What do you mean by "Total # of tires for the size"?

Here's your data set up in a consumable format:

DROP TABLE #Table1

CREATE TABLE #Table1 (DealerID INT, RepairID INT)

INSERT INTO #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)

DROP TABLE #Table2

CREATE TABLE #Table2 (RepairID INT, Tire VARCHAR(15))

INSERT INTO #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')

Here's a query built upon your data which may go some way towards what you're trying to do:

SELECT DealerID, Tire, RepairsOfThisTyre, RepairCount = SUM(RepairsOfThisTyre) OVER(PARTITION BY DealerID)

FROM (

SELECT DealerID, Tire, RepairsOfThisTyre = COUNT(*)

FROM #Table1 d

INNER JOIN #Table2 r ON r.RepairID = d.RepairID

GROUP BY DealerID, Tire

) d

ORDER BY DealerID, RepairsOfThisTyre DESC

How close is this to your requested result?