its prob aswell that you didn't go any faster as you'd prob kill yourself at this time of year with the roads being so frosty and if you didn't kill yourself you would have definately killed the engine,doing a top speed run on a rst is risky even with a perfectly set up engine but doin it with one thats hasn't been rollin road tuned is risky,very risky
anyway my advice get it set up and attempt your top speed either at a better time of year or even better at a airfield or something