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I'm trying to identify the short story(?) in which cars are designed to be slower than they appear to be driving - louder engine noises, higher speedometer readings than actual speed.

This is, I believe, in several Cyril Kornbluth works, but it's certainly in The Marching Morons.

The psychist climbed down into the driver's seat and did something with his feet. The motor started like lighting a blowtorch as big as a silo. Wallowing around in the cushions, Barlow saw through a rear-view mirror a tremendous exhaust filled with brilliant white sparkles.

"Do you like it?" yelled the psychist.

"It's terrific!" Barlow yelled back. "It's—"

He was shut up as the car pulled out from the bay into the road with a great voo-ooo-ooom! A gale roared past Barlow's head, though the windows seemed to be closed; the impression of speed was terrific. He located the speedometer on the dashboard and saw it climb past 90, 100, 150, 200.

.....

They seemed to be traveling so slowly, if you ignored the roaring air
past your ears and didn't let the speedy lines of the dream-boats fool
you. He would have sworn they were really crawling along at
twenty-five with occasional spurts up to thirty.

....

Barlow stiffened as he realized the rush of air past his ears began
just a brief, unreal split second before the car was actually moving.

....

"The automobiles have a top speed of one hundred kilometers per hour -
a kilometer is, if I recall my paleolinguistics, three-fifths of a
mile - and the speedometers are all rigged accordingly so the drivers
will think they are going two hundred and fifty."