As per the quote below, a car loses about 0.08g of tread per km.

Compared to a car, a bike tyre is about the same diameter, 10% of the width (~20mm), 28% usable tread depth (~2mm), has 50% less wheels, and can travel 10% the distance (~10000km).

This suggests a (very approximate) tread loss of 0.08 * 10% * 28% * 50% / 10% = ~ 0.01g per km for bicycles.

For replacing longer car journeys less typically travelled by bicycle, rail transport is the best solution and removes the issue of tyre wear.

Quoting [deleted] in r/theydidthemath:

Using the same assumptions as above (215/60R16 tires, 7mm of tread loss over 100,000 km), I estimate the loss of tread by volume from each tire as follows:

Cylinder with a diameter of 664 mm and a height of 215 mm has a volume of 74,412 cm3. Cylinder with a diameter of 664-(2x7)=650 mm and a height of 215 mm has a volume of 71,307 cm3. The volume difference between a new and worn out tire is 3105 cm3.

Typical land to sea ratio of tires is 60-70% land, depending on the type of tire. If we go with an about average value of close to 65% tread, we get the lost rubber volume of about 2000 cm3 or 2,000,000 mm3 over a single tires lifespan.

Each revolution of a tire loses about 0,04 mm3 of tread, which, according to Wolfram Alpha, is a bit less than the volume of a medium grain of sand.

If we look at the entire car with 4 tires over a kilometer of road, we get 80 mm3 or about 0,08 grams of tread lost per car per kilometer.