Better gas mileage - PeachParts Mercedes-Benz Forum

pawoSD · #1 07-28-2009, 02:48 PM

Quote:

Originally Posted by BobK

depends on a bunch of factors. What car, what road, what wind direction and speed, how much stop and go, what tire inflation? During the 55 years, it was figured they could have saved more gas if they had keep the speed limit at 70 and mandated 4 more psi in all tires. My SDL got better mpg at 75-80. Found that out when I was making almost daily 200 mile round trips when mom was sick. Went from 23 to 27 mpg. My old '74 Datsun 260Z barely got 20mpg at 55, but got 26.5 at 85mph. Whole lotta factors involved...but that doesn't fit in the safety nazi or car haters agenda.

That would be impossible to get 6.5 more mpgs going 30mph faster....that defies pretty much every law of energy and physics.

In my diesel I have seen the ultimate best mileage going 50-55mph and coasting all the way to stops (as little brake usage as possible) and gentle steady accelerations. I've seen as high as 30.4 in my 300SD.....my W124 can go a bit faster and still get optimum mileage, it seems to peak around 68-72mph.....on cruise control with no headwind and no A/C on....I can get around 26. Usually its around 22-23 though. The diesel gets about 23-24 at 73mph.

However, there are no cars that will get 6+ mpg higher going 85 than if they were going 55. Not possible. Wind resistance goes up dramatically as well as tire rolling resistance and everything else.

WVOtoGO · #2 07-28-2009, 03:12 PM

Quote:

Originally Posted by pawoSD

That would be impossible to get 6.5 more mpgs going 30mph faster....that defies pretty much every law of energy and physics.

I’m thinking that would require a rather high torque (think diesel) engine running at near redline at 55 and into overdrive and running at very low RPMs for the 85, still delivering enough energy to turn the drive shaft/tires.

On a flat surface as well.

I read someplace that the energy required to gain X mph more speed is doubled for the next (and every) X mph gain after that.
Wind resistance is a wicked thing.

BobK · #3 07-28-2009, 03:43 PM

Tires were Michelin XWX 195/70-14. Best street legal tires money could buy at the time. Tire presure was pretty high, rolling resistance was pretty low. Car was small and fairly clean to the air. Six cylinder (156.5 ci) with twin Hitachi SU carbs. They were terrible for fuel economy at lower speeds. Car weighed 2645 with me in it. Factory HP was 162. I improved the air cleaner for lower resistance and installed headers with 2.25 inch pipe back to turbo Corvair muffler. Probably picked up a few HP but not many. Remember, not all cars had super tight closed loop fuel/emissions management systems back then, and tuning multiple carbs was an art (or black magic if you wish). Car got what I claimed and not just once. This was in the height of the 55 and 85 was well, an act of faith. Believe what you want. As I recall, 85 was slightly over the torque peak for that engine.

WVOtoGO · #4 07-28-2009, 03:56 PM

Two things I remember about my cousins ’71 240Z other than it being a heck of a fun little car to rip around the back roads of the San Joaquin valley orchards of Linden CA.
1. You had to add oil to the carbs (dash pots, I think they were called) or it didn’t run worth a darn.
And
2. It did run a lot better at 75+ than it did at 25-30.

That was one funnn little car for a teen like me back then.
Thanks for the memories.

BobK · #5 07-28-2009, 04:06 PM

My cousin had a '72 240Z that he bought off an old lady and then blueprinted the engine. Ran super smooth and got over 32 MPG on a run from OKC to Cincy. Convinced me to buy my 260Z. They really loved to run high speed. The early cars actually had English SUs, I think. In '73 they went with the Hitachi copy and it ran like ...well, it didn't do as well. And yes, you had to put oil in the carbs. It was a type of shock to keep the internal pistons in the carb from jumping up too quickly when you mashed the pedal. I did a ton of experiments with different oil in the carbs and different spark plugs in the engine. I think I bought one set of every brand of plugs made for the car and several heat ranges in a few. NGK BP5ES turned out best all round plug.
Funny note: as I recall, they had an aluminum brake drum with a steel insert for the friction surface. Drum brakes on back, how far we have come...oh wait, some companies still use them.

mpolli · #6 07-28-2009, 04:15 PM

Quote:

Originally Posted by pawoSD

that defies pretty much every law of energy and physics.

Well not EVERY law, but the aerodynamics ones yes!

mpolli · #7 07-28-2009, 04:21 PM

Air resistans increases at the square of the speed, so twice as fast is 4 time the drag (for example). Wikipedia has a good chart of energy usage/losses in a car:

http://en.wikipedia.org/wiki/Fuel_economy-maximizing_behaviors

(scroll about 2/3 down) Engine inefficiency is the big killer overall, but in terms of energy that actually gets to the drive wheels, aerodynamics are the biggest MPG problem on the highway.

Gasoline IC engines are more efficient under fairly heavy load, about 75% as a rule of thumb. This is why pulse-and-glide works.

tyl604 · #8 07-28-2009, 04:34 PM

So guys, no one noticed my snide remark about the value of our time being ignored by folks who want us to drive 55 mph?

Eskimo · #9 07-28-2009, 04:56 PM

Here's a link to the article mentioned in the opening post.

WVOtoGO · #10 07-28-2009, 07:28 PM

Quote:

Originally Posted by mpolli

Air resistans increases at the square of the speed, so twice as fast is 4 time the drag (for example).

I am very aware of that formula with regard to aircraft.
I’m not so sure how it applies to automobiles though, with regard to formulating required power to increase speed. Sure, the basics of wind/air resistance being related at the square of the speed is the same. That's a set law. But, I’d think that with the automobile, the power requirements as speed increases would be at quite a bit greater rate than those of an aircraft. Aircraft don’t have down forces increasing their weight/surface friction with the ground, etc.

pawoSD · #11 07-28-2009, 04:59 PM

Quote:

Originally Posted by mpolli

Well not EVERY law, but the aerodynamics ones yes!

That is mainly the one I was getting at.

But it also goes hand in hand with needing less energy to go faster.

Though, I can understand with a carb'd engine there is a possibility it wastes fuel much worse at some speed than another....