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Old 08-26-2010, 09:18 AM   #1 (permalink)
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Celica GTS - '02 Toyota Celica GT-S
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Turbocharged Saab hit 99.9 Mpg highway, 51 Avg after fill.

Well, for those who thought you needed a naturally aspirated or electric assist engine to achieve unheard of mileage, think again!

Vehicle: 1998 Saab 900 S 2.0L Turbo 5 Speed Coupe.
MPG Modifications: Gutted interior, only 2 front seats (reduced overall weight by about 200lbs. Hi-flo filter + intake located right next to the engine (gets crazy hot.)

Trip: Belleville, NJ --> Pittsfield, NY = 180 miles each way over mountainous terrain.
Conditions: High humidity, partly cloudy in the low 80's.
Fuel: BP Ultimate (93+ octane rating on pump.)
Techniques employed:
- Acceleration on boost gauge remaining in -10 in/hg of vacuum. (gently depressing 50% throttle max.)
- Speeds not exceeding 65 mph using throttle. (GPS said 61, Speedometer said 63 typically.)
- Drafting behind large vehicles where roadway was full of cars... (never do this on the open road)
- Engine off neutral coasting down every incline. (Speeds vary, please be aware of local laws and speed limits.. lets just say "I got really far" after many of the hills with the engine still off. Vehicle brakes do not work with the engine off, keep this in mind if you are doing this at highway speeds. Simply let out the clutch to restart the car in 5'th gear once speed reduced to below 55 mph.)
- Neutral coasting during in town driving (it's much easier to maintain slow rolling speeds due to less wind resistance.)
- Always parked facing next direction of travel. (avoid unnecessary back-ups and turn around)
Luck factor: There was virtually no stops or traffic in front of me for at least 350 miles of my total trip. I chose good times to travel, and misses traffic on purpose.

It took 2.5 hours of constant downhill coasts I watched the MPG gauge go from the high 50's to the mid 70's for a while. Finally at the bottom of a hill it had maxed out at 99.9 mpg where it remained for the next 30 minutes before it's descent back down to earth at around 35 mpg (construction zones and traffic forced me to slow down out of my "sweet spot."

Stopped at the BP gas pump I had used to fill before leaving, mileage was reset to 0. Upon arrival it was 410 miles, the tank took 8 gallons of gas to fill back up. = 51.25 avg / mpg tank.

The plan for this vehicle was to be fun to drive and perform double duty as my race car, and gas mileage experiment. While some may argue burning up gas for fun at the racetrack negates all fuel savings, I tend to disagree. I find saving fuel to be a great challenge and equally as satisfying as running 13 seconds in the 1/4 mile or running a road course. The benefits of this vehicle aren't in the dollars saved, if money were my main concern.. I wouldn't drive an automobile. My Saab just goes to show "it can be done." You can have a fast / fun car that also sips fuel when off boost to the point that hybrid owners may be envious of.

I have recently purchased a laptop and run a program which basically controls all parameters of the ECU and how it operates the vehicle's engine. I'm working on several new maps, some for increased hp and performance, and some for ultimate fuel conservation. There's a fine line between sipping fuel and pre-detonation when it comes to turbocharged engines though. When reducing the amount of fuel going into the engine, one must also reduce the throttle input to match, or risk catastrophic engine failure.

This, my friends.. is my eco-modding story.



(The top row of pixels are out on my information display, but you can see for yourself.)

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Old 08-26-2010, 09:31 AM   #2 (permalink)
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This is so far beyond what would be expected from the above vehicle with the above techniques that I must doubt the accuracy of your measuring techniques. 99mpg is an obviously erroneous figure. Even the 50mpg fill is suspect. Can you do two 50mpg tanks in a row?
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Old 08-26-2010, 10:10 AM   #3 (permalink)
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I typically get in the mid 40's with mixed city / highway. The key to my success here has everything to do with a low compression engine staying out of boost, and spending just as much time coasting with the engine off, as I do with the engine on. 51 mpg is my record high for this car, last fill was 359 miles / 8.119 gallons of fuel = 44.21 mpg. Prior to that was 423 miles / 11 gallons of fuel = 38.45 mpg.

I take trips upstate NY every few weeks, so of course I will report back next time I return home with new results. I believe If traffic conditions remain the same, I could potentially pull of another 50 tank.

The common misconception most people seem to have is that if you have a turbocharger, you can't get good gas mileage. When in fact, turbocharged engines are more economical off-boost in most cases. 8.4:1 compression requires much less fuel consumption than my Subaru with 9.5:1 naturally aspirated.

Staying out of boost is as simple as not going more than 40% throttle in the first 2 gears, and less than 50% throttle in 3-5 gears. In essence, when I'm driving for fuel conservation, I'm driving only as though the engine were naturally aspirated. I let the 5 speed transmission gearing do the uphill work for me by downshifting to 4'th gear keeping the rpms at 2,600 where this engine makes peak torque - but I stay out of boost as I try to never exceed 10 in/hg of vacuum. At 65 mph on the highway the engine is very near peak efficiency from what I've observed thus far from my records.

The vehicles mpg calculation is not an instant reading, it doesn't fluctuate quickly at all, it climbs very slowly. On this particular occasion I was able to coast for a few miles at a time many times in a row as I rolled through the mountains.. climbing back up-hills was a breeze achieved by my downhill speeds. Now, if one were to discredit the factory gauge reading and call it instant, regardless of any error.. the final fill / math shows that at over 50 mpg the car was getting plenty of time with infinite mileage as I coasted with the engine off.
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Old 08-26-2010, 10:44 AM   #4 (permalink)
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There are a few things worth noting here.

Smaller turbocharged engine are definitely more efficient than larger engines, even when on boost in certain circumstances. It just mainly depends on if you hit fuel enrichment under boost or not and if the engine has to pull ignition timing back. However, the 2.0L isn't a small engine to start with for a car of that size. If you look at the EPA ratings for the 2.0L turbo and 2.0L NA, the NA gets better mileage as would be expected.

A lower compression ratio engine is generally less efficient. You can't compare an AWD car to a FWD car, theres more losses in the AWD due to the drivetrain.

I'm as doubtful as robertsmalls here. My guess would be either you had a good amount of downhill coasting (and the trip back would be much lower), or you had a short fill, or both. I'm not trying to rain on your parade, but that vehicle at that speed just doesn't equal that mileage.
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Old 08-26-2010, 11:52 AM   #5 (permalink)
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Good job getting 50 mpg. I might suggest running low octane fuel since you are still operating in the intake vacuum side of things.
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Old 08-26-2010, 09:22 PM   #6 (permalink)
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I'm not sure what a short fill means, but I went from a full tank / 0 miles on the trip OD to filling until the pump clicked off. After squeezing another dollar into it filling it to the brim, you could consider it about 8.4 gallons which would still equate to 48.80 mpg.

I assure you there's no smoke & mirrors. This has everything to do with the driving techniques here and my relationship with my automobiles. Quite frankly, I hoped to use less gas than I did. I achieved a high of 47 mpg in my dodge Neon with 160,000 miles, about 37 in my Subaru Legacy with 204,000 miles..... so my Saab which tips the scales at 2,700 lbs getting as high as it did really isn't as difficult as it sounds. Each way on the trip was roughly 180 miles, I had the engine off for an estimated 40 miles each way as I rolled down hills effortlessly (and up and over the next one fairly quickly with the speeds I obtained going down hill. Out of my 410 miles, perhaps 70-80 of them were with the engine not running at all. The rest were at (what I call) peak engine performance. 410 -70 = 340. 340 / 8g = 42.5mpg. That ought to help put it in perspective a bit. Free mileage = outstanding numbers. I believe anybody can do it, I'm no pioneer just love to experiment.

Any other theories on why / how I may have miscalculated or what could potentially have skewed my results? I'm always eager to take on other perspectives. Thanks
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Old 08-26-2010, 09:29 PM   #7 (permalink)
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I believe his figures are possible.

Nice work Drive Stick.

His car has fairly good aero and he is using the terrain to his advantge very well.

Especailly if his gearing is allowing him to use lower revs to maintain speed.

Reducing his vehicles weight also allows him to use less energy to climb, as well as less sectional density on the downhill sections which would reduce his terminal downhill speed.

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Old 08-26-2010, 09:31 PM   #8 (permalink)
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current or avg?

My wife's bone stock 2010 equinox very often shows 99.0 mpg on the instantanious mpg readout. No big trick--get to speed , let off the gas. A long downhill coast would keep it there a good long time.

I have not been able to get a trip average mpg any where near the 50's. You are to be congratulated
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Old 08-26-2010, 10:07 PM   #9 (permalink)
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Coasting down and building speed for the next uphill run is the way to do it for the best mileage. Most use their cruise control which does just the opposite....
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Old 08-26-2010, 10:32 PM   #10 (permalink)
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I do find it hilarious when someone tells of success that there are 10 people waiting in line to tell him that he mis-filled his car, that he had wind at his back, he didn't do aba testing, the 3 or more tanks that he filled were all short? and didn't give true results. It is quite frustrating. If he is in a hilly or mountainous terrain, getting double the EPA rating is fairly easy. Plenty of people are getting 25-30% better on flat terrain. Believe what you want, but you dont need to always call BS to feel important.

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