Indicative Revs at 60MPH

[GregSoz]

Hi Guys,
have just purchased a 68 Auto 280SL and want confirmation that the gearing is correct ie that the car is in 4th as it SEEMS to rev high around 3100RPM at 60MPH. I know they are high reving but is this OK?

Also is it OK to run PREMUIM PULP (above 95 octane) with no lead additive?
thanks

Greg from South Oz Australia

[hands_aus]

Hi Greg,
Welcome aboard. Any pics?

Yes to both your questions. The revs are spot on.
You will get used to the noise level of the engine.
I was told to turn my radio up, but the speaker is so old. I actually like the sound of the engine now.

The search function works very well. So if you have any questions about your car do a search. Most topics have been discussed in depth but we always welcome a challenge.

Because of the differences in the models of our cars, a lot of people have their cars model and year in their signature. This helps responders provide specific information to your car.

cheers

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[jeffc280sl]

To be accurate you need the following information.  Rear axle ratio-stamped on the differential-should be 3.69 or 3.92 and your tire diameter.  Final transmission gear ratio is 1.00:1.

These programs will help.  Figure the tire diameter and then input the info into the rpm/mph calculator.

http://www.1010tires.com/TireSizeCalculator.asp?action=submit

http://www.angelfire.com/fl/procrastination/rear.html

Jeff C.
1970 280SL 4-speed

[JamesL]

Greg

Allowing for 40 year old error - and accepting the advice below - yep. Sounds about right.

Do a search on the site for RPM but mine runs:
3000rpm - 60mph
3500 rpm - 70mph
4000rpm - 80mph
and this afternoon

4500rpm - 90mph

Mine was built in '69

And I was also told when I bought mine that MB over specced the valves etc such that they run perfectly well on unleaded. Mine has never been converted as such and runs all day, very happily, on 94/95 octane unleaded. Sometimes as a special treat, 98

[GregSoz]

Thanks Guys,
greatly appreciated. Will post some pics on the weekend, only picked it up Monday night and haven't had the joy of taking for a spin/photo in daylight hours.
GSoz

[glennard]

Speed = rpm/50???  Depending on diff. ratio, tranny, tires, etc.  Or 50 times speed(in mph) equals rpm.  50 might be 45 to 55.  Important thing is mpg.

[lurtch]

Hello Greg - - Yes, you have correctly determined that our 113's are geared way too low.  This is so because of a flawed decision back in the early development of the car.  MB wanted their new  vehicle to have fast acceleration because the other cars in its performance/price class were the Jag XKE , and the Corvette ( both of which will unquestionably leave our cars behind in a cloud of tire smoke ). There are two remedies: Live with it - -  or replace the rear end with a  3.27 ratio  from a 280 SEL 4.5. My 230SL has this set-up and I am very happy with it. Fortunately for me one of the previous owners had this done years ago. Regards, Larry in CA

[Naj ✝︎]

lurtch,
 

quote:or replace the rear end with a 3.27 ratio from a 280 SEL 4.5. My 230SL has this set-up and I am very happy with it. Fortunately for me one of the previous owners had this done years ago. Regards, Larry in CA

So how many MPH do you get per 1000 RPM?


naj

68 280SL

[lurtch]

Hi again - - - I guess I left out a few details?  

The conversion constant  with the 3.27 differential is 1.15 . - - Because I did not change the speedo gearing the indicated road speed is about 60MPH  while the true and calculated road speed is 69 MPH @ 3400 RPM.

Regards, Larry in CA

Download Attachment: 3400-RPMs-equal-70-MPH.jpg
49.89 KB

[dwahi]

Hi All

So following an engine tune I'm curious to find out more about the axle ratio my car may have.

I get 4000rpm at 70mph, is that correct? 280SL Auto

Thanks

[Bonnyboy]

There is an older thread - "Highway RPM's " and there is a graph that shows speeds vs rpms for the different rear end ratios - its pegged mine perfectly - 3.69 rear end -   3200 rpm at 68 mph.   

4000 rpm at 70 mph seems too high - Even with a 4.08 the graph says 3800 rpm at 70 mph.

There are several threads and pictures in the archives showing where to see the rear end ratio stamped on the rear end.  That may be a good place to start.

[JamesL]

Have you got a GPS thingy - as that would get yor speed spot on.

[66andBlue]

Use the graph in this post
http://www.sl113.org/forums/index.php?topic=16765.0 - Reply #2

[dwahi]

Great, thanks a lot. Ill go out again and then recheck the graph against my readings. Thanks a lot for your help.

Regards

[Garry]

What you really need to do is get under the car and look at the flat area on the differential housing and from the stamped ratio number you will know if it is the correct one or a has been replaced with a lower ratio one.

Garry

[twistedtree]

Are both the 4-speeds and automatics 1:1 in the transmission?