Modified the order by so that it listed output in descending order by RepairsOfThisTire by DealerId, which what you original indicated what you also wanted.

CREATE TABLE dbo.Table1 (DealerId INT, RepairId INT);

INSERT INTO dbo.Table1 VALUES

(1,1),(1,2),(1,3),(1,4),(1,5),(1,6),(1,7),

(1,8),(1,9),(1,10),(2,11),(2,12),(2,13),

(3,14),(3,15),(3,16),

(4,17),(4,18),(4,19),(4,20),(4,21),(4,22),(4,23),(4,24),(4,25),(4,26);

CREATE TABLE dbo.Table2 (RepairId INT, tire VARCHAR(15));

INSERT INTO dbo.Table2 VALUES (1,'225/65R17 102T'),

(2,'225/65R17 102T'),(3,'225/65R17 102T'),(4,'235/60R18 102V'),

(5,'235/60R18 102V'),(6,'235/60R18 102V'),(7,'235/60R18 102V'),

(8,'205/55R16 89H'),(9,'205/70R15 89H'),(13,'225/65R17 102T'),

(14,'235/60R18 102V'),(15,'235/60R18 102V'),(16,'235/60R18 102V'),

(17,'235/60R18 102V'),(18,'235/60R18 102V'),(19,'235/60R18 102V'),

(20,'235/60R18 102V'),(21,'235/60R18 102V'),(22,'235/60R18 102V'),

(23,'235/60R18 102V'),(24,'235/60R18 102V'),(25,'235/60R18 102V'),

(26,'225/65R17 102T');

select * from dbo.Table1;

select * from dbo.Table2;

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 dbo.Table1 t1 left join

dbo.Table2 t2

on t1.RepairId = t2.RepairId

) t

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

group by DealerId, tire

order by DealerId, RepairsOfThisTire desc, tire;

go

DROP TABLE dbo.Table1;

DROP TABLE dbo.Table2;