Adding a scooter?

I've wondered how many miles I'd have to go on a scooter to get my average mpg up over 40 mpg. I haven't taken calculus, so here's my hand-cranked answer (Thanks to Excel).

Total miles: 150			Gas Used
			car mpg	scoot mpg
	Car miles	scoot miles	20	60	carbon car	carbon scoot	total gas	Cost of gas	Total MPG	Total Carbon
	150.00	0.00	7.50	0.00	146.73	0.00	7.50	$ 22.50	20.00	146.73
10%	135.00	15.00	6.75	0.25	132.06	4.89	7.00	$ 21.00	21.43	136.95
20%	108.00	42.00	5.40	0.70	105.65	13.69	6.10	$ 18.30	24.59	119.34
30%	75.60	74.40	3.78	1.24	73.95	24.26	5.02	$ 15.06	29.88	98.21
40%	45.36	104.64	2.27	1.74	44.37	34.12	4.01	$ 12.04	37.39	78.49
50%	22.68	127.32	1.13	2.12	22.19	41.51	3.26	$ 9.77	46.07	63.70
60%	9.07	140.93	0.45	2.35	8.87	45.95	2.80	$ 8.41	53.53	54.83
70%	2.72	147.28	0.14	2.45	2.66	48.02	2.59	$ 7.77	57.90	50.68
80%	0.54	149.46	0.03	2.49	0.53	48.73	2.52	$ 7.55	59.57	49.26
90%	0.05	149.95	0.00	2.50	0.05	48.89	2.50	$ 7.51	59.96	48.95
100%	0.00	150.00	0.00	2.50	0.00	48.91	2.50	$ 7.50	60.00	48.91
Scoot to work 3 * a week, save $40 a month.