--> 测试数据：[tb]
IF OBJECT_ID('[tb]') IS NOT NULL DROP TABLE [tb]
GO
CREATE TABLE [tb]([Carid] VARCHAR(1),[jydate] DATETIME,[jysl] INT,[jygls] INT)
INSERT [tb]
SELECT 'A','2012-06-02',30,1200 UNION ALL
SELECT 'A','2012-06-12',35,1400 UNION ALL
SELECT 'A','2012-06-22',32,1600 UNION ALL
SELECT 'B','2012-06-02',34,1200 UNION ALL
SELECT 'B','2012-06-24',33,1400
GO

--> 测试语句：
select a.Carid,convert(varchar(7),a.[min_jydate],120) as 月份,
(sum_jysl-b.jysl)*100.0/(end_jygls-begin_jygls) as 平均油耗
from
(
    select Carid,min([jydate])as min_jydate,sum([jysl]) as sum_jysl
    ,min([jygls]) as begin_jygls
    ,max([jygls]) as end_jygls from tb
    group by Carid,convert(varchar(7),[jydate],120)
) a
join tb as b on a.Carid=b.Carid and a.min_jydate=b.jydate
/*
Carid 月份      平均油耗
----- ------- ---------------------------------------
A     2012-06 16.750000000000
B     2012-06 16.500000000000

(2 行受影响)


*/