F.D. Ratio change on 1983 240D. First report.

[cjlipps]

I changed the stock 3.69 ratio for a 3.07 ratio from a 300D turbo. My commute is around 40 miles several days a week and around here it's pretty flat and at fairly low altitude so I thought the rpm drop at highway speed would be more comfortable and not hurt driveability.
The first two gears (automatic) are a little too low anyway and now they are about right. Third is good to about 50 mph (gps) but once it shifts to 4th it struggles to speed. However, once attained, 65-75 mph is fairly easy to hold, even up the (mild) hills and the engine noise improvement is dramatic. Now an indicated 59 mph is a gps 70 mph so you get the idea of the experience. (A speedo change is in the future) I actually think the 0-50 time is improved noticeably but the 0-60 time is about the same (which translates to an abysmal 50-60 time). I'll report on any significant fuel economy change.
Would I do this in a hilly environment or at higher altitude? Probably not. So far I don't regret it though and my car is about due for a valve adjustment and I have a dragging front brake so it may work out even better once those are corrected. I dread summer with the additional a/c compressor drag on the highway though.
Time will tell......

[Stretch]

Interesting - what size tyres are you using?

[leathermang]

Did you carefully measure the mileage you were getting with the stock rear end?
What was it ?

[t walgamuth]

I have advocated 346 as max for a 240. Glad you are satisfied with your choice! I would not worry about struggling to reach cruising speed. I'd hold it in third to get up there and just maintain with fourth.

[pawoSD]

I think 240's are pretty peppy with a 3.69.....I bet a 3.07 + manual trans would be "acceptable".....living with a 240d in the first place requires a different mindset.

Meanwhile, I just adjusted the valves today on my 617 to squeeze a bit more performance out of it, and it worked. Runs even better now.

[cjlipps]

Army, they are the stock 185/75R14 tires IIRC.
Leathermang, I check mileage regularly and it's usually around 25mpg in cold weather and peaks at 28ish in hot weather. I haven't actually put the calculator to it and averaged my numbers but that's ballpark.
PawoSD, amen on the different mindset. This morning I actually PASSED two cars (one on a two-lane road) so it's looking promising.
With only about a day and a half on the alteration I think it's going to be alright around town but will need some attention on the highway until up to speed. Once up to speed the change is very pleasant. Picture a cabin noise level of 59 mph with a road speed of 70 mph.
I'll report more later. I'm going to fill up and zero the gps today. Should need filled up by Sunday at which point I'll have some results even if inconclusive. If I have time I'll adjust the valves Saturday.

[charmalu]

I haven`t driven a 240 with an automatic, nor have I dirven one with a 3:07 Diff.

I would imagine a 240 with a 3:07 and auto would work better than one with a 4-spd manual. especially when starting on a hill. you have the torque convertor slippage to get you going.

I was doing some diddling aroung with the calculator, and if I worked it out correctly, the difference from a 3:69 to a 3:07, is 20.2%.

I changed my 240 from a 3:69 to a 3:58. got it out of a Euro W123 230E. had the 150mph spedo, so couldn`t pass it up. I do see a small decrease in my get up and go some.
I got a GPS for Christmas, so with this tank Iam on we`ll see what we get. Iam running 195-70-14 tires, and the GPS shows the spedo almost spot on. But the odometer is off a bit.

I think the 3:58 is a 3% difference.

Charlie

[ForcedInduction]

If you're worried about acceleration you could always find a 200D or 220D diff, its a 3.92.

[cjlipps]

Originally Posted by ForcedInduction

If you're worried about acceleration you could always find a 200D or 220D diff, its a 3.92.

It's too late to worry about acceleration in a 240D. I bet the 2.92 would sure help the "hole shot" though.

After another day with the 3.07 I like it better all the time. The speed is a little harder to manage than before because there is less engine braking going downhill so you tend to speed up and there's less grunt to get you up the hill so you can't afford to let it drop off approaching the hill. All in all, the cabin noise is MUCH better and the 0-50 is better also. The worst part is once you get into 4th (I let it shift about 50) it scratches and claws for every mph up past 60. If you can go above 60 or below 50 it's great. I also did a top speed test on a deserted flat road and over about 2 miles it topped out at 82 (gps). I filled the tank and did a fuel economy check and came up with 25.7 but it was only about 100 miles so not a very good representative sample. Tomorrow I'll drive about 400 miles so I'll know more.
I also took the one front wheel off and freed up the sticking caliper so that might unleash an extra pony or two. If there's time tomorrow I may do a valve adjustment before departure. Can't hurt and it's due anyway.

[winmutt]

[QUOTE=t walgamuth;2636451 I would not worry about struggling to reach cruising speed. [/QUOTE]

Its a 240, I assumed he already had the patience for it!


Maybe try adjusting shift RPM?

[t walgamuth]

Right, hold third til you get up to speed then shift to fourth.

[ForcedInduction]

Originally Posted by t walgamuth

Right, hold third til you get up to speed then shift to fourth.

I was just thinking the same this afternoon.

If you're shifting to 4th at 50mph then you're only revving to 2500rpm. 3rd gear is good for 75mph@4500rpm with a 3.07. Hell, 50mph is barely past the top of 2nd!

[pawoSD]

I've driven a manual 240 and an auto.... surprisingly the auto seemed faster, maybe it was just not having to shift.

[winmutt]

Originally Posted by ForcedInduction

I was just thinking the same this afternoon.

If you're shifting to 4th at 50mph then you're only revving to 2500rpm. 3rd gear is good for 75mph@4500rpm with a 3.07. Hell, 50mph is barely past the top of 2nd!

240d.

[vstech]

mutt. 240d with a 307 rear would be same rpm's as a 300d turbo... just not the same power...

at all...