Manual shifting of an automatic tran

[Mercedes Fred]

The speedometer on my '88 300e has roman numerals I, II, III (sans serif) at provocative locations. Can one safely manually shift the trans to sate one's urge to go fast? Is that the purpose of those curious numbers?

Furthermore, the nomenclature of the roman numerals does not seem to match the shift. does 2 on the shifter = I on the speedometer, 3=II and III=D?

III on my 300e is set at 160kmh; this is ok?

anon!

fd/

The marks on the speedometer indicate the suggested maximum speed in the various gears. You can go a bit higher, but then the rev-limiter in the fuel pump relay shuts the fuel off momentarily. The "I" mark is for first gear (even though your shifter doesn't have a "first gear" marking since it is only actuated from the kick-down switch). The "II" mark is for second gear...and so on. They don't give one for fourth gear (though some of the old diesels had a "maximum" mark)...I guess it's because at top speed your attention had best be on the road. The marks are also handy for knowing when it's okay to down-shift to a lower gear without blowing a chunk out of the block.
Randy D.

[bobbyv]

those marks on the speedometer are the max speeds for each corresponding gear.

when upshifting, you have a tach that you can use, to ensure that you do not exceed the redline. When downshifting however, it is more difficult to predict what engine speed will result if you downshift to a lower gear, at a given vehicle speed. More importantly, you cannot quickly tell if redline will be exceeded when the downshift is performed.

those speedo marks therefore are for downshifting. In my 5spd 190e, the "III" marking for 3rd gear is at (about) 100mph. If I am at 4th gear at 110mph and braking for a turn (or an off-ramp), and am planning to downshift to 3rd, I simply look at the speedo needle - when it gets below 100mph, it is safe to shift to 3rd gear, at which point it will be at redline in that gear. This is especially important if you are performing a heel-and-toe downshift (braking while downshifting).

there is usually no mark for the top gear because there is no other gear to downshift from.

But even in my 5spd 190e, there is no mark for 4th gear. This is because the car is not redline-limited in 4th, i.e., I can attain top speed in 4th with the engine below redline. Rather, it is drag-limited or governor-limited (or both). This means that i can safely downshift from 5th to 4th at any speed.

however, if I can take my car down a steep enough slope with the wind at my back and make it exceed the rated top speed while in 5th (but still below redline), then I cannot quickly tell what speed above the rated top speed will it be safe to downshift to 4th, when required, since there is no mark for 4th on the speedo.

same is true if you are coasting down that steep incline at a speed higher than the rated top speed with the transmission in neutral . You cannot quickly tell if it will be safe to place the transmission in top gear at that speed because there is no marking for it in the speedometer.

[Mercedes Fred]

perhpas those markings on the speedo are meant exclusively for manual trans and they didnt bother to remove them for the automatic?

the beaters i have driven for just transportation all made it clear thhat one ought not use any other selection than "D" on the transmission, unless posed with a steep grade, etc. However, my '68 GTO has an automatic that was clearly meant to be shifted either by user or by the trans. The 300e shifter seems notched in a manner similar to the old GTO, as in steps without an "idiot button" to prevent it from moving, hence implying that it ok to manually shift the automatic, like the transmission was designed for it. However, I dont want to screw around wth it unless im sure...

anon,
fd/

[longston]

Just so no one gets confused, I have been told that the roman numerals indicate shift sequence points, not gear numbers. In other words, "I" indicates the first shift point to second, "II" the second shift point to third, and so on.

There is nothing wrong with manually shifting an automatic if it is done properly. It's better done with a tach, and is done properly if you stay below the redline. The idea is to keep the engine in it's opimum power curve for better handling and performance. I manually shift my 300SD to keep the revs above 2500 RPM in order to have the turbo working. The trick is to note where the transmission shifts gears automatically in terms of speed and RPM, and then know just how far you can extend that range. The ideal is to match the speed of the crankshaft with the driveline by adjusting road speed and engine revolutions.

If you're unclear on the procedure, or just want to get the straight scoop from a real pro, go to your local library and see if they have any books by Bob Bondurant or Jackie Stewart. In fact, there's an excellent video by Jackie Stewart also.