Big tires may be needing a 35kg servo.
But most of the servos rated at that is nowhere that stong.
But other than that you may need a external BEC as your now powerful servo rob the power needed for your esc and receiver.
I'm running a 25kg servo in my Hot Rod without a bec.
Working fine, but I really need to install a BEC.
When your servo dies, it's already to late to fix it.
A BEC make sure the servo is getting the power need so it won't run hot.
Hot circuit board in servos is a killer for them.
R.I.P Ole (8th Oct 2012)