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.
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.