Constant speed, best MPG guidelines

Andrei Rinea

Veteran Member
Joined
Jan 21, 2002
Location
Europe, Romania, Bucharest
TDI
VW Tiguan 4Motion 2.0 TDI 170HP (engine CBBB)
If I drive at a constant speed ("highway") which is the best speed and what gear is best ?
Does it have to do with the RPM of maximum torque? I seem to have good MPG at 1900 RPM (max torque for my 1Z engine) in 5th gear? Are there better options?

And if max speed is a problem, let's say in town at long constant trips when I can't exceed 70 km/h. Should I keep it in 5th at 1500 rpm (or so) or in 4th at 1800??!

Please describe me some guidelines? What is best? A higher gear no matter what low rpm I get or the rpm has to be in a certain range? Any input is verry welccomeeeeee!
TIA!
 

Rammstein

Top Post Dawg
Joined
Sep 3, 2002
Location
Québec city baby, Canada
TDI
Golf 92 D
Check this page out: fueleconomy.gov

Check out the little graph under "Observe the speed limit".

The best graph ever done to show how fuel economy changes versus the speed you’re going at.

It goes down around 55-56 mph(88-90 km/h). There's a plateau between 47 mph (76km/h)and 55-56 mph(88-90 km/h). Also a peek at 30 mph (48 km/h).

These are the speed you'll be going at, with the tallest gear engaged, in order to maximize your fuel economy.

Read the whole link, very informative.

Rammstein.

[ January 12, 2003, 19:17: Message edited by: Rammstein ]
 

Brioscooter

Veteran Member
Joined
Dec 3, 2002
Location
Baltimore, Md.
TDI
2004 and 2005 Golf TDIs
It seems that 40 MPH in 4th gear would be the most efficient, as that puts the engine's RPMs at the torque peak. Also, wind drag isn't so much of an impediment at that speed or below.
However, most people don't travel at that speed on the highway. It is too slow (legally), and very dangerous.
55 MPH (88 KPH) in 5th gear would put you at the same RPM sweet spot, but wind drag would be a factor at that higher speed. This leisurely pace will also get you run over in my area (Baltimore/Washington DC).
I have gotten fairly decent fuel economy 47+ MPG)at my usual speeds of 65-70 MPH. This includes a lot of stop and go traffic. A road trip would bring better than 50 MPG economy, no doubt.
 

Brioscooter

Veteran Member
Joined
Dec 3, 2002
Location
Baltimore, Md.
TDI
2004 and 2005 Golf TDIs
It seems that 40 MPH in 4th gear would be the most efficient, as that puts the engine's RPMs at the torque peak. Also, wind drag isn't so much of an impediment at that speed or below.
However, most people don't travel at that speed on the highway. It is too slow (legally), and very dangerous.
55 MPH (88 KPH) in 5th gear would put you at the same RPM sweet spot, but wind drag would be a factor at that higher speed. This leisurely pace will also get you run over in my area (Baltimore/Washington DC).
I have gotten fairly decent fuel economy 47+ MPG)at my usual speeds of 65-70 MPH. This includes a lot of stop and go traffic. A road trip would bring better than 50 MPG economy, no doubt.
 

SwimmerDave

Veteran Member
Joined
Sep 1, 2002
Location
Decatur, GA
TDI
2014 JSW 6MT
I would prefer to travel/commute at 1900rpm in 5th gear, but going 55mph would get my ass run over in Chicago. About the slowest I can safely travel is 65mph in the far right lane. Even then, I'm getting passed by 99% of the traffic on the road. However, I am getting about 50mpgs on the highway at that speed.
 

TDIinTX

Veteran Member
Joined
Sep 12, 2002
Location
Dallas, Texas
TDI
2002
As far as city driving goes, one way of thinking of it, from an economical standpoint, is that there are basically four elements, other than it's own fuel-burning engine, that makes a vehicle move. Being pushed, being pulled, momentum, and gravity. Out of these four, the only one we really have any control over in typical driving situations is momentum. I don't claim that any of this is revolutionary by any means, but going a step further, it is a law of physics that it is easier to keep something from starting than to stop it. For example, imagine you're moving a refrigerator on a dolley: you won't have too much of a problem keeping the refrigerator from tipping over backward as long as you don't let it go too far. But once it's past a certain point, it will require considerable strength to keep it upright. The same goes with a moving vehicle, if you think of taking off from a dead stop as stopping stillness as opposed to starting motion. In driving, by maintaining momentum as much as possible (i.e., swerving around a slowing vehicle rather than slowing behind it, and staying out of the right lane in urban driving to avoid this situation repeatedly), you are preventing the loss of motion. The reason cruise-control is so effective at maximizing fuel economy is because is allows you to easily sustain the energy of momentum that the car is carrying along. Nothing you're not already familiar with, I know, just a different way of looking at it.
 

Chris Hinds

New member
Joined
Jan 13, 2003
Location
Crawley, UK
Experience driving the Passat (Estate) 110 varient in the UK suggests that keeping the RPM as low as possible in driving without labouring the engine is the key to achieving good economy. I frequently drive round town at 30mph in 5th gear (barely 1000rpm) with no problems. The Passat has a built in trip computer and today I recorded the following (instant economy) read outs at 30mph (readings on a virtually flat road, speed set using cruise control)

3rd Gear, just under 2000rpm, 33.2mpg
4th Gear, 1350rpm, 49.2mpg
5th gear, 1100rpm, 78.0mpg

The real key to economy though is conserving momentum. My personal best on a journey was over a 15mile journey with an already warm car. The area is not known for being flat in any way (North Devon, UK) and I achieved 74.8mpg according to the computer. In the journey I only braked once and then only lightly. Anticipation and keeping plenty of distance between you and the car in front is the biggest help to economy.

Chris

N.B. all figures in Imperial MPG
 

Brioscooter

Veteran Member
Joined
Dec 3, 2002
Location
Baltimore, Md.
TDI
2004 and 2005 Golf TDIs
Like everything else, obtaining the best fuel economy comes down to a balance.
Driving around at 30 MPH will get you hit just about anywhere other than out in farm country.
Driving fast enough to match the flow of traffic so as to NOT get hit will use more fuel.
Driving fast enough to get a speeding ticket will use still more fuel.
To me, it isn't worth causing an accident to squeeze every last mile out of a tank. I try to keep within 5-10 MPH above the speed limit, which means I am only getting passed by about a third of the traffic. I can still get in the high 40 MPGs, and about 700 miles out of each tank.
Diesel is still relatively inexpensive in the US at $1.50 +/- per gallon. A nice balance.
 

SpamJ

Veteran Member
Joined
Sep 6, 2001
Location
Cable, MN
TDI
Jetta Wagon 2002 Silver
Remember, the speedometer reads high, so when you think you're cruising at 70 MPH, your really doing a touch more than 66. Makes for better fuel mileage though. But also help explain why everybody is going MUCH faster.

I know, they still drive fast, but run a GPS and you'll get a better idea of how fast.

Sam '02 Wagon

Who set's his cruise for 4.5 MPH over the speed limit and doesn't have to worry about a 400 dollar Valentine.
 

gdr703

Veteran Member
Joined
Jun 13, 2002
Location
Vancouver, Canada
TDI
Golf 2 door 2002 Indigo
As I understand it the best fuel efficiency for the tdi engine is attained at about 1750 rpm and 80% power.
5th gear gives the least residual drive train drag.
Then it follows that the best mpg should be in and around 1750rpm in 5th.
(Or in 5th at 1100 to 1700 rpm.)

hth.

[ January 14, 2003, 09:40: Message edited by: gdr703 ]
 
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