2005 Dodge Magnum SXT - Auto>Manual Conversion- 10.3% decrease in fuel consumption
 

This past March, my Dodge Magnum decided to shred apart its 4-speed automatic transmission. Shortly afterward, I decided to swap the 4-speed out for a 6-speed NSG370 transmission. I placed the Magnum back on the road with the new transmission this past October, and start gathering fuel consumption data for it in November.

2015 (with 6-speed NSG370)

Date	Odometer (mi)	Fuel (gal)	FE (US)	FE (L/100)
11/09/2015 19:56	350.6	15.443	22.7	10.4
11/20/2015 16:39	353.6	14.455	24.4	9.6
12/07/2015 08:04	364.9	15.719	23.2	10.1
12/17/2015 19:35	380.1	15.479	24.6	9.6
Total:	1449.2	61.096	23.7	9.9

2014 (with 4-speed 42RLE)

Date	Odometer (mi)	Fuel (gal)	FE (US)	FE (L/100)
11/10/2014 00:00	342.2	15.122	22.6	10.4
11/20/2014 18:48	277.9	13.470	20.6	11.4
12/05/2014 17:14	284.0	14.154	20.1	11.7
12/15/2014 18:26	313.8	14.501	21.6	10.9
Total:	1217.9	57.247	21.3	11.1

Note: I also changed the rear end from 3.636 to 2.87, in order to compensate for the top gear change from 0.69 (42RLE, 4th) to 0.84 (NSG370, 6th). However, the resultant decrease in engine speed from this swap (about 3.9%) cannot account for the 10.1% decrease in fuel consumption.

I read the title and thought you started using MORE fuel, glad i was wrong! contrats! I'm a bit jealous, i always wanted to drive a chrysler 3.5 with a manual trans.

what are the rpms at 60?

Thanks! These 3.5L engines do seem to like being harnessed to a manual very much. Automatic transmission really sap a lot of power.

About 1780 RPM. I also had to change out the rear diff. It was a 3.64 ratio, now it's a 2.87 ratio.

Here's a short vid, using Torque Pro and Track Recorder, shortly after I returned the Karen-mobile back to the road.

[youtube]yrAfN-1irGE[/youtube]

....so h0w do you know it WASNT the rear gear change that improved the mileage.....and why not include that info in the first post......

Fair question.

Top gear on the dead 42RLE had a ratio of 0.69. Top gear on the replacement NSG370 has a ratio of 0.84. This represents about a 21.7% increase in propshaft rotation speed, for a given vehicle speed. If I were to have left the original 3.636 rear end in place, that should have resulted in a definite increase in fuel consumption.

The FDR was 3.636, while it is now 2.87. This represents about a 21.1% decrease in required pinion input speed, for a given vehicle speed.

These two changes almost cancel each other out, and the net decrease in engine speed (about 3.9%) does not account for the 10.1% decrease in fuel consumption.

Put it another way - Engine speed at 60 MPH was about 1850. Now, it's about 1780. There is no way that this change in engine speed (~3.9%), by itself, would have resulted in the measured decrease in fuel consumption.

Because I forgot? I only regarded the rear end swap as a necessary adjustment to counter the larger top gear ratio of the installed NSG370, nothing more. I figured that the 3.9% drop in engine speed that I calculated back in April of last year, would have given me a very modest 2% decrease in fuel consumption. This is based on the rule of thumb that a percent change in fuel consumption will be about half the percent change in the rear end ratio. The 2% change would have been rather hard to measure as it is just above noise.

You do bring up an interesting idea - What if I had left the original 3.636 rear end in place? Engine speed would have been about 2250 RPM at 60 MPH, which would have resulted in a net increase in engine speed of... 21.7% from when the 4-speed was still installed. I would have expected a 10.9% increase in fuel consumption just from swapping in the NSG370, if I were to have treated the engine speed increase as a result of swapping in a (nonexistent) 4.43 rear end.

Nice mod. I bet its a lot more fun to drive now too.

Oh, yah! I like to think that the Karen-mobile now has performance that approaches the available 5.7L V8, while having fuel economy that approaches the available 2.7L V6.

Shortly after I placed the Karen-mobile back on the road, I accidentally gave too much gas when starting in 1st from a dead stop. I left a nice, pretty black tire stripe for about 20 feet on dry pavement. Obviously, not good for either tire life or fuel economy. I shudder to think what would have happened, had I left that 3.636 diff in.

The lower weight of a manual transmission and the absence of the torque converter slip should be taken into account for the final results, but what about the gear spread of the 4-speed automatic and this 6-speed manual? Is it the same transmission used in the Wrangler and the Sprinter, right?

The 42le and 42rle were really just a modern take on much older transmission design. The increase is probably a result of no converter slip when accelerating, no pumping fliud, etc. Plus now you're essentially doing mini pulse and glides whule shifting.

The Jeep Wrangler used this transmission on the 3.8L V6 engine (which was what made this swap possible - the 3.5L and the 3.8L V6 bellhousing bolt patterns are nearly identical). The Chrysler Crossfire also used this transmission, but with a radically different bellhousing bolt pattern.

42RLE gears

1st	2.80
2nd	1.55
3rd	1.00
4th	0.69

NSG370 gears

1st	4.46
2nd	2.61
3rd	1.72
4th	1.25
5th	1.00
6th	0.84

The gearing appears to be more closely spaced together on the 6-speed. That would help with fuel economy during acceleration, particularly with city driving.

I think that the two largest power drains were torque converter slippage and pumping losses through the transmission oil pump.

Not sure if transmission weight itself would have played much of a part. I lost maybe 50 lbs or so from the swap.

Heh, that's true, too.