Convert the diameter from millimeters to centimeters.

The given diameter is $870 \text{ mm}$.
Since $1 \text{ cm} = 10 \text{ mm}$, we divide the diameter in millimeters by 10.

$$\text{Diameter in cm} = \frac{870 \text{ mm}}{10 \text{ mm/cm}} = 87 \text{ cm}$$

We are given that $1 \text{ inch} = 2.54 \text{ cm}$.
To convert centimeters to inches, we divide the value in centimeters by 2.54.

$$\text{Diameter in inches} = \frac{87 \text{ cm}}{2.54 \text{ cm/inch}} \approx 34.2519685 \text{ inches}$$

Rounding 34.2519685 to two decimal places gives 34.25.

$$\text{Diameter in inches} \approx 34.25 \text{ inches}$$

The diameter of the tank in inches, rounded off to two decimal places, is $\boxed{\text{34.25 inches}}$.