In Reply to: Question about proof meeter posted by Dave Smith on February 23, 2019 at 17:44:05:
The A.S.A.E. ( American Society of Agricultural Engineers ) standardized things back in the day. First, the standard established for PTO speed is 536 RPM and thus was rounded off to 540 RPM. The established ASAE engine speed becomes 1580 RPM. These numbers are used to calculate readings with. Ground speed will vary depending on which gear is used. Also, the ASAE standard for belt pulley speed is 3000 to 32000 ft. per minute, or, 1316 RPM on Proof-meter. The Proofmeter (Tachometer) is detailed in the later 8N Operator’s manuals and subsequent NAA and Hundred Series model Operator’s Manuals. It is probably a chapter often skipped over but you have to read it and understand what it is saying. Here is the paragraph from the NAA manual and is the same as in the 8N manuals:
“The hour meter portion of the Proof-meter represents the hours of engine operation. One hour shown on the hour meter represents one clock hour of tractor operation at 1580 engine RPM. Engine speeds below 1580 revolutions accumulate hours slower than clock hours. Engine speeds in excess of 1580 RPM register faster than clock hours.”
I think Dogman is pretty close in the explanation. It also makes me wonder when I see an ad if the seller says, "...tractor has only 400 hours since engine rebuild..." if he is just reading what the Proofmeter shows or did he actually keep records of actual hours the tractor ran? The numbers are not the same thing as many are led to think.