azoufan

So along with the head and cam swap in the August issue of MM&FF, there is also a 6 speed trans swap article. D&D Performance has a 6 speed "swap kit" for the 05+ Mustang and they have a whole writeup on the replacement procedure. $5K for the whole thing including a one piece driveshaft.

Specs are a .50 final gear ratio for 6th gear compared to .68 in the factory 5 speed. I looked around and found a calculator to show me the difference in RPM's between the 5 and 6 speed transmissions and came up with this:

Gear - 4:10
Tire Height - 27"
Trans Gear - .68 (stock 5 speed)
MPH - 80
RPM = 2776

Gear - 4:10
Tire Height - 27"
Trans Gear - .50 (D&D 6 speed)
MPH - 80
RPM = 2041

Now those actual RPM values may differ slightly, for instance I believe that at 80 MPH my tach actually shows 2900'ish, but regardless a drop of 700+ RPM in 6th gear at 80 MPH is substantial.

So help me justify spending $5k on a new 6 speed transmission. Based upon those numbers, price of gas at $3.25 (a little higher than it is today, but we'll get there for an average soon enough ), average MPG of 19.5, and the fact that I drive approximately 50 miles per day, about how much will I be able to save in gas by dropping highway RPM's at 80MPH by 700? Yeah I know, I don't drive at 80MPH all the time, it's just a baseline to get some numbers flowing. Pretend I drive 80 all day every day. How would I go about figuring that out?

Your help is greatly appreciated!

Bingo

Just put some sub-3.00 gears in the rear and you'll be set.

azoufan

Yeah I know, make up your mind, install 4:10's or try to get good gas mileage. Look, I didn't say it was a logical way of looking at it, it's just a way of looking at it. I'm never selling the car, so if I can work out how much gas I will save over say the next 5 years or so and can somehow (on paper anyway) work out that the transmission will pay for itself, then the WAF (Wife Acceptance Factor) goes way up. You've always got to have an angle my friend...

Bingo

Definitely...that's logical thinking. Better than some people who go out and buy NEW cars for "better fuel economy". Heck, it'll take >10 years to make up for the cost of the car in savings in fuel, and the person would probably have sold the car by then.

Personally, I wouldn't go the 6-speed route 'cause I don't do much highway driving. But if you're crusing around 75 most of the time, a $5-6k tranny swap may be worth it. Plus I'm sure you could get $1k at least for your 5-speed......

In your case, I'd just figure you'd get probably a 10% increase in economy - maybe 2 mpg. If you drive 18000 miles a year, that's a saving of about 82 gallons (assuming you went from 20 mpg to 22 mpg ave). 82 gallons x $3.25 in gas = $266. I don't think it would be worth it.

Of course you could see a 20% increase in economy - I don't know - but I don't think a transmission swap would be justified.

2MFF

$5000 will buy you 1538 gallons of gas at $3.25. A trans will never save you that. Especially a fake 6 speed

pville piper

A fake 6 speed? One thing about D&D, they don't sell fake anything. But 5 grand is a bit much when they sell a kit for every other stang at $2800.

Cheese302

why do people call it a "fake" six speed. There is nothing ke about it, oh are you talking about two overdrive gears? well they are still two gears, or two seeds. so there are six speeds. AND if you throw some crazy gears in the rear end, you are going to go faster. and then the fuel economy would be better.


is 5 grang justified for fuel economy alone... no, but the fact that it is now a six speed, that your car will be faster with closer ratios, and the increase in fuel economy... i would do it when i have the cash. but i am kinda twisted i just want six gears

Boomer

Wonder if he means fake as in... 4speed + 2 overdrive
instead of a 'real' 5speed + 1 overdrive

RRRoamer

Exactly! I have a 1981 Ford F100 pickup that I have owned for the last 13 years. And it gets about 11 mpg on a good day... But only a good day!

When gas started going up a couple of years ago, I decided to do the research on a Golf TDI (if I am going to get a car payment, I might as well get it attached to a car that got VERY good fuel economy!).

When I did the math on costs (insurance, fuel, car payment, etc.) I quickly found that at 40 miles per day, gas would have to get up to $4 a gallon just to BREAK EVEN with continuing to drive my old beat up pickup! Strangly enough, you can buy a LOT of gas (even at $4 a gallon) for $300!

So, I dropped the whole new car idea. Until I took a 2005 GT out for a spin around the mountains over Thanksgiving 2004....

2MFF

It is just a 4 speed + 2 overdrive gears as opposed to a 4 speed with 1 overdrive gear. Not much benefit, especially for such an expensive mod. I'm not trying to bust any one's *****, I just want to make sure people don't lay out 5 grand without understanding the numbers.

Here is an excerpt from a post we had last year at this time regarding the T-56

http://forums.bradbarnett.net/showthread.php?t=24547

Bingo

You're just supporting azoufan's point...he wants it for economy, which 2 overdrives would indubitably help with. I don't think he plans on going 214 miles per hour, so 2 overdrives would be a benefit. Besides, with such a high gearing, his 4.10s won't be so detrimental on the highway. I kind of wonder why he has 4.10s, though, if he's so worried about fuel economy.

azoufan

You wonder why I have 4:10's? You need to meet me to understand me.

Basically, 4:10's completely changes the driving dynamics of the car stop to stop and around town. I just LOVE the way the car drives with the 4:10's. The downside is that I spend a good deal of time on the freeway going to and from work, so dropping my RPM's from ~3K to ~2K or so would be like having my cake and eating it too. You see, I want, expect and demand the best of both worlds, no matter how ridiculous the request may be.

Now my parents just moved to Vegas, so I can see alot of 4 hour roadtrips in my future, which having the extra gear would be really nice. Around town, the setup is awesome. On the freeway for 4 hours, 4:10's and SLP Loudmouths = a horrible roadtrip. Since no one is willing to give me a GT500 for free, I'm trying to think of ways to get good low end and a nice highway drive. Yeah I know, impossible, but for a while it was sounding feasible.

Oh, and I'm not "worried" about fuel economy (I wouldn't have even bought this car if that was the case). I was just trying to rationalize the purchase of a $5K transmission for no other reason than it would be cool to have one.

Cheese302

i just dont understand how it having two overdrives means it is different from having two gears. it IS a six speed transmission. yo are making no sense to me. do you understand how the trans works, these are not anything like the old electric overdrives on triumphs and stuff. if you swap your rear end, you get the same net output as changing the ratio inside the trans.

the first overdrive is lower than stock which takes up the slack of the stock spacing between gers. and a .50 6th is just awesome. ther is no way i would want to swap the stock 5 speed to a .50 overdrive while leaving 4th 1.00.

2MFF

Not really. You have to look at the whole gearset, not just one gear. His expected gain of a few hundred rpm at cruise is not likely to give any real measure of economy

2MFF

With the 2.XX first gear ratio you will lose that kick in the pants, but you can have great economy with the better overdrive.

You want a cool transmission? Look into a Richmond 6 speed. True 5 forward gears (close ratio) and 1 overdrive.

2MFF

Yes, I understand how they work. Do you?
Do you really think a .50 is that different from .62?
Do you really think the double overdrives can be used anywhere other than cruising on a highway?
What speed would have to be at to utilize the .50? Hint: there is a point where the rpm can be too low for the specs of the engine

Look at the gearset and compare the overall ratio for a Viper spec T-56 wth 4.10 rear gear vs a TR-3650 and a 4.10 rear. Tell me why the T-56 would be worth the $5k swap.

Overall gear ratio for first gear:
T-56: 2.66*4.10 = 10.91
TR-3650: 3.38*4.10 = 13.86

Overall gear ratio for final overdrive:
T-56: 0.50*4.10 = 2.05
TR-3650: 0.62*4.10 = 2.542

As you can see, while the overdrive will affect cruising rpm, the lower first gear will also affect acceleration. Trade offs have to be made. Do you want acceleration or fuel economy?

So at 80mph you would have:
T-56: (80*2.05*336)/27 = 2040.89 rpm
TR-3650 (80*2.542*336)/27 = 2530.70

How many miles of cruising at 500 rpm less to recoup the $5k?

Next compare the T-56 with 4.10 vs the TR-3650 and 3.55. Tell me why the T-56 would be worth the $5k swap.

First gear:
T-56: 2.66*4.10 = 10.906
TR-3650: 3.38*3.55 = 11.955

Overdrive:
T-56: 0.50*4.10 = 2.05
TR-3650: 0.62 * 3.55 = 2.201

Again, explain to me the benefit?

If you guys want I can make a couple charts like this

http://www.2mfffabshop.com/mycar.htm

so you can see the differences numerically.

Again, I'm not trying to bust anyone's *****, I just want to make sure people understand the pros/cons of such a big dollar decision.

firestang70

[quote=azoufan]You wonder why I have 4:10's? You need to meet me to understand me.



. On the freeway for 4 hours, 4:10's and SLP Loudmouths = a horrible roadtrip. quote]
I am trying to understand you. I have SLP LMs and 4.10s= fun freeway rides and great long trips. 4.10s keep the revs past the point where my drone (1700-2000). So mine is actually more pleasureable on the highway. The tranny swap doesn't seem worth it. Good luck with whatever you choose.

06GT

AZOU: I don't know about you not driving 80mph all the time, I hit 100 at LEAST once whenver I'm on the 101/202 or 60

S197 GT

I love your approach on this one...LOL!

"Hey babe, I found a way to save money by swapping out my trannie. No seriously, not only do I save money, but I get an extra gear for free if I buy it right now!... Thanks baby. I love you too."

ford4v429

this sounds a bit like my "I heard you can get an extra 2 mpg with a shaker scoop"...the wifes not buyin that one either (yet)