w123 axle ratios
 

I have a 78 300d, and an 83 240d, I drove the 240d on the highway and it was a cruiser!! it felt so much happier at 75 than I remember my 300d being

My 300d is in the shop but am I correct in thinking that the axle ratio in the 240d must be lower hence the lower highway RPM?

Your 78 300D has a 3:46 Differential ratio
Your 83 240D has a 3:69 Differential ratio.

The 240d is a lower ratio, which means the RPM will be higher than the RPM`s of the 300D at the same speed.

The lower the number of the Differential gear ratio, the higher geared it is.

The 240D`s all used a 3:69, but the last year of the W123 MB used a 2:88 ratio which is way higher.

For every time the engine rotates 3.69 times, the rear wheels go around one time.
For every time the engine rotates 2.88 times, the rear wheels go around one time.

The low geared rear ends will get you off the line quicker, but the engine will be racing at the top end.


Charlie

so is the transmission why my 300d feels like the engine is at a much higher RPM then? Basically the 240d handled the highway with ease, my 300d seems like its working at lot harder

The 300D would be an automatic, what is the 240? auto or manual?

The 300 is about 88HP and the 240 is 67HP so you would think the 300 would be faster.

Could be the Torque Convertor slipping, lack of valve adjustments, fuel filters needing changing, air filter.
How many miles between the two?

Our 80 240 with 354K and manual trans will walk away from my BIL 82 with 140+K and auto trans.

I haven`t driven a none turbo 617 so don`t have any experience with them. I do know our 240 is pretty peppy for what it is.


Charlie

Could be, if your 300D auto-box is slipping badly, or not upshifting to 4th gear.
Keep in mind that other factors, like bad motor-mounts, condition of the drivetrain and body, or anything that transmits excessive noise and vibration into the cabin can make it feel like it's working alot harder. Also, the older automatics in these cars don't have a lock-up torque-converter, contributing to some extra slippage and noise in top gear.

I used to have a very tired & rusty automatic 240D that was nearly intolerable on the highway. While my current 240D, with it's solid body, factory replacement engine and stickshift, is relatively pleasent at speed.

Happy Motoring, Mark