Since 1900, the Chicago Cubs have the best ERA by a team in a season, with an ERA of 1.73 in 1907.

	TEAM	ERA	SEASON	G	W%	W	L	GS	SO	CG	SHO	SV	IP	H	ER	R	HR	BB	HBP	TBF	WP	K/9	BB/9	K/BB	HR/9	K%	BB%	WHIP	BABIP	RA	H/9
1	Cubs	1.73	1907	155	.704	107	45	155	586	114	31	8	1373.1	1,054	264	390	11	402	38	5,315	25	3.84	2.63	1.46	0.07	11.0	7.6	1.06	.244	2.56	6.91
2	Cubs	1.74	1909	155	.680	104	49	155	680	111	32	11	1409.1	1,094	272	389	6	364	38	5,494	30	4.34	2.32	1.87	0.04	12.4	6.6	1.03	.247	2.48	6.99
3	Cubs	1.75	1906	155	.763	116	36	155	702	125	28	10	1388.1	1,018	270	381	12	446	53	5,427	20	4.55	2.89	1.57	0.08	12.9	8.2	1.05	.239	2.47	6.60
4	Athletics	1.79	1910	155	.680	102	48	155	789	123	24	5	1421.2	1,103	282	441	8	450	55	5,503	39	4.99	2.85	1.75	0.05	14.3	8.2	1.09	.261	2.79	6.98
5	Athletics	1.93	1909	153	.621	95	58	153	728	110	27	3	1378.0	1,069	296	416	9	386	65	5,385	31	4.75	2.52	1.89	0.06	13.5	7.2	1.06	.253	2.72	6.98
6	White Sox	1.99	1905	158	.605	92	60	158	613	131	15	0	1427.0	1,163	315	450	11	329	31	5,542	39	3.87	2.07	1.86	0.07	11.1	5.9	1.05	.253	2.84	7.33
7	Naps	2.02	1908	157	.584	90	64	157	548	108	18	5	1424.1	1,172	319	459	16	328	34	5,488	24	3.46	2.07	1.67	0.10	10.0	6.0	1.05	.253	2.90	7.41
8	White Sox	2.03	1910	156	.444	68	85	156	785	103	23	7	1421.0	1,130	320	479	16	381	38	5,513	31	4.97	2.41	2.06	0.10	14.2	6.9	1.06	.259	3.03	7.16
9	Cubs	2.04	1905	155	.601	92	61	155	627	133	23	2	1407.1	1,135	319	442	14	385	51	5,500	21	4.01	2.46	1.63	0.09	11.4	7.0	1.08	.253	2.83	7.26
10	White Sox	2.05	1909	159	.513	78	74	159	669	115	26	4	1430.1	1,182	326	464	8	340	52	5,560	29	4.21	2.14	1.97	0.05	12.0	6.1	1.06	.261	2.92	7.44
11	Pirates	2.07	1909	154	.724	110	42	154	490	93	21	11	1401.2	1,174	322	448	12	320	48	5,438	14	3.15	2.05	1.53	0.08	9.0	5.9	1.07	.254	2.88	7.54
12	Naps	2.09	1906	157	.582	89	64	157	530	133	27	4	1412.2	1,197	328	482	16	365	48	5,560	23	3.38	2.33	1.45	0.10	9.5	6.6	1.11	.257	3.07	7.63
13	Phillies	2.10	1908	155	.539	83	71	155	476	116	22	6	1393.0	1,167	325	445	8	379	43	5,400	26	3.08	2.45	1.26	0.05	8.8	7.0	1.11	.258	2.88	7.54
14	Robins	2.12	1916	156	.610	94	60	156	634	96	22	9	1427.1	1,201	336	470	24	372	38	5,725	34	4.00	2.35	1.70	0.15	11.1	6.5	1.10	.253	2.96	7.57
15	Americans	2.12	1904	157	.617	95	59	157	612	148	21	1	1406.0	1,208	331	466	31	233	29	5,448	22	3.92	1.49	2.63	0.20	11.2	4.3	1.02	.259	2.98	7.73
16	Pirates	2.12	1908	155	.636	98	56	155	468	100	24	9	1402.1	1,142	331	474	16	406	52	5,584	19	3.00	2.61	1.15	0.10	8.4	7.3	1.10	.243	3.04	7.33
17	White Sox	2.13	1906	154	.616	93	58	154	543	117	32	4	1375.1	1,212	326	460	11	255	32	5,345	33	3.55	1.67	2.13	0.07	10.2	4.8	1.07	.267	3.01	7.93
18	Senators	2.14	1918	130	.563	72	56	130	505	75	19	8	1228.0	1,021	292	408	10	395	28	4,954	35	3.70	2.89	1.28	0.07	10.2	8.0	1.15	.252	2.99	7.48
19	Cubs	2.14	1908	158	.643	99	55	158	668	108	27	12	1433.2	1,137	341	461	20	437	39	5,611	34	4.19	2.74	1.53	0.13	11.9	7.8	1.10	.251	2.89	7.14
20	Giants	2.14	1908	157	.636	98	56	157	656	95	24	18	1411.0	1,210	336	455	26	288	35	5,544	18	4.18	1.84	2.28	0.17	11.8	5.2	1.06	.261	2.90	7.72
21	Browns	2.15	1908	155	.546	83	69	155	607	107	15	5	1397.0	1,151	333	478	7	387	63	5,446	22	3.91	2.49	1.57	0.05	11.1	7.1	1.10	.261	3.08	7.42
22	White Sox	2.16	1917	156	.649	100	54	156	517	78	21	21	1424.1	1,236	342	462	10	413	35	5,645	12	3.27	2.61	1.25	0.06	9.2	7.3	1.16	.263	2.92	7.81
23	Giants	2.17	1904	158	.693	106	47	158	707	127	21	15	1396.2	1,151	337	476	36	349	35	5,561	29	4.56	2.25	2.03	0.23	12.7	6.3	1.07	.251	3.07	7.42
24	Phillies	2.17	1915	153	.592	90	62	153	652	98	20	8	1374.1	1,161	332	463	26	342	31	5,478	20	4.27	2.24	1.91	0.17	11.9	6.2	1.09	.256	3.03	7.60
25	Cubs	2.18	1918	131	.651	84	45	131	472	92	21	8	1197.0	1,050	290	393	13	296	22	4,843	19	3.55	2.23	1.59	0.10	9.7	6.1	1.12	.257	2.95	7.89