Or a matter of economics? Record at half the speed and you have double the amount of tape!
Sure, but for a professional studio, this shouldn't be an issue!
Oh-oh, don't forget: Kraftwerk had no money at that time. Conny Plank was not their engineer, he was the producer of Autobahn, so probably it was HIS money they spent!
Oh, come on, that would mean that Conny Plank was on the verge of bankruptcy, wouldn't it. I don't know the open reel tape price in 1974, but with a high quality tape (say Maxell XL-1) today, I'd just save about 20 Euros by reducing the speed. And if this was an issue, I wouldn't even be able to offer some cups of coffee to the musicians in the studio. But maybe you're right, I can't verify it as I wasn't there - it just seems to me that what Dirk meant was that Conny Plank chose 7.5 ips for sonic reasons.