You can do as fast as the car will allow, but I wouldn't do that for long or often. I was able to get my 77 240D auto up to 95mph but usually it did fine going about 75mph, anything above that and the temp would start to creep up, not sure if the cooling system was in top shape though.

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Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.