If it's for practise, club meets etc then try some Schumacher full blocks on the rear. They work quite nicely and last forever. Not as much side bite as minispikes so will never be as quick but as long as you balance the car out with some less grippy fronts then they're pretty good. Work best on long pile astro but have been told they are ok on worn short stuff too. Done 100s of laps on mine round telford and still going strong
http://www.racing-cars.com/pp/Old_Ca...cal/U6786.html