"Every 18 months computers double in speed"

18 months = 1.5 years

the model T  was widely available in 78 years ago.

there have been 78/1.5 = 52 equivalent generations

this is a speed up factor of 2 ^52 = 4.5 x 10 ^ 15

the model T  traveled about 20 mph

if cars improved in speed at the same rate computers do,
a car today would go 4.5 x 10 ^ 15 x 20 = 9 x10^16 miles per hour
there are 3600 seconds in an hour, so this car would travel

2.5 * 10 ^ 13 miles per second

light travels at 186,000 miles per second

thus the car travels 135 million times the speed of light.