What RPM at 60?

[stinkgod]

My 89 300te runs 3000 Rpm at 62 mph. This seems high. I know I'm getting all 4 gears, and i do get 23-4 mpg on the highway.

Not that i can afford it, but aren't there high geared diffs for these?

zack

[las769]

That sounds about right my 90 300sel reads the same thing. double your tach reading should be your speed

[lkchris]

This is normal.

The engine doesn't make enough power at lower rpms to pull 60 mph, because if it did that's the way Mercedes would have designed it.

[cjlipps]

Because I am at 75 at 3000 rpm in my 1990 300E (and that's a verified, timed speed). Is it possible that your tach and/or speedo are inaccurate? I guess maybe my tach could be inaccurate....
Chuck.

[stinkgod]

In a test, in the early '90s Car & Driver noted that the 300 did not accelerate as well as the other cars in the test because the axle ratio had been raised (numerically lowered) a couple of times.

Would it be feasable to swap rear ends or gears?

Zack

[PaulC]

Are you comfortable with acceleration abilities under 70 MPH? A taller gear will usually have a negative impact on this aspect of performance. Don't forget that a wagon is typically geared lower than a corresponding sedan in order to accomodate the heavier unladen weight of a wagon, and to keep the engine in a fatter section of it's torque curve at lower speeds for towing and/or heavy cargo loads.

As you have apparently surmised, making a ratio change to pick up a few more mpg or a few less rpm at cruising is a loser financially.

[S-Class Guru]

Ah, my favorite pet-peeve with the heavier 6-cylinder cars.
I love my 300SE, but if I could find that transmission designer, I would
ring his bell!

Yes, apparantly your TE has the same 3.45 diff ratio as the 300 SE/SEL.
This is the biggest engineering goofball that Mercedes ever pawned off on us.
They built a 4-speed tranny, but some silly rule about being able to pull a zillion tons up the side of the Alps, forced them to put in a super-low first gear. This made the 300E jumpy and harsh, so they decided to have it take off in 2nd.
All this wasn't too bad with the taller 300E diff gear; but when they dropped the 300 engine in the S-class, the poor old dog wouldn't start off in 2nd so well (or haul that zillion pounds up the Alps). So, the easy fix was just to gear the poor engine down until it screamed its guts out on the highway.
Effectively they gave us a 3-speed non-converter-lock tranny with a drag-race gearing.
As a result, the wear and tear on the engine and all the accessories is huge.
No wonder we have water pumps, fan idlers, alternators, etc; failing too often.
And the gas mileage is awful also, for a 300E engined car.

My engine would easily pull my 300SE down the road in 4th with 25% taller gearing. Sure, it might have to downshift occasioinally to pass or take a steep hill, but so what - all modern cars do that.
Why couldn't they just give us 4 workable gears and a good diff gearing?

So, what we have is a great car with a willing engine, hamstrung by a horrible transmission design and even worse diff gear selection.

I've threatened a few times to re-gear the diff, and do one of the 1st-gear-start mods, then tweak the speedo to match. However, the 1-2 shift is designed to be pretty harsh in the SE, so I keep backing out on the mod.

Ah, I feel better, still love the old screaming gas hog though.

DG

[cjlipps]

Mine is definitely a slug off the line and at lower speeds and doesn't start to feel right until about 70. If I didn't drive so much on the highway I would like a lower rear end gear. Apparently they need a ratio between yours and mine.

[Jackd]

Because I am at 75 at 3000 rpm in my 1990 300E (and that's a verified, timed speed

The 1990 300E had a 3.07 differential. 3000rpm at 75mph is not possible with a 3.07 differential.

[Rockman59]

As I recall back in my NHRA hot rod days.....the tach reading was about 1/2 the speedometer speeding when using a 4:10 positraction rear end. 60 mph equaled 3000 rpm. With the 4:56 gearing at 60 mph the engine was turning about 3500 rpm. So if you are running 62 mph at 3000 rpm your car has some pretty low gearing, probably close to 4:10...

[S-Class Guru]

Rockman, the tire diameter of the older cars makes a big difference.
My SE came with the small 15" 205/65. At 62 mph it turns 3000 rpm, and it absolutely has a 3.45 diffy. At first I assumed it had about a 3.90, but I checked it and got 3.45, then found 3.45 stamped on the case.

As a side note, a car with a 3.07 and same size tire would do approx 69.6 at 3000 rpm.

I like my Corvette gearing better - 3.42, but with a 50% overdrive in 6th. 1500 rpm at 75mph, and 30 mpg with 400 hp. Too bad they can't build a body/interior like Mercedes.

DG

[softconsult]

Yes, '92 300E 205 x 15's 70 mph is right at 3,000 rpm.

Steve
'92 300E Sportline 242K

[Rockman59]

I like my Corvette gearing better - 3.42, but with a 50% overdrive in 6th. 1500 rpm at 75mph, and 30 mpg with 400 hp. Too bad they can't build a body/interior like Mercedes.
________________________________________________________________
I bought a new Vette in '85, the second year of the new body style and advanced engines and transmissions. That car would consistantly pump out
28 mpg at 70mph. Of course no one would ever believe it...unless they too had a Vette. Just goes to show you can make some big horsepower and still have decent fuel economy if you do it right.

[lkchris]

Originally Posted by Rockman59

I like my Corvette gearing better - 3.42, but with a 50% overdrive in 6th. 1500 rpm at 75mph, and 30 mpg with 400 hp.

The Mercedes engine is only 183 cubic inches and therefore doesn't have the power to pull the car 70mph at 1500 rpm.

Double that rpm and it's like having 366 cu inches, isn't it?

[cjlipps]

Originally Posted by Jackd

The 1990 300E had a 3.07 differential. 3000rpm at 75mph is not possible with a 3.07 differential.

Why not? Are you saying the rpm is high or low?

A useful formula:

RPM=[(336)(FD ratio)(trans ratio)(MPH)]/tire diameter in inches. Of course with a non locking torque converter you have to assume some slippage (maybe 3-5% - not sure what a good number is here).

You can also solve for the other variables by algebraic manipulation.