Can anyone help me figure out what the cruising RPM is for a '66 122S with M40 and stock rear end? Problem is, I can't seem to figure out what rear end ratio this car has. The tag is gone, and the 'net has conflicting info about which years had which ratios.

Trying to calculate cruising RPM for 60, 65, 70 and 75 mph. Going by engine noise alone, I would guess 3000, 3200, 3500, and 3800, respectively.

Thanks,
John

I bought a $30 tach at the auto parts store, easy to hook up. Placed it next to the A pillar so it is not in direct line of sight. No holes in dash, a short strip of steel attached to a screw that holds down the windshield surround. Wires go down from the dash in the door jamb and under the dash.


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My name is Klaus and I am a V♂lv♂holic

Klausc had a very sane response to an idiot question buy a tach.

"Klausc had a very sane response to an idiot question buy a tach."

Hey, "motiv8"...fuck you!

...perhaps it is not very polite to call a complete stranger an "idiot" from behind your computer screen?

Sorry for the choice of words. I was describing the question. The engine RPM can not be accurately determined by the sound of the engine while traveling down the road.If you use the calculator at the link I added and use 4.1 for the diff ratio and your tire size found on the tire side wall it will tell you the speed at different RPMs or RPM at different speeds. But if you have no way to measure speed or RPM what can you do? Buy a tach.

Good point. A tach is in order. I just wanted to have a reasonably accurate guesstimate of engine speed, which you guys have helped me with....Thank you!

input diff gear ratio,tire size and if OD which model

www.mintylamb.co.uk/gearspeed/

If your car is a sedan, Phil's calcs are right on. If it is a wagon they should be increased
about 11% because it would have 4.56 gears in the differential
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George Downs Bartlesville, Oklahoma

It would normally have come with 4.10 gears from the factory. So, with the stock tire diameter:

60 MPH = 3260 RPM
65 MPH = 3530 RPM
70 MPH = 3800 RPM
75 MPH = 4075 RPM

That's exactly how my 144 goes with the 4.10 rear end.
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1974 144GL, 1996 940 GL LPT

Easiest way to determine that is to jack up the rear wheels & rotate the driveshaft while counting how many turns it takes to spin the rear wheels one turn.
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1999 E320 1994 F150 (awaiting a new engine) 1989 560 SL 1994 945T (daughter's) 1986 242Ti (awaiting a B230FT transplant) 1968 GT500KR (under slow restoration) 2007 HD Nightster (son's ride)