Packard Forum Index » Pre-War (1899-1942)

Does anyone know the idling RPM of a 245 cu. in. 6 cylinder. I just installed a tachometer and it is reading 900 RPM. The owners manual states that the idle should be set for 6 miles an hour on level ground. That's how I have the idle set now and it sure doesn't sound like it's anywhere close to 900. I have contacted the seller and he said that he would need a base line RPM in order to zero the tach. I have no complaints with the seller as they have been very cordial and helpfull.
Jim Long

Probably the tach is just reading double?

You can figure this number yourself if you know the rear axle ratio and tire radius. And whatever gear you are in when that is measured (if applicable).

I have an Actron Multimeter that reads RPM by connecting the leads to the Coil and ground, and then setting the dial to either 4/6/8.

It works well for double checking Tachs and other engine items as it has various engine diagnostic functions, all in a standard multimeter format.

Sounds like your tach isn't the right one for engine cylinder count.

For what it's worth the service manuals seem to want the idle speed to be between 400-500 rpm on all the engines so I would presume the 6mph would fall somewhere in that range. 500 is given as the upper limit to set an idle speed as that is when the centrifugal advance starts to come in. Anything faster would affect adjusting the timing or checking other tuneup specs such as fuel pump output etc. If you don't have a meter like Kev's which is capable of measuring RPM I believe Harbor Freight and Amazon sell a relatively inexpensive visual tach that works by reading a piece of reflective tape you would place on the vibration damper..

Since there are so many variables affecting an actual RPM then without a meter and assuming the car is fairly stock, if you provide a year probably some of the math proficient people on the forum could take the typical axle ratio and tire size of the six in the model you have and see what RPM would work out to the 6mph you have set. That would at least give a ballpark number you could provide to the tach vendor.

Chalk mark on tire and floor. Push car backwards or forwards one complete tire revolution, mark floor again. Measure the distance between the two chalk marks. Or jack wheel off the ground enough to run a tape measure around the middle of the tire tread. That's tire circumference, close enough, because you could argue tire slightly expands after driven several miles. mph = rpm x tire circumference in feet, divided by rear axle ratio x 88.

If trying to determine mph/rpm at road speed and your car has overdrive, multiply rear axle ratio by .72 in the above.

That seems an overly complicated way to measure circumference. Measure the diameter and multiply by pi.