why don`t cars have to be "run in " anymore

[kaw69]

Why can you drive new cars at fast speeds without it damaging them and making them sluggish.
On the same point does it damage a new car to drive it for the first 1000 miles at a lower speed

[Hymie]

In olden days, manufacturing tolerances were not what they are on today’s engine parts (pistons, bores, bearings, journal sizes, gear teeth etc). Natural wear of the parts (by running in slowly) improved the mechanical ‘fit’ of the parts – and resulted a longer engine life. Thrashing a new engine could result in damage due to the moving parts not being as good a fit to one another as they otherwise might be.

That said, I would still advise keeping the revs on a new or reconditioned engine below 4,000 for the first 1,000 miles or so, to increase the life of the engine and minimise unnecessary wear.

[annie0000]

When I got my last new car (3 years ago), it was recommended to keep below 3000 revs for a number of miles

[DTCwordfan]

because they use a small amount of alkylate fuel between the factory and the delivery point to temper the engine in - this is something akin what goes into F1 cars

[mike11111]

It was a common sight until the mid-sixties to see a car with the sign, "Running in - please pass", stuck to the back. Although the message was valid, the subtext read, " I have to drive this slowly because I've just bought a brand-new car and I don't want to collide with your second-hand jalopies, thank you very much, poor people!"

[TWR]

Any sensible driver that looks after his / her car will take time to bed everything in within the engine Kaw69, there is nothing worse than flogging a new car only to find the thing breaking down.