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  #1  
Old 09-20-2014, 09:29 AM
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300D NA Cruising RPM

Don't have a tach on my 1981 300d/NA. For ya'll that have a tach, what's my RPM cruising at 70MPH. 4spd auto.

Pat
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  #2  
Old 09-20-2014, 09:42 AM
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Only way a na would have a tach would be if they installed one. Only the turbo cars got tachs however there are rpm calculators online. Put in your tire size and use 3.46 as your final drive ratio. Add 200rpm to the number the calculator spits out.

Likely around 4300rpm
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  #3  
Old 09-20-2014, 09:45 AM
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It depends on the tyre size and whether you still have the correct 3.46 final drive ratio.

On standard size 14 inch wheels and say 195/65R14 tyres the revs are about 3500 at 70 mph

(Gear box top gear ratio - 4th - is 1:1)

Many calculators are available on ye olde interweb such as this one

Engine RPM Calculator
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Old 09-20-2014, 12:17 PM
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Much appreciated
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  #5  
Old 09-21-2014, 02:57 PM
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Too high for my tastes. It does not hurt the engine though. Changing the rear end and speedo head for a little taller rear end is not that bad. May even help fuel milage a little.

My 77 300d is a possibility as it seems powerful enough to take a reasonable rear end and speedometer change. I forget the next taller reasonable ratio available. In combination with a little larger tire size the differance should be noticeable if needed.

Funny car that one of ours with about half the radiator fins rotted out with age. It still did not overheat on the highway. I have a spare good rad to replace that one though.

There was a rise in coolant temperature going up serious grades that the heater running during them took care of well. It was a little hot in the cab that summer day though when doing it.
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