Descargo de responsabilidad enorme: Esto viene con no garantía, pero si He entendido tu problema y No me equivoco en alguna parte, considera lo siguiente.
Tu mesa siendo
CREATE TABLE `foo` (`id` INTEGER, `date` DATE, `mileage` INT);
Una forma posible es crear primero una vista con la fecha de la última lectura del mes pasado (que puede convertir en otro SELECT anidado si así lo desea):
CREATE VIEW `past_month_readings`
AS (SELECT `id`,
max(`date`) AS `latestreading`
FROM `foo`
WHERE MONTH(`date`) = MONTH(NOW())-1
GROUP BY `id`);
Seguido por
SELECT `currentmileages`.`id`, `currentmileage`-`previousmileage` FROM
(SELECT `foo`.`id`, `mileage` AS `currentmileage`
FROM `foo`
JOIN (SELECT `id`, MAX(`date`) AS `latestreading` FROM `foo` GROUP BY `id`)
AS `baz`
ON `foo`.`id` = `baz`.`id`
AND `foo`.`date` = `baz`.`latestreading`
)
AS `currentmileages`
JOIN
(SELECT `foo`.`id`, `mileage` AS `previousmileage`
FROM `foo`
JOIN `past_month_readings`
ON `foo`.`id`=`past_month_readings`.`id`
AND `foo`.`date` = `past_month_readings`.`latestreading`)
AS `previousmileages`
ON
`currentmileages`.`id` = `previousmileages`.`id`
para
+------+------------+---------+
| id | date | mileage |
+------+------------+---------+
| 1 | 2015-06-15 | 1234 |
| 1 | 2015-07-15 | 1444 |
| 1 | 2015-07-25 | 2000 |
| 2 | 2015-06-01 | 100 |
| 2 | 2015-06-20 | 200 |
| 2 | 2015-07-20 | 300 |
+------+------------+---------+
esto resulta en
+------+------------------------------------+
| id | `currentmileage`-`previousmileage` |
+------+------------------------------------+
| 1 | 766 |
| 2 | 100 |
+------+------------------------------------+
