What is the model designation of your car? It'll be numbers begining with 124023 or 124030 on a plate ahead of the radiator.

If your speedometer and odometer read correctly then you probably have the right differential. Or at least a matched speedometer and differential.

You can try a another differential ratio but you'll either get better acceleration and worse fuel mileage or better fuel mileage and worse acceleration.

Seems to me that when someone isn't satisfied with acceleration and fuel mileage either the engine needs service or that person bought the wrong car.

Sixto
95 S420
87 300SDL