Fuel Economy Question.....
02-27-2002, 10:36 PM
#1
Administrator
Thread Starter
Fuel Economy Question.....
Ok, not sure if this has ever been answered before, but I know how someone finds out what their fuel economy is, and how many miles per gallon they're getting. BUT, my question is this. Does it really matter what speed your going in order to calculate your fuel economy? Like lets say you get 28 miles per gallon highway. I assume that would be going the standard speed limit. BUT, what if you do like 100 mph for a long distance. Would you still get 28 miles per gallon, or would your fuel economy drop because you were going faster and making the engine work harder? 
Just something I was thinking about when I got into work this morning at 5 am. rolleyes.gif *lol*
Just something I was thinking about when I got into work this morning at 5 am. rolleyes.gif *lol*
02-27-2002, 10:51 PM
#2
Senior Member
Join Date: Aug 2001
Posts: 198
Likes: 0
Received 0 Likes
on
0 Posts
I've noticed that when I drive at or near the posted speed limit, 70, I get around 27mpg for a tankfull. When I set the cruise at 90 or 95 I average much less. Right around 21 or 22 mpg for a tankfull. I attribute it to this: The faster you go, the more rpm's you're running. The more rpm's, the more a/f mixture your engine sucks in.
Not to mention the extra fuel it takes to make the power to get up to that speed in the first place.
I know it's affected by how hard I'm stomping on the fun-pedal in town too though.
I hope this logic makes sense, as I'm sitting at 27.5 hours with no sleep. rolleyes.gif
Not to mention the extra fuel it takes to make the power to get up to that speed in the first place.
I know it's affected by how hard I'm stomping on the fun-pedal in town too though.
I hope this logic makes sense, as I'm sitting at 27.5 hours with no sleep. rolleyes.gif
02-27-2002, 11:12 PM
#3
Super Moderator
Join Date: May 2001
Location: Rancho Palos Verdes, CA
Posts: 11,851
Likes: 0
Received 2 Likes
on
2 Posts
Vehicle: 2008 Toyota Prius 2006 Suzuki SV650S
The force of the wind (aerodynamics) goes up by 4 every time you double your speed. So, regardless of RPM, the engine has to work 4 times harder to push the car through the air at say 100 MPH vs 50 mph.
To give you an idea. Car and Driver some time back tested a turbo corvette. It topped out at 198 MPH. They asked the builders (calloway) why they didn't just "boost" up the engine to top out at 200 or over 200. The Calloway engineers informed them that the car would need an extra 35 HP to reach that extra 2 MPH. Rather that mess with their engine...the Calloway engineers went back to the drawing board and came up with a new aerodynamics package, that allowed the vette to reach 201 MPH...with the same HP.
Speed GREATLY affects gas mileage. Not only because of the higher RPM, but because of the increased load on the engine due to the car forcing it's way through the air.
Want great gas mileage? Shift at 2000 rpm and don't go over the posted MINIMUM speed limit. (just stay in the right hand lane please!)
[ February 28, 2002: Message edited by: Random ]
To give you an idea. Car and Driver some time back tested a turbo corvette. It topped out at 198 MPH. They asked the builders (calloway) why they didn't just "boost" up the engine to top out at 200 or over 200. The Calloway engineers informed them that the car would need an extra 35 HP to reach that extra 2 MPH. Rather that mess with their engine...the Calloway engineers went back to the drawing board and came up with a new aerodynamics package, that allowed the vette to reach 201 MPH...with the same HP.
Speed GREATLY affects gas mileage. Not only because of the higher RPM, but because of the increased load on the engine due to the car forcing it's way through the air.
Want great gas mileage? Shift at 2000 rpm and don't go over the posted MINIMUM speed limit. (just stay in the right hand lane please!)
[ February 28, 2002: Message edited by: Random ]
02-28-2002, 12:47 AM
#5
Senior Member
Join Date: Jul 2001
Posts: 749
Likes: 0
Received 0 Likes
on
0 Posts
Yep, it's all about drag. For anyone who's interested, Drag force = Cd*(0.5*rho*V^2)*S
where
Cd is the drag coefficient
rho is the density of air
V is the velocity
S is the wetted surface area
Corvettes have one of the lowest Cd's of any production car (~0.26). We're pretty low too at around (~0.32). Viper's are awful at 0.45 and Pickup trucks top out around 0.5. These figures are from memory, so forgive them if they're off a bit.
Body kits can improve aerodynamics, but not in the traditional way. Most kits lower parasitic drag under the car (with ground effects). This lowers venturi drag, which is caused by the interaction between a fast moving plate (car) and a stationary plate (road). This creates a low pressure region on which sucks the car backwards. Lowering the car and minimizing the distance on the sides and front lessen this parasitic drag. Theoretically, raising the car up really high lowers the drag too, but then you seriously affect handling
Venturi effect is actually a good thing for providing downforce and helping high-speed cornering. However, one of the first rules in aerospace is: whenever you create lift (or in this case, downward lift), you also create drag.
Here's a good website on race aerodynamics: http://www.nas.nasa.gov/About/Education/Ra...r/glossary.html
ac
[ February 28, 2002: Message edited by: Curtas ]
where
Cd is the drag coefficient
rho is the density of air
V is the velocity
S is the wetted surface area
Corvettes have one of the lowest Cd's of any production car (~0.26). We're pretty low too at around (~0.32). Viper's are awful at 0.45 and Pickup trucks top out around 0.5. These figures are from memory, so forgive them if they're off a bit.
Body kits can improve aerodynamics, but not in the traditional way. Most kits lower parasitic drag under the car (with ground effects). This lowers venturi drag, which is caused by the interaction between a fast moving plate (car) and a stationary plate (road). This creates a low pressure region on which sucks the car backwards. Lowering the car and minimizing the distance on the sides and front lessen this parasitic drag. Theoretically, raising the car up really high lowers the drag too, but then you seriously affect handling
Venturi effect is actually a good thing for providing downforce and helping high-speed cornering. However, one of the first rules in aerospace is: whenever you create lift (or in this case, downward lift), you also create drag.Here's a good website on race aerodynamics: http://www.nas.nasa.gov/About/Education/Ra...r/glossary.html
ac
[ February 28, 2002: Message edited by: Curtas ]
02-28-2002, 07:06 AM
#9
Super Moderator
Join Date: May 2001
Location: Rancho Palos Verdes, CA
Posts: 11,851
Likes: 0
Received 2 Likes
on
2 Posts
Vehicle: 2008 Toyota Prius 2006 Suzuki SV650S
QUOTE
Originally posted by Iago:
Second rule in aerospace: with enough thrust, ANYTHING will fly.
Second rule in aerospace: with enough thrust, ANYTHING will fly.
Enter the original JATO rocket car
02-28-2002, 10:06 AM
#10
Senior Member
Join Date: Feb 2002
Posts: 463
Likes: 0
Received 0 Likes
on
0 Posts
Hey for you guys who want a very accurate way to figure out gas milage, just set your trip button when you fill up to a full tank. Don't reset the button till the next fill up but before you reset, you can calculate your mpg this way....
1. After filling up, take the amount of fuel you filled your tank with and divide it INTO the distance you traveled indicated on your trip. Say you traveled 280 miles so far and you fill your car up with 10 gallons of fuel. 280/10=28 mpg.
Cool eh?
1. After filling up, take the amount of fuel you filled your tank with and divide it INTO the distance you traveled indicated on your trip. Say you traveled 280 miles so far and you fill your car up with 10 gallons of fuel. 280/10=28 mpg.
Cool eh?