maximum MPG at what MPH?

Dean_S

Veteran Member
Joined
Mar 29, 2001
Location
Lawrence Kansas
TDI
BMW 535d x-drive
Yes, power required varies as the cube of the speed. But, if you go twice as fast, you will get your trip over in 1/2 the time. So the energy required for a given trip will vary as the square of the speed.

The point of maximun torque is the point of maximun efficiency? Perhaps from a thermodynamic point of view, but this real engine's torque peak is determined by a computer, not thermodynamics. Where is the post that showed the efficiency peaking at around 2600 rpm? Yes, that was for a euro engine, not a castrated unit for the north american market.
 

green+blue

Veteran Member
Joined
Apr 3, 2001
Location
so.ill.
Sure power required at the wheels increases as a cube of speed but IMHO the light airplane analogy fails in one key aspect. Transmission of power from engine to wheel. Now my flight time is limited to little Cessnas, Pipers and Aeroncas, but none of them had a variable ratio gearbox or a variable pitch propeller, if you wanted more speed you increased throttle, in my Jetta when I want more torque at the wheel I have two options, I could try driving at 5000 rpm in first or I could shift up to a higher gear and keep the engine running at (or just a tad above) its peak rpm. I remember the power curve on TDI has a big 'sweet spot' starting about 1950. I think the reason that I have personally found greatest mpgs at 2100-2200 or so rpms is that the computer control is compensating for the power lost in transmission to the wheels and is providing fuel at the lowest rate consistant with the engines torque curve considering the additional energy required to spin the drive ect... This is just a guess based on driving two Jetta TDIs over the past nearly three years. If my hunch is correct then 5/6% of the power is lost to mechanical drag. But, since the increased torque AT THE WHEELS due to taller gearing is about 100%, 50%, 50%, 33% as you go from 1-5th, the increased wind resistance would have to be considerable for you to not get higher mpgs at higher mphs AT THE SAME RPM.
 

goldandblack

Veteran Member
Joined
Feb 13, 2001
Location
Boilerville USA
TDI
TDI 97 Passat
GSXR600,
This has nothing to do with RPM'S and fuel milage....I see you are from Hill AFB and last month we had two of your F-16 pilots here at our freedom festival. Captain Shepard sure put on a show and what the F-16 can do. He came over to the crowd at the airport and talked to us before he went up. If all the fly boys are like him, we are in good shape. The Navy Blue Angels were the stars of the festival but Captain Shepard sure did credit to the Air Force. If you know him, tell him he made a hit with us in Evansville Indiana.
 

Oldman

Top Post Dawg
Joined
Feb 3, 2001
Location
Leander,TX,USA
let me say as my best guess 40 MPG in 5th gear. CarTest software free on the internet can help calculate this BTM.

My wife's 5 speed auto Vovo has a MPG computer. Surprise that best MPG is kind of leaner from 45 to 55 MPH stead state (36 MPGs). MPH below that MPG falls off, MPH above that MPG falls off at about the same rate i.e. MPG at 35 MPH = MPG at 65 MPH, I estimate this to be around 32 MPGs steady state.
 

Old Navy

Top Post Dawg
Joined
Jun 15, 2001
Location
Ozark Hill's in Missouri, USA
TDI
None now, .
Like jayb79 my '00 NB Tdi loves to idle at 28 MPH down the main street of our town. It's a brick road about 2 miles long with out a stop with a 25 MPH speed limit, then goes to 35 mph and changes to asphalt for another 2 miles before coming to stop sign. Durring the 35 mph section I engage the cruise control and maintain 35 mph. That amazes some of my occasional passangers.

My best milage before I retired was 73 mpg. I was on flat back country roads going to work. It was 50.1 mile commute and the ecomony run was done for 5 days and includes 3 stop lights, 2 stop signs and two yeild signs. It was early fall with lows in 50's F to high of mid 70's. When I told my co-workers they were agast.

The following week a fellow teacher who also commuted the same route as I tried to out run the NB and was telling his students later (autobody class) that he was out run from 60 mph to top end by my little "4 banger diesel Bug". That's the fastest I have ever gone (110mph)in bug. It was still excellerating but getting a little lite on the ground so I backed off.

His V-6 Jimmy at 60 mph couldn't match my bone stock Tdi.

Couple of days later the school director had words to the effect that we should not talk like that around the students. Wonder why?
 

Old Navy

Top Post Dawg
Joined
Jun 15, 2001
Location
Ozark Hill's in Missouri, USA
TDI
None now, .
Like jayb79 my '00 NB Tdi loves to idle at 28 MPH down the main street of our town. It's a brick road about 2 miles long with out a stop with a 25 MPH speed limit, then goes to 35 mph and changes to asphalt for another 2 miles before coming to stop sign. Durring the 35 mph section I engage the cruise control and maintain 35 mph. That amazes some of my occasional passangers.

My best milage before I retired was 73 mpg. I was on flat back country roads with 55 mph speed limit going to work. It was 50.1 mile commute and the ecomony run was done for 5 days and includes 3 stop lights, 2 stop signs and two yeild signs each way. It was early fall with lows in 50's F to high of mid 70's. When I told my co-workers they were agast.

The following week a fellow teacher who also commuted the same route as I tried to out run the NB and was telling his students later (autobody class) that he was out run from 60 mph to top end by my little "4 banger diesel Bug". That's the fastest I have ever gone (110mph)in bug. It was still excellerating but getting a little lite on the ground so I backed off.

His V-6 Jimmy at 60 mph couldn't match my bone stock Tdi.

Couple of days later the school director had words to the effect that we should not talk like that around the students. Wonder why?
 
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