Zeke, biodiesel has lower BTUs than D2 and may have something to do with the 20 mpg. What MPG are you getting with D2?

Graminal95, are you really getting 34 mpg? An increase of 7 mpg with new nozzles? If true, that's gonna be my next project!!!

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

How are your tires? Have you check their pressure lately? Probably have but just thought I would ask. Also how is the wear on the tires? You might have an alignment shop check out the alignment.

Also have you ruled out brakes dragging. You could have a caliper seized and a pad is dragging slightly. If your brake hoses are old then can cuase restriction in the system and not allow the pads to release off the rotors like they are supposed to. Brake hoses swell on the inside when they get old. Which allows you to appply the brakes but it doesn't allow the brake fluid to flow back as easily and can cause the pads to drag. You're parking brake could also be dragging. Try adjusting the parking brake off a little and see if that helps.

Biodiesel does have less BTU's but it should only drag down milage a few mpgs, not clear down to 20.

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I ran my 240D 4 speed manual on B99 and it got 33mpg on the freeway at 68mph for 350 mile stretches. I doubt the fuel is the problem.

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1974 240D 3.0L 4(M) - 250K -OM617.910 + 300D FW + Rajay turbo power!
1977 6.9 3(A) - 84k