I recently had my car's gearbox and flywheel replaced to fix a noise. Since collecting the car, I've noticed a significant lack of acceleration, but only in 6th gear. On a flat road, it takes 10-12 seconds to accelerate from 60 to 70 mph. On a medium incline on the motorway, the car actually decelerates from 70 mph, even with the accelerator fully pressed.

This problem started immediately after the repair. All other gears accelerate normally, and there are no other unusual sounds. I'm wondering if this is more likely the result of an error during the installation or a new, unrelated failure. I was told that the installed gearbox was a match for the car. I've contacted the garage to have them look at it, but having little knowledge of cars beyond the basics, I was hoping to get your thoughts on the likely cause.

Thanks Dustty