Biggest difference between GP & AMA Bikes?

Compression.

Seven Millimeters, bracing/gussets & Foot pegs

Riders that know how to ride them in sand.

For starters, AMA bikes run on gas while MXGP bikes use petrol.

The handlebars of the englisch where on the back

Lots of carbon fiber subframe/air boxes. Suspension settings that are typically softer with faster rebound.

Torque settings are in N/mm, not Lbs/Inch, also tires are called tyres and the air pressure is in Bar not Psi. You’re welcome.

I believe there are differences in compression ratios and in the mxgp series they are allowed to modify the frame in ways that you can't in ama. Not sure the specifics on either. Of course there could be other differences other than how they set the suspension up, which is somewhat different even from ama outdoor set ups.

Factory frames. The bike Herlings spanked everyone on at Ironman a couple of seasons back was nothing like his usual ride.

They are usually a CLICK off.

One tooth bigger on the rear sprocket ,10cc less fork oil and the ability to rail the out side of corners on a track that isn't immaculately groomed.

The GP bikes use silencers on the end of the exhaust, whereas the AMA bikes use mufflers.

First and foremost, its the speed they travel..

Backrest, louder pipes, fatter rear tire.

In Europe the bike has to be set up right or it’s a big “whoops”
In USA the bike has the be set up for the whoops ( at the expense of everything else)

In Europe the Yamaha 450 is a championship winner

In AMA it’s a blue pig

GP bikes can't be sold after a few rounds because they're deteriorated too much for riding so fast. AMA bikes are sold to C riders

AMA requires production frames, swing arms and engine casings. The FIM allows full prototypes, and there was once a time when works bikes didn't share a single part with a production bike (ask Decoster).

The sky's the limit on MXGP bikes, but production bikes are so advanced now (and have been for a while) that there's not much point in designing a one-off bike from scratch just for the GPs. But I agree with what someone posted about custom frames -- they have the freedom rework the frame geometry to match a rider's proportions and riding style. Even the Pro Circuit bicycle teams do that.

As a minimum, the MXGP teams get new models before they're available to the public. You might remember that the now-defunct Suzuki MXGP team had the new RMZ a year early, and the Wilvo Yamaha team did the same with the new YZ. Can't do that in the AMA.

The GP bikes have lots of stickers with funny spelt names and logos

GP bikes are way tricker. I remember an Eric Geboers (god bless him) interview. He talked about having a uniquely specd and sized frame for his RC500 along with all kinds of other stuff like cranks that had holes drilled through them for tungsten plugs to alter balance. Nowadays though I think the biggest difference is that the GP bikes have an extra gear or two.

Just kidding

GP bikes win the Motocross des Nations.
AMA bikes don't.

Objection, speculation.

GP bikes sound different due to their accents.

They use Aluminium frames instead of Aluminum, aside from KTM/Husky of course.

GP bikes are made to fit the rider in almost every aspect. AMA bikes require the rider adapt to whatever is thrown at him.