All modern Power Wheels come with 550 size motors but can also accept a 775 size in the factory gearbox. This is not the case with all brands and most can only accept 550 size. Generally 550s run fast, have less torque/power and run hotter and can be more prone to failure while 775s run slightly slower, have much more power and run cooler which is why we recommend 775 motors at 24v.
550 size motors have a can diameter of ~1.5" and can length of ~2.25". Overvolting 550 motors to 24v (which are typically rated for 12-14v) with an ESC is often fine overtime but longevity ultimately depends on ambient temperature, motor airflow, duration of constant use, rider weight, terrain type, etc. Typical usage on a flat lawn in most cases will not result in a failure of the factory motors or gears. Overvolting without an ESC will create very high amp spikes which will exceed 100 amps (2,000+ watts) briefly, which is extremely bad for the motor and will greatly add to the internal heat build up. If the motors experience more than average use with an ESC or with a factory setup on 24v the motors can overheat to the point of failure which is around 160*F (internally). Once the motors fail they will typically emit magic smoke and stop working. Once the motors are removed and inspected a motor that has failed from overheating will either be seized (failed) or rotate roughly (failure imminent) and will smell strongly.
775 size motors have a can diameter of about 1.75" and can length of ~2.6". They have more power/torque (~+2x over 550) and run cooler at higher voltages. 775 motors require new pinions (550 and 775 motors have different shaft diameters) which match the gearbox (info below) and new bolts which are larger than the 550 size. Spacers were required when mating a 775 with the old #7 gearbox (discontinued around 2010) but are not needed with the newest generation Power Wheels gearboxes known as 7R.
Unfortunately there is no stronger than factory Power Wheels gearbox available on the market and the first gear is least likely to fail, not the most. But, the current generation of Power Wheels gearboxes (7R) are stronger than the previous (#7) which was discontinued around 2010-ish. The easiest way to tell them apart is the new 7R's have groupings of numbers stamped on them with a hole which denotes the internal gears, the example below is a 21-22-23. The older #7 gearboxes will simply have one number stamped on them denoting the motor pinion size.
Please also note in this photo that the Power Wheels factory gearboxes have mounting holes for 550 and 775 motors (although Power Wheels does not offer 775 motors in their cars- thanks Mattel!)
Gearbox Quick Facts
7R Power Wheels gearboxes come in 3 varieties, 15-16-17, 18-19-20 and 21-22-23.
All cars leave the factory at the same 2.5/5mph speed but they do not all have the same sized tires so as a result there are different geared gearboxes. Cars with large tires (Dune Buggy for example) have smaller pinion/slower 15-16-17 gearboxes while cars with small tires (Mustang for example) have a larger pinion/faster 21-22-23 gearboxes.
All 7R gearboxes are the same except for the position of the first internal gear in the gearbox (which is denoted by the hole punched in the gearbox as shown above). Depending on the size of the pinion gear the location of the first internal gearbox gear is in a different location so that the larger motor pinion gear meshes correctly. This means that you can swap the first, second, third or final gear from any 7R gearbox into another 7R gearbox. But, you MUST match the pinion gear to the gearbox. For example in the 21-22-23 gearbox above but you must use a 22t aftermarket pinion gear (even though it may ship with a 21, 22 or 23 pinion). You cannot take a slow 15-16-17t gearbox and attempt to fit it with a motor with a 22t pinion.
The factory uses a non-standard pinion gear pitch which is not available in the aftermarket. Simply put, this means that a 15-16-17 gearbox will ship from the factory with either a 15, 16 or 17t pinion but when buying an aftermarket pinion you must choose the middle # or in this example 16t.
Again, all gears in the gearbox can break and in our experience the largest gears are most likely to break while we have never broken the smallest gear in the gearbox. Ever. The only issue we have ever experienced with the first gear was the gear melting on the gear post which happened on our cars once when the center post became dry so always make sure there is grease on all gear posts (the factory grease is the best to use). Finally for easy gear changes we recommend using the wheel retainers which can be found in our store.
Avoiding Broken Gears
The new 7R factory Power Wheels gearboxes are the strongest and best gearboxes to use. There are no stronger aftermarket Power Wheels gearboxes available anywhere now or in the past. We find that uneven terrain that has ruts, bumps or other sudden drops seem to break the plastic gears much more than any other factor by far. We torture test our ESC setups with the most powerful 775 motors available (330w 150mNm) and have never broken a gear during a hard (55amp 1,300 watt) even overloaded 24v ESC start so motor power during our use has never broken a gear.
Adding rubber traction to rear drive wheels will typically result in gear failure in almost every case and is not recommended while adding rubber or other traction to the front tires is recommended and will reduce road noise and understeer.
Since the internal gears inside every 7R gearbox are the same we recommend buying a spare factory gearbox without a motor ($20-30) to use for parts in case a gear breaks which is an average cost of $5-6 per gear.
In short, gear failure is always possible and something that all Power Wheels modders have always dealt with. The frequency of gear issues depends on the level of abuse which includes weight overloading, overvolting without an ESC, rear rubber tires and most importantly uneven terrain.