My 78 300SD does 0-60 in 11.9 secs
 

Want to know how? Good, so do I! I'm baffled. But I'm sure the big brains on this forum can figure it out! :) Here's the situation:
- New-to-me W116 300SD with 120k miles. Nice condition, but sat a lot last several years due to elderly original owner. I've done nothing so far except change fluids, adjust valves, check hoses. I'm sure injectors, turbo, etc. need attention still.
- Car has incredible acceleration: 11.9 secs to 60mph, measured multiple times with GPS and stopwatch. Chirps the tires on shift from 1st to 2nd. A real hoot to whip through the country roads.
- Transmission shifts smoothly through all 4 gears; no flaring or slipping, etc.
- No obvious engine mods (e.g., no Doktor Bert-esque tuning of cam, timing, ALDA, etc.)
- Tach is accurate (confirmed with touchless tach)
- Speedo reads 15-20mph high (e.g., reads 79 when GPS shows 60)
- High speeds require excessively high revs - e.g., approx 4,000 rpm at 70 mph. I'm afraid to push it above 78 as it sounds like it'll rpm itself to pieces. So highway cruising at modern speeds is virtually impossible.

I know what you're thinking, as I thought the same thing -- some acceleration junkie must have replaced the standard 3.07 rear diff with something significantly shorter like a 3.69 (and not recalibrated the speedo of course). So, I crawled under the rear to look at the stamp on the diff and it says. . . 3.07. Huh?!

I'd greatly appreciate folks' thoughts on what might produce the above characteristics in this car. And if anyone wants to drag race, just holler :D. Thanks!

11.9 is just barely faster than stock specs... Of 12 seconds, so, not a ton faster.

If the casing states 3.07, it's possible the ring and pinion were swapped from a taller diff.


The tach is the mystery, 4K at 70 is screaming for sure... I can't imagine how the tach could be off, if the gps has verified the speeds... With a 3.07 unless the torque converter is seriously shot.

Tire size?

Thanks for pondering this with me.

Tires 205/70 R14.

If torque converter were badly shot, presumably I'd see a number of other symptoms, right? Tranny shifts smooth and strong, no slips, no clunks, no leaks, etc....

Keep the ideas coming!

Trans fluid might be hotter with a worn tc, and shift rpms would be higher... Might froth the fluid...

Since you've verified the RPM is accurate, that the speed reads consistently off, and you know the tire size, then the last item to compute is the diff.

If you're driving at 70 mph at 4000 rpm with 205/70/14 tires, your diff should be around 4.303 according to this site:

Tire Size, RPM, Speed, and Differential Ratio Calculator

And yet the diff is stamped 3.07....

This is the stock spec. It's what your car is supposed to do when maintained.

Dkr.

Regarding acceleration, yes, I understand a perfectly tuned and maintained W116 300SD can (should) accelerate like this. This car, however, having sat virtually unused, is pretty far out of perfect tune right now, so I'd expect it to perform a bit more sluggishly.

Regarding higher speeds and rpms, surely 4,000 rpms at 70 mph is not stock spec, right?

my 82 300D runs 34-3500 rpm at 65 on the highway, bone stock as far as I can tell, shifts beautifully, firm but not hard, and everything has been very very well maintained, seriously I need to post some pics, especially considering what I've got into both cars honestly.

but the jump from 65 to 70 makes very small rpm difference though, maybe 3600 at 70ish

My other 82 (this gets real confusing with registration and insurance having 2 nearly identical cars) which is getting a 4 speed swapped in very soon has a non functional tach, but sounds about the same, I want to find some 2.88s or even the 2.47 and swap into both cars.

My 83 300D runs around 3,400 RPM at 75 mph, 3,000 RPM at 70 mph and 2,800 rpm at 65 mph. It does have a stock 3.07 and 205/60/15 tires.

Which is probably why one of the responses earlier said the internals may have been swapped. Is there a way for you to confirm the ratio (other than reading the stamp)?

The other thing I can think of is the transmission has been changed and the gearing is not stock anymore. Double check the transmission # on the side.

To confirm ratio of diff, can you jack rear wheel off the ground. Chalk mark the drive shaft, shift in n. turn the rear wheels by hand and count revolution of the drive shaft? Don't do this until someone confirms it will work. Mike

Btw. My diff is wrong for my 300d turbo engine. I love it

I confirm, it will work
 

I mark the RR tire on the sidewall at the bottom and the DS on the same side. I haven't done that since working with 12 bolt Chevys back in the 70s.

If you want to know exact what gear is in the differential without removing the cover that's the ultimate test. It eliminates transmission concerns and tire sizes.

So, hows the fuel consumption going?