1968 RPMs at 60 MPH ?

[FRITZ68]

My '68 280SL with automatic seems to be running about 3800 RPM at 60MPH.  
Does that seem about right?   This is about twice the RPMs of my new car.
My speedometer in K is no longer working and the speed was taken off a GPS, so may not be exactly accurate.  Either  way, it seems a bit high

Fritz

[66andBlue]

It depends on the gear ratio in the differential but for a 4.08 it seems to be in the ball park.
Take a look at the graph here:
http://www.sl113.org/forums/index.php?topic=18239.msg127406#msg127406

[FRITZ68]

Very nice chart - thank you.  I will do another test, but according to the chart with a 4.08 it should be about 56-57 MPH at 3000 RPM and at 3500 RPM should be about 65 MPH.  Mine is reading considerably higher RPMs, but starting in 4 automatic, it shifts twice, which is correct per my understanding of the automatic transmission.  I can see the reason that some like to change to a different ratio rear end, but plan to leave mine "as is".  I guess we are spoiled with our newer cars with 6-speed and 7-speed automatics with a resulting much lower RPM at highway speeds.    However, from what I read on this forum, the 280SL engine runs nicely at the higher RPMs.

Fritz

[ejboyd5]

What size wheels and tires are you using?

[FRITZ68]

I purchased a new set of Coker Radials 185 R14 tires.  As I recall they were slightly larger than the tires that were on the car at time of purchase.
We have the car on our lift and hope to find a data plate that shows the rear axle ratio and if found, I will report.

See pictures of these tires - they were supposed to be the closest possible tire to match those installed originally and they look good and ride great!

Fritz

[66andBlue]

185 R14 is the the original size and therefore correct.
Do not look for a plate but for a stamped imprint on the diff housing. You may have to use sandpaper to take some surface paint of to see it. It should show "4,08" or a similar number.

[Benz Dr.]

185 R14 is the the original size and therefore correct.
Do not look for a plate but for a stamped imprint on the diff housing. You may have to use sandpaper to take some surface paint of to see it. It should show "4,08" or a similar number.

I was always under the impression that 185 70 14 is a bit too small. Most cars I see use 195 which a bit taller tire.

[FRITZ68]

Thanks for all of the very clear and informative pictures and other info.  We were looking for a plate today on the differential, and could not locate.  The car is now out of our shop, but can be put back on our lift next week and we will take another look, now that we know there is no plate.  

As for the tire size, as I recall I carefully researched this on this forum and elsewhere at the time and was assured by Coker that it was the exact same size and tread as when car was new.  Would a 195 tire be any wider?  (There is not a lot of clearance to the inside of the tire and if a 195 tire would be wider, it could create another problem). If the 185R14 is the correct size as original, would the graph previously furnished be correct?   I printed two copies of this graph and will do some careful research in the near future to see how it compares. The tires on the car at time of purchase were slightly smaller (not available to measure) than the 185R14 so I felt at the time that this was the correct size.

Regardless of the RPMs or rear end ratio, this 280SL is a fun car to drive and will be greatly enjoyed by my wife.

Fritz