Fuel Economy

From TDIClub

• EPA fuel economy numbers

In case you are wondering, EPA fuel economy numbers can be found on http://www.fueleconomy.gov . These are derived from a 1970s-era test (gentle acceleration and braking within the capabilities of 1970s cars, and highway speed averaging 48mph); reported numbers for 1985 and newer cars are reduced by 10% for city and 22% for highway from the lab results.

• Coasting (down hills, or when approaching red lights)

Please DO NOT turn off ignition or shift to neutral when coasting in order to save fuel, because for TDIs downhills are free! Yes, that's right. When there is no fuel requested, no fuel is injected! Like most modern fuel injected cars, no fuel is injected when the engine is being turned by the wheels when coasting in gear. Shifting to neutral forces the engine to take fuel to maintain idle (and may be dangerous because you do not have any engine braking), and turning off the ignition is dangerous since you may lose power assisted steering and brakes.

• 
• Maximum torque occurs at around 1800 RPMs. This means greatest fuel efficiency. This is where you want to be when driving at a constant speed at any gear. Maximum horsepower means least fuel efficiency. The sweet spot for spirited driving with good mileage from each tank is somewhere in between.

The TDI engine is capable of producing 30HP as low as 1300 RPM, more than necessary to pull the car along a level road. However, at this point EGR is at maximum, and you stand a good chance of a clogged intake. Please use this advice wisely. Remember, the TDI was designed for the highway. It has a formidable turbo, and is not to be lugged around at low RPMs. This may cause a surge in the turbo pressure. Best method to use: accelerate quickly to your desired speed, staying in boost, and using a heavy right foot. Once there, stay there and be mindful of road conditions that may be to your advantage. Don't forget about the Italian tune-up.

• Drive your own road.

Drive responsibly, share the road. Drive purposefully, at your own speed. Select your route to avoid stop streets, slow roads, and fast highways. Opt to drive on roads with good road surfaces. Opt to drive at a time of day that avoids rush hour or traffic build ups. Know what the speed limit is, and drive appropriately. Do not drive in the pack. Do not hinder other road users, let them pass. Do not let other road users intimidate you. Do not let another road user tailgate you, drive the speed limit. Do not tailgate, use the 2 second rule.

• Gears.

Use 5th gear whenever you can. Use the highest gear possible at all times. Dont be afraid to change gear. Get used to it. Change up, change down. The best time to change gear, is the first time you think about it. At 40mph the engine uses 20% less fuel in 5th than in 4th to produce the 6 hp needed to keep the car moving on a flat road. But, that is getting close to the lugging condition, so be prepared to change gears should it be necessary. In fact the engine uses 20% more fuel to produce the same HP at any speed in 4th than in 5th. Clearly 5th gear is the best gear for mpgs.

• Brakes.

A good way to convert fuel into heat and dust. Avoid using them. Adjust the speed of the car using your right foot only. This means you have to read the road ahead, and drive smoothly. Use the brakes only: at the end of your journey; at stop streets; at red lights; for policemen; pedestrians; and idiots; Avoid all other uses of the brake pedal. Excessive use of the brakes costs you in fuel, in brake pads, in rotors, and in time to get these items replaced.

• Cruise control.

Use the cruise, it does a pretty good job, except when driving downhill. Get used to using the cruise control as part of your driving technique, applying it when appropriate.

• Right foot.

The TDI engines fuel efficiency, (converting the BTUs in the fuel into HP at the wheels) varies from a high of 45% to a low of 12%. The 45% occurs with a heavy right foot, the 12% occurs with a light right foot. At 1750 rpm the range is 45% to 15%. At 3500 rpm the range is 36% to 12%. Therefore: use a heavy right foot at low rpm, whenever you can. To get better mpgs you need to use the engines best fuel efficiency. The TDI is fuel efficient when you use a heavy right foot. It is fuel inefficient when using a light right foot. Use the some or none technique, ie a heavy right foot or no right foot. - Drive with purpose. Romp up hill, and coast down hill.

• RPM.

Keep the RPM down. But, be careful to not lug the engine, which might occur below 1300 rpm. Lugging is a condition where the power required is more than the power available from the engine. Typically in a gasser this is stated as keep the revs above 2000 Most 2litre gassers have 30hp available at 2000 rpm. The TDI has 30hp available at 1300rpm. Therefore the equivalent statement for the TDI is keep the revs above 1300 rpm to avoid lugging. When slowing down keep the car in 5th until the rpms are less than 1000 rpm, then change down, as this will minimize the engine braking effects. When driving on a flat road, with only the driver in the car it will pull in 5th from 1100 rpm without lugging, and on a downhill even at lower rpm. Get to know your cars capability in this rpm range. But, dont forget to exercise the car from time to time through the whole rev range. Say once a week, when accelerating on a highway on ramp, hold second gear and stomp on the go faster pedal and let the rpm go through to the top. It blows out a whole bunch of crud, and keeps the engine sweet. The engine responds well to a touch of variety. By the way, the 90HP TDI can produce 20HP at 1000 rpm, 30HP at 1300, 41HP at 1500, 56HP at 1900, 71HP at 2500, 82HP at 3000, 87HP at 3500.

• Cold Start.

Observing the mpgs I return when I make 1 journey of 200 miles, vs 10 journeys of 20 miles, indicates to me that there is a cold start penalty equivalent to 3 miles. I think this cold start process takes place over the first 10 miles. The temp gauge shows the engine coolant is up to temp in 2-5 miles, but, I am of the opinion that this process is not complete at that point, theres more too it. For the first 10 miles of a journey the TDI engine (not just the coolant) has to get to operating temperature, as does the gearbox, wheel bearings, tires, brakes, etc etc. It takes +/- 10miles, regardless of conditions, or how you drive. The only thing that can be done is to reduce the losses, ie keep them closer to 20% rather than 30% (2milesworth rather than 3milesworth). In other words drive careful during the first 10 miles. Do not blast off from a cold start, or you will experience a cold start penalty that is larger. The glow plugs operate to help warm up the car, but only if you keep the rpm below 2200. If you exceed 2400rpm the glow plugs shut off. So keep it below 2200 rpm til the temp guage is off the bottom three marks. I have wondered about the TDI heater or Zerostart device that keeps warm water in the radiator, and how this might help to reduce the Cold Start Penalty.

• Driving off.

Get in the car, fasten seat belt, start the engine, and drive off. Do not let the engine idle, to warm up. The engine warms up quicker when its under load, and the mpgs benefit because you are using the fuel to go somewhere. The lube oil in the engine is up to pressure and distributed throughout the engine in the first 5 seconds or less.

• Getting up to speed.

5th gear is the economy gear. Your goal is to get into 5th gear as fast as is reasonably possible. Use first gear to get the car rolling. Within 4 cars lengths you should have gone through 1st, and 2nd, and have engaged 3rd. Do as much of your driving as you can in 5th. You will be changing gear below 3000 rpm. So flip your way through those gears in the 1800-3000 rpm range. Target to get into 5th at 50mph and 1800rpm on a flat road. Higher rpm of course on an uphill, lower on a downhill. Sometimes youll need to pull away harder, do it pedal to the metal in the highest gear possible.

• Speed.

The optimal speed, on a flat road, is around 55mph. (+/- 1800 rpm in 5th) The higher the speed the more effort it takes to move the air out of the way. (As airspeed doubles, the force of the wind quadruples, any yachtsman will tell you that.) The optimal speed is not 30mph because of the engines lousy fuel efficiency at real low rpm. However this optimal speed is often not possible because it is too fast (legally) for an urban road speed limit, and too slow (sensibly) for a highway. Travel at a speed fast enough to allow the continual use of 5th gear, and as slow as road conditions permit. Be careful, do not hinder other road users. Tailgating a big truck can give significant reductions in wind resistance but is really not sensible. Keep your distance, observe the 2 second rule, you will get some drafting effect at 60 mph and faster. Less than 50mph, its just not worth it, anyway. As a rule of thumb these numbers are very possible: 40mph = 80mpUSgallon 50mph = 70mpUSgallon 60mph = 60mpUSgallon 70mph = 50mpUSgallon 80mph = 40mpUSgallon

• Slowing down.

Dont use the brakes. The brakes are very good at stopping the vehicle, and in converting fuel into heat and dust. Avoid using them for slowing down. Over-run is the method of choice. It is the safest. At low speeds the engine braking effect is minimal. In some locales coasting and freewheeling are illegal. Over-run is where the car is in gear, no right foot, no brake, the car is traveling along subject to engine braking effects. The engine consumes no fuel. Coasting is where the car is not in gear, the engine idling, the car is traveling along the road without any engine braking effects, but the engine is consuming fuel to keep it idling. Free-wheeling, is where the car is not in gear, engine is off. How do you steer? As there is no power steering. And how long will the brakes remain effective for? Not a good idea. Coasting is of use under certain conditions (>50mph). The amount of fuel used (0.5 litre an hour) may be less than the engine braking effect. It depends on the road conditions. If in doubt use over-run. When using over-run allow the rpm to drop to 1000 before changing down, as this will minimize the engine braking effect.

• Stopped.

Turn the engine off if you think you are going to stand still for more than 10 seconds. No use burning fuel to go nowhere. Restarting the engine uses as much fuel as 10 seconds worth of idling.

• Driving Uphill.

This is where you can operate the engine close to its most efficient, take advantage of it. Just dont overdo it and get into a lugging situation. Lugging, and changing down is not good.

• Driving Downhill.

Driving the car gently down a hill is a very inefficient use of the engine. This engine converts fuel to power most inefficiently when asked to make a couple of hp only. The difference can be as much as 3 times as much fuel consumed to produce a HP. (Ie 15% efficiency instead of 45% efficiency) Do not use the cruise when going downhill. Let the car over-run. Do not use the right foot to maintain speed going down a gentle slope. If the hill is not steep enough for the car to maintain speed, let it slow down, (or knock it out of gear so it will maintain speed careful, maybe illegal). Do not maintain a light right foot to keep the speed up. Go into a downhill, adjust the cars speed (faster or slower) so that it can go down the hill with no right foot, and no braking required, and at the bottom of the downhill come out at your desired speed. Coasting in gear is always preferable for mpgs when your speed is below 45mph. Above 50mph it may be preferable for mpgs to coast out of gear (engine is idling, consuming fuel) assuming that you can handle the extra speed without using the brakes. And, assuming its not illegal to coast out of gear in your area. The engine idling consumes, say 0.5 litre per hour, thats enough fuel for 2.5HP. Above 50mph the engines braking effect is more than that. If the road conditions, and traffic are such that you can coast out of gear, then coasting can show a better MPG at speeds above 50 mph. If in doubt over-run, (coast in gear). Then you wont have burnt the fuel just to idle the engine while coasting, that is in case you have to use the brakes.

• 2 second rule.

Dont tailgate. Leaving a larger space avoiding having to react to slight changes in speed by the vehicle ahead (the added braking that one does when tailgating wastes fuel). In addition, leaving a larger space allow you to have more visibility of the road and traffic conditions ahead, allowing you to anticipate slowdowns and speedups, allowing you to be pro-active instead of reactive in your driving. Use the 2 second rule to check your following distance (but don't focus just on the one vehicle in front of you all the time). When the vehicle in front of you passes some mark, count; One crocodile, Two crocodile, and then you should pass the same mark. Its a simple rule to check yourself when driving in traffic, and especially at higher speeds, that you are maintaining a reasonable distance between yourself and the vehicle in front. In some places people may argue that if they leave a 2 second gap others will cut in. Thats got nothing to do with your driving, do not get lured into driving in someone elses style. Decide on, and stick with, your own driving habit. Can you drive an alternate route, or at a different time of day? Do not drive in the pack, drive your own road.