This is a bit long to fit in a comment, so:
It would be cool, but, ultimately, you've got to get a feel for the rev differences between gears in your particular vehicle. Once you get in the habit, you won't be looking at the tach, anyway.
That said, you could always put a couple of magnets on the flywheel (180* apart) and use a Hall effect sensor. Plug that into an Arduino. Calculate the RPM from the sensor, and update whatever gauge you have.
There are a number of examples out there of doing this with stepper motors, etc.:
If you get a good sensor, you could potentially do this without the magnets by reading the teeth on the outer edge of the flywheel. (And, I'd be really, really careful about attaching anything to the flywheel or clutch plate.)
Edit That'll give you engine speed, not input shaft speed. Oops.
Another option would be to put the magnets on your driveshaft. If you had a way to tell what gear the car is in - which you can calculate based on engine RPM and vehicle speed or driveshaft speed - you can do some basic arithmetic and extrapolate the flywheel RPM.
There are a lot of options here, but, ultimately, I don't see this being useful beyond the bling factor.
Now, if you wanted to build an "automatic" manual, and have the computer control the clutch and rev match for you, this isn't a bad place to start.
Ultimately, the best place for the sensor is near the input shaft, but you'd need something on the shaft to sense. You could also do this optically, I suppose. :/