I also posted this question in StackOverflow. Here is the solution they came up with:

select DealerId, Tire, count(*) as RepairsOfThisTire,

max(cnt) as TotalRepairs,

count(*) * 1.0 / max(cnt) as PercentOfTotal

from (select t1.*, t2.Tire,

row_number() over (partition by t1.DealerId

order by (case when t2.tire is null then 1 else 0 end),

t1.RepairId

) as seqnum,

count(*) over (partition by t1.DealerId) as cnt

from table1 t1 left join

table2 t2

on t1.RepairId = t2.RepairId

) t

where seqnum <= 0.8*cnt and tire is not null

group by DealerId, Tire

order by 1, 2;