At what RPM?
 

I have a 6 cylinder 1989 300SEL [W126].

Can anyone tell me at what RPM should the car be at when she is CRUISING at 60mph and at 70mph?

My tach shows around 3100-3200 RPM at 60mph and around 4000 RPM at 70mph. Does this sound right?

I am wondering if there is an overdrive gear or perhaps gear four is not kicking in.

Thanks in advance.

Sounds like you are reading the correct RPM for 4th gear as the rear end has a 3.46/1 ratio. That is one of the lowest ratio for a late model gas BENZ.

Thanks MB Doc.

That ratio doesn't do too much for cruising at highway speeds. I tend to like the lower revving engines at higher speeds.

The engine seems loud. The six cylinders in my car just seems to work way to hard to cruise above 60mph.

Thanks again for your reply.

At 70mph my tach reads 3400rpm.

The rev's sound normal for a Mercedes. I have a 88 260 and had trouble getting used to the high revs on the highway too. No other makes I've owned were like that. I did eventually get used to it and apparently the engines are built to take it. Another oddity is that some cars, like mine have a second gear start making the transmission essentially a three speed instead of a four.

If 4th gear is 'drive', i.e. 1:1, and the rear end ratio is 3.46:1, then assuming a tire size of 195/65R15 - probably smaller than actual in this case - the engine speed at 60 mph would be 2800 rpm. At 70 mph it would be 3260 rpm.

Now, I don't know how accurately you noted your engine speed or road speed, but note that 6/7 X 4000 rpm = 3429 rpm for 60 mph, which is not what you indicate. I would recheck your figures, and if they are accurate, than either you have problems with your instruments, or your transmission is slipping.

Steve

Thanks for all the input.

Having rechecked the tach, I note 60mph=3100 rpm and 70mph=3500 rpm. Figures seem to be in line with those mentioned by the senior members of this board.

The noise I mentioned may be more akin to loud fan noise emanating from front of engine area. It comes on under load at around 40 mph and gradually gets noiser as speeds increase. Any ideas on this issue.

Thanks again.

"The noise I mentioned may be more akin to loud fan noise emanating from front of engine area. It comes on under load at around 40 mph and gradually gets noiser as speeds increase. Any ideas on this issue."

This would be your thermostatically clutched engine fan. It should only come on if your engine is running hot. Check the air passages through the radiator and A/C condenser fins - they may be bent or blocked with insects and road debris. A reverse power wash with a hose may clear them.

Steve

Steve: Thanks for the tip. I'll check the condition of the radiator and condensor and will update with details.

RPM
 

Knuckles,
Sounds to me the RPMs are about what they should be. There is no overdrive or higher gear it just runs that way.
Bill Lewallen

Picked up a 1990 for my Dad. Was doing like 3900 @ 80mph, pulled over and started off again to make sure. Went from 2nd-1st-2nd-3rd-4th, yup, all there. Car has 146K so I guess it can handle it. My wifes miata did like almost 5k at 80, the A/C would hum. Don't get in a tivey, I live in 70mph zones, 80 happens.