Some tires that don't balloon (like the sweeps) will help to slow you down which lowers the load. 1 inch of increased tire diameter equals 6mph in your case, more than a single tooth of pinion change makes. According to the gear calc your top speed should be around 48mph with 1 inch added diameter, kind of surprised the Neewer can't handle it.
Worth mentioning, a switch to a 50T HPI trophy truggy spur with 13t should bring top speed down enough with your current tires. Up to you which route you want to go. I'd probably switch to the 50t or larger spur though, gives more gearing options and allowing larger diameter tires too.
Last edited by Maj_Overdrive; 01-12-2016 at 09:07 AM.