I've driven PLENTY of 123's and 126's, the 124 is TOTALLY a different animal. in great tune, with the suspension dialed in, the 124 from 87 will just SUCK UP the interstate! I think if you drive the 124 like you drive the 240D you will get better economy, but you won't drive it the same way... you'll fly in the car. you actually have to WATCH the speedometer to know how fast you are going... you can actually hear your radio, along with your cel phone, and anyone in the car trying to talk to you.
BUT
the 240D is a very simple klatta klatta car, and the 87 is not.
it's almost silent to drive and idle.
the cooling system is actually important to the car, and needs severe maintenance to keep from grenading... a failed belt can ruin the motor.
the vacuum pump has a plastic cage that WHEN it fails, will grenade the motor....
the fan clutch WILL fail about every 75Kmiles... keep up with it.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!