My records seemed to indicate we had not changed the chain since just before the 3 ring circus in 2012, which seems a long time, but probably correct.

I used the KMC single speed chain and needed 102 links of the 110 supplied. The photo above shows the length of the old chain and the equivalent point on the new chain so it has obviously stretched.

I had a new 38T chain ring which I installed and you can see below the wear on the old one compared to the new one underneath.
The rear sprocket is also new 16 teeth which came with the new hub.



I had considered using a 36 tooth but as it is well under the 2.5 ratio Rohloff recommends (here) for Tandem i.e. 16 x 2.5 is 40 which is the lowest they recommend for tandem. With 38T we are a drive ratio of 2.375.

Gear Ratios	Ventana with Rohloff Hub
Crank Length	175	mm	not considered			175	mm	not considered		Crank Length
Chain Ring	38	Teeth	front			36	Teeth	front		Chain Ring	1.0555555556
Sprocket	16	Teeth	rear			16	Teeth	rear		Sprocket
Drive ratio	2.375	recommended	not lower than 2.5			2.25	recommended	not lower than 2.5		Drive Ratio
Wheel circumf	2.07	m	estimate fo 26”			2.07	m	estimate fo 26”		Wheel circumf
Gear	IGR*	metres per crank revolution	gear inches assume 26” wheel	Kilometres per Hour at below RPM		IGR*	metres per crank revolution	gear inches assume 26” wheel	Kilometres per Hour at below RPM	Gear
				120					120
1	0.279	1.372	17.2	9.9		0.279	1.299	16.3	9.4	1	1.0555555556
2	0.316	1.554	19.5	11.2		0.316	1.472	18.5	10.6	2
3	0.360	1.770	22.2	12.7		0.360	1.677	21.1	12.1	3
4	0.409	2.011	25.3	14.5		0.409	1.905	23.9	13.7	4
5	0.464	2.281	28.7	16.4		0.464	2.161	27.1	15.6	5
6	0.528	2.596	32.6	18.7		0.528	2.459	30.9	17.7	6
7	0.600	2.950	37.1	21.2		0.600	2.795	35.1	20.1	7
8	0.682	3.353	42.1	24.1		0.682	3.176	39.9	22.9	8
9	0.774	3.805	47.8	27.4		0.774	3.605	45.3	26.0	9
10	0.881	4.331	54.4	31.2		0.881	4.103	51.5	29.5	10
11	1.000	4.916	61.8	35.4		1.000	4.658	58.5	33.5	11
12	1.135	5.580	70.1	40.2		1.135	5.286	66.4	38.1	12
13	1.292	6.352	79.8	45.7		1.292	6.017	75.6	43.3	13
14	1.467	7.212	90.6	51.9		1.467	6.833	85.8	49.2	14