exceed 80-82mph, the car starts to consume more gas. at 90mph I get approx. 24-25mpg. It always poke my curiosity! For the rpm versus fuel economy, when doing like 20mph, the DIC says something about 15L/100kms when at 1250rpm just like if the gear was geared so the engine has to deliver power to maintain the speed... still a mystery to me...
 

PT1

Messages
5,746
Location
near the mistake
To do a real comparison you have to measure the fuel accurately....forget the DIC. Plus most fuel tanks are not ful when the punp clicks off and can vary by as much as 2 gallons depending on the sensitivity of the filler backpressure setting on the filler.
 
Top