Tach time reading higher than actual hours

[markazzarito]

I recently flew a leg from northern NV KLWL to Orange county KSNA. When logging the flight i noticed tack time was almost 1/2 hour more than actual flight time or hobbs. I always thought this worked the other way around considering tach time is a product of engine revolution and engine revolution slows during taxi takeoff and landing. I was cruising at 10,500 ft and while i normaly run 25 rpm WOT at altitude i had the power setting pretty low for much of the flight due to very rough air. The tach showed 4.9 hours while my gps ipad and clock all showed 4.5 hours. I went back and checked a few old flights and found this to be common. Does anybody here have experience or insight as to why the M20c would run higher in terms of tach time vs hours? 

Edited September 30, 2016 by markazzarito
Spelling edit

[MB65E]

Yes, most of our Tachs record at 2300 rpm. So it will records more time at 2500rpm. Others have posted a formula that can convert tach to actual flight time.

Personally, I wouldn't worry about it or try to correct it. It's just the way it is.  When/if I sell mine I'll be sure to let the buyer know about the change. If I was looking at buying another, I'd apply the same thought process. 

Also, it will always keep the airplane in an over serviced condition vs under serviced.

I was at SNA 2x yesterday. I like OC. You get north of the 20's there and people turn into assholes in LA county. 

fly safe!

-Matt 

[Hank]

My C isn't like that. I often cruise at 2500 and find tach time to usually be 0.2 less than actual by my watch on a trip this long. 

[jetdriven]

I think the proper recording speed is 2566 RPM, but often a 2300 RPM unit gets swapped in at some point , which will over count.  That was my situation

[carusoam]

The recording speed is probably written or printed in ink on the side of the can.

A mirror and a light while the access panels are open may work.

Or, compare at the settings Byron posted would be easier.

The mechanical tach is really a revolution counter, with it's output/display calibrated in hours...

Best regards,

-a-