Want to check the engine speed in my 240d, which has no tach. Bought wine of the photo sensor tachometers, but am looking for the best spot to put the reflective tape.

Seems to me that putting the tape on a pulley is far easier than putting it somehow down on the crank pulley and then having to get a clear shot down so low.

I was thinking the black smooth round surface that the fan is attached to, as it is right up top, smooth, easy to clean and stick to, etc.

But then I need to be sure to know the ratio that the ratios are known to get an accurate reading.

Anyone know? Ideas?

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)