All correct.

However, I don't like the term "aerodynamic wall".

There is a limit of speed based upon available horsepower. It's up to the engineer to get the engine to produce that horsepower at the limit of speed. If he does it badly, the engine will be running 500 rpm below its maximum horsepower and the vehicle won't achieve the maximum speed for which it is capable. In this scenario, your "aerodynamic wall" would be 5-6 mph less than the capability of the engine/vehicle combination.

Now, you'd have to redefine your "aerodynamic wall" because it falls at a different speed.

The numbers make this counter intutive - ignore the numbers - ignore the word ratio - in fact I suggest not to use the term "gear ratio" with the words higher or lower - think of higher gear or lower gear

Gear Ratio Terminology

Gear ratio - Wikipedia, the free encyclopedia

Does that help?

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

"Higher" could be "taller" or it could be "numerically higher". Both are opposite.

So, the correct terms are "taller" and "shorter". The 3.46 is "taller" than the 3.92.

Those that use "higher" and "lower" are guaranteed to cause confusion, as evidenced on here all the time.

I would agree that as we're talking about a 240D the term aerodynamic wall - as in the sound barrier - does not really apply...

...However, I think that aerodynamic drag could very well be the reason why the maximum speed in two quite closely spaced top gears would be the same in the real world.

I found quite a nice web page (that is actually more concerned about mpg than top speed in our green conscientious times!) here =>

Velocity & air drag

This must be more so with the introduction of them there new 6 speed manual transmissions - I drove a Renault (says he as he spits on the carpet in disgust!) that just seemed to just make more noise in 5th - I got fed up with it and just went from 4th to 6th like any good luddite would...

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

The maximum speed cannot be the same with the exception of the sole condition that I detailed above.

All anecdotal evidence to the contrary is caused by other variables that are not controlled.

Brian is right that it is too hard to control all variables when testing top speed in real life. Looking at the second graph in my first post, the top speed of my car would only change between 77 and 80 mph (when changing the diff quite significantly). This was for a perfectly flat road with no wind. Any difference in wind or grade would absolutely distort those values.

Mercedes picked a 3.92 diff for my car so that engine was at its peak power production (which occurs at 4200 rpm) at a vehicle speed that required its peak power production. In other words, if it had picked a 4.5 diff, 4200 rpm would correspond to a lower vehicle speed, where the 44kW are not needed for flat highway driving. Basically, it would effectively redline and run out of rpms (as Brian explained earlier). If MB had picked a 3.5 diff, the vehicle would never achieve 4200 rpm (or 80 mph for that matter) for flat highway driving, as the required power to move would exceed the power produced. So basically, they tried to hit a sweet spot with their decision of a 1:1 gear ratio in 4th gear combined with a 3.92 diff.

I will definitely look over the previous threads on changing diff ratio. Somehow I missed that.

Remember, they must always compromise. They decide on the tires and the wheels and then they must select the diff. They don't have unlimited options for the ratios. Whatever is on the shelf is what they must use. So, they are going to be slightly off the perfect ratio, by definition.

What's interesting is that they are not always consistent on the gassers. The M103 powered W126 get a 3.46 ratio and the V8's get a 2.47. The V8 would need to attain 150 mph to achieve approx. 5000 rpm and I don't believe it can do it. The M103 only needs to achieve 107mph to reach it's 5500 redline. It can certainly do that............and possible more.

They took a diff that was too tall for the V8 to save on fuel economy. They reversed themselves with the M103 and selected a diff that was extremely short to give the vehicle "satisfactory" driveability around town. Then they go and take first gear out of the equation on the vehicle. Stupid, if you ask me.

Well the engineers had their parameters.

any design whether it be a building a whizzer motor bike or our revered mercedes benzes is a combination of compromises.

In my car the only explanation for the top speed being the same in fourth or fifth is that as the RPM rises so does the horsepower but only up to a point. The torgue is no doubt developed at a lower rpm.

The amount of power available to push the car through the air is constant if the air density and so forth is constant. So in fourth for example the hp is probably a bit more at lets say 4500 rpm and the torque a bit less.

In fifth the od is about 18% so the rpm will drop maybe 800 at which point the torque probably dominates the hp a bit and it all balances out.

Brian always likes to argue with me about my fuel mileage experiences but his experiences are with gas engined chrysler products from 30 years ago so as it relates to diesels I believe he mainly has his formulas to argue. I have changed the gear ratios in probably a dozen vehicles and my findings are pretty consistent. He'll say my methods are not scientific but I have kept records for the last 20 years in all my vehicles and I will stand by them.

In these MB diesels and in my suburban diesels if you drop the engine speed 20% you realize nearly an exactly corresponding increase in fuel economy.....20%.

It is not as straightforward with gassers. the economy has never fallen when gearing them "higher" but it does not always correspond as precisely as with the diesels.

If Brian has changed his gear ratios I will be happy to stand corrected.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Obviously, you haven't paid any attention when I changed the diff on the SD from 3.07 to 2.88. The fuel economy benefit was about 1 mpg...........hardly the 6.8% that you would claim for such a change.

Others have changed from 2.88 to 2.47 and noticed no change in fuel economy.

So, I'll take your stand as corrected. Diesels don't benefit from the percentage change of the ratio. I'm sure yours don't either but we've had that discussion already.

This statement is telling..............you have no understanding of horsepower and torque and the relationship between the two.

Hmmm, lets see. 307 / 2.88 = 1.065.

Lets say you were getting 27 mpg highway x 1.065 would equal 28.77. That is less than 2 mpg.

What did you find as your results?

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I already explained that I got about one-half of that amount, as predicted.

I think you overstate. I know what my car will do. Come on out to Indiana and I will give you a ride.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Yes, I "overstate"...........others think the same..........I'm an "overstater".

Guilty.

Actually I have an excellent understanding of the two.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.