Postgresql querying trends

Firstly apologies if this is a duplicate, I am fairly new to SQL and so Im not sure what the correct terminology to use in my searches So I have a database which records motor races, with the following simplified schema race_table ========== race_id PK race_date timestamp average_speed Decimal max_speed Decimal drivers ======= driver_id PK driver_name text date_started timestamp driver_races ============ driver_id FK race_id FK If each driver has ~1000 races spread over 2/3 years How would I go about querying the overall % change (positive of negative) in their average race speed for a given date range, for example % Change in first 6 months Joe Smith - 5% increase Andy James - 4% increase % Change in first 12 months Joe Smith - 8% increase Lewis May - 6% increase UPDATE: More detail on % Change The % Change could be calculated using linear regression, (least-squares-fit would be suitable), the average change is effectivly the y-difference on a line-of-best-fit, where each point is a race, x is the race_date and y is the average_speed for that race. Postgres's regr_slope will give the gradient of the line which is effectivly the same as the %change SELECT regr_slope(r.average_speed, EXTRACT(EPOCH FROM r.race_date)) as slope FROM race_table as r, driver_races as dr WHERE dr.race_id = r.race_id AND d.driver_id = 1 This gives the figure I want, but I now need to apply is against all users, sorted by 'slope'