Call hourly rate = HR. Assume that I can guess my monthly usage in hours, which I call $g$. Beware that the fixed fees are presented in different units of time, so first convert everything into months, because it's easier to estimate car usage monthly than yearly. Then Simple's monthly fee $ = 45/12 = \$3.75$. My problem strategy is to compare 2 plans at a time, by sampling without replacement and ordering, with $\frac{3!}{1!(3- 1)!} = 3$ comparisons in total. In each comparison, I'd determine the breakeven number of hours between the 2 plans, then by comparing it with $g$, I'd instantly know the cheaper plan. Is this the simplest method? How would I execute it?
The last row above displays the daily mileage limit, at or below which the HR remains constant and independent of km travelled. Penalties exist for mileage above the limit, which I won't exceed so I ignore them. Autoshare confirms that for any particular car at the same location, Simple's HR (whatever it is) is always exactly $-\$1$ than that for Metro and Metro Plus. I omit the row for rates 'per 24hr' because if you need a car for 24 consecutive hours, car rental (eg Budget, Enterprise) is cheaper than car sharing. Anyhow, FYI, for a given car at the same location, Simple's 'per 24hr' rate is always $-\$5$ than Metro's and Metro Plus's.
My first attempt compares Simple and Metro. Call the breakeven number of hours $h$. Then Simple's monthly cost = Metro's monthly cost $\implies (9.25 \text{ to } 13.25)h + 3.75 = (8.25 \text{ to } 12.25)h + 15$.
$1.$ The stated HR are stated as a closed interval, so how do I quantify them?
$2.$ How do I adjust for the greater mileage offered by Metro Plus?
Update 2015 Jan 6: I show user Ross Millikan's calculations here.
you would have to use the car 135 hours per year to compensate for the added annual fee. - Is this just $\frac{\text{Metro's Annual Fee} - \text{Simple's}}{\text{HR difference between Simple and Metro}} = \frac{\$15 \times 12 - \$45}{$1} = 135$ hours?
The added km on Metro plus come at a price of 120/year - Is this just Metro Plus's Annual Fee $-$ Simple's = $\$(25 - 15) \times 12 = \$120$?
You define a usage model, which for your examples must include hours and km per month. Your model should be a probability distribution of how you will use the cars. The output will be a probability distribution of costs. The simple answer is to take the expected value of each distribution and take the least. Harder is if one service will charge you an enormous amount for going over some number of miles, but you have to sign up with one and only one. Then you are insuring against wanting that many miles. Welcome to real life.