M6: 3.42 vs 3.73 differential

**tici** · 11-01-2005, 02:30 PM

Last weekend I put a T56 in my originally automatic TA.
The rear end is still a 2.73 and it sucks! (of course).
I'll soon install a shorter ratio but I don't know which one is better.
What are the pro and cons of 3.42 and 3.73 gears?

6th gear manual with 3.73 has the same RPM as the original 4th gear with the A4. Using a 3.42 it will be a little longer: less rpm and a better mileage.

What do you suggest?

Our highways are for 120 - 140 km/h (75 - 87 mph). Is the 3.73 too short?

I saw many M6 peoples installing a shorter gear set as stock. Why?

Thanks - Stefano

**HotRodV6** · 11-14-2005, 04:51 PM

Well, most 6 speed cars go with a 4.10 rear gear, it gives great performnance and with the 6th overdrive you can still get some good milage on the freeway. 3.42 wouldnt be much better than what you have now, and since your going to do the swap im would do 3.73 minumim, and best bet would be the 4.10 gears.

Good luck. Wish we had that kind of speedlimit here in the states.

**Hurley711** · 11-14-2005, 05:02 PM

I believe the rule of thumb is 4.10 for M6 and 3.73 for A4. I swear I've read that several times before. Get a stall convertor while you're at it.

**GatorSS** · 11-18-2005, 10:20 PM

Depends on how much highway driving you're going to do.

**tici** · 11-19-2005, 08:52 AM

Highway is 3/4 of what I drive, that's why I was thinking at a low ratio.
the M6 with a 3.73 in 6th gear has the same RPM as the A4 with the 2.73 in 4th, so I won't go higher than that.

**7camaro7** · 11-19-2005, 09:28 AM

I have a question, can an LS1 with 3.42 (a4 or m6) be just as good as an LS1 with 4.10(a4 or m6) on acceleration AND top speed?

sonnyred · 11-19-2005, 12:38 PM

Originally Posted by tici

Highway is 3/4 of what I drive, that's why I was thinking at a low ratio.
the M6 with a 3.73 in 6th gear has the same RPM as the A4 with the 2.73 in 4th, so I won't go higher than that.

I always thought 3.73 or 2.73 refered to first gear only and sixth gear was 0.50 on all m6

**SS#1531** · 11-19-2005, 01:49 PM

I'd go for the 3.73's. That's what I have in my M6 car. It's a good compromise. You'll still have great acceleration, and get decent fuel economy. At 85mph I still get about 24mph, or 38.6 kilometres per gallon, which if I did my math right, is 8.77 kilometers per liter.

sonnyred · 11-19-2005, 02:02 PM

I agree.

**GatorSS** · 11-24-2005, 08:36 PM

Since it's 3/4 time highway driving, I'd lean towards the 3.42.

**98_Formula** · 11-24-2005, 08:56 PM

definitly 3.73's, i think if u upgraded to the 3.42's you'd just want more and wanna swap out to the 3.73's ne ways. u'll still get good gas milegae, and if u worry bout that just think of me i'm going to putting 4.10's in my A4, ... gas mileage... whats that?

**tici** · 11-24-2005, 09:36 PM

we pay now 1.70 - 1.80 swiss francs per liter
about 5.15 - 5.45 US$ per gallon

so mileage DOES count...

**Killer_bluebird** · 11-24-2005, 09:37 PM

3.73's Gets's my Vote. I just had them installed on my TA and I was surprise at the difference. During street driving the engine revs a lot quicker the car has more low end, as far as gas milage I rarely bang through all the gears I use 1st, 3rd, 5th on the city with no need to use the other gears, on the Highway I can't remember exactly but the engine still revs less than 2,000rpms at 75mph. As far as the track I don't 1/4 mile my car often (4.10s would be better) but for the road course it is perfect.
My .02

Good Luck.

**GatorSS** · 11-24-2005, 10:04 PM

Originally Posted by tici

we pay now 1.70 - 1.80 swiss francs per liter
about 5.15 - 5.45 US$ per gallon

so mileage DOES count...

With those prices, 3.42 is looking real sweet!

**7camaro7** · 11-25-2005, 10:49 AM

so when you put 3.73's in and take out 3.42's, it affects your overall gas milage? The way I understand it is that those are only the first gear's gears. Why would it have anything to do with gas milage after you shift?

dubS6 · 11-25-2005, 01:07 PM

Originally Posted by 7camaro7

so when you put 3.73's in and take out 3.42's, it affects your overall gas milage? The way I understand it is that those are only the first gear's gears. Why would it have anything to do with gas milage after you shift?

b/c at 80 mph or so you are at like 2000 rpm w/ 3.42's and with 3.73's you would be at like 70 at 2000 rpm. so at the same rpm or usage of gas you would be going slower with taller gears.

as far as the gears go i would never go below 3.73 with the power you are making. with the supercharger i think 3.73 would be a good gear b/c it holds you in boost longer. I went from 3.42's to 4.10's and only lost about 2 mpg on the highway with my h/c 420hp car. last week i actually got 24 mpg but that was a first. gears make a night and day difference with that much power.

**hammertime** · 11-25-2005, 01:26 PM

Originally Posted by 7camaro7

so when you put 3.73's in and take out 3.42's, it affects your overall gas milage? The way I understand it is that those are only the first gear's gears. Why would it have anything to do with gas milage after you shift?

When you change the rear axle gear, it changes the reduction ratio after the transmission, so all ratios are affected, not just the first gear.

Also, keep in mind - all LS1 era T56's have a .50:1 6th gear. That means your 4.10 gears feel like a 2.05 in 6th.

75 mph w/ 2.73's and .7:1 overdrive = 1875 rpm
75 mph w/ 4.10's and .5:1 overdrive = 2010 rpm

4.10's will give you much improved acceleration out of the hole for a whopping 135 rpm over a 2.73 A4 car.

Go 4.10's!

**hammertime** · 11-25-2005, 01:33 PM

TICI - just saw that you are supercharged...

Are you drag racing this car? 1/8 mile, 1/4 mile? What rpm is your peak HP? What is your trap speed? Ideally, you would want your car to just pass peak HP rpm as you run through the timimg traps.

**tici** · 11-26-2005, 02:37 PM

Originally Posted by hammertime

TICI - just saw that you are supercharged...

Are you drag racing this car? 1/8 mile, 1/4 mile? What rpm is your peak HP? What is your trap speed? Ideally, you would want your car to just pass peak HP rpm as you run through the timimg traps.

This is a daily drive: 365days a year.
I installed the supercharger because I wasn't happy with the stock power, with the additional 100 RWHP I'm more than happy.
It has a stock behavior under 3000 rpm, then suddendly you feel "a certain difference"

Max torque at 4800 rpm, max power... at 5800 rpm it was still rising but then we stopped logging.
No drag raging. 2-3 spirited trips every year to Germany on the freeways. A couple of races on some rings with friends (Italy, Germany or France).
Normal speed on the highway is 80 mph that's why I was thinking that 3.42 would be a good idea.
I know: for acceleration 3.73 would be better, but the time I spend on racing is 1% of the total driving time so maybe it isn't so impostant to get the maximum out of it.

**GatorSS** · 11-26-2005, 06:05 PM

Originally Posted by tici

... I know: for acceleration 3.73 would be better, but the time I spend on racing is 1% of the total driving time so maybe it isn't so impostant to get the maximum out of it.

That is the reason I would go with the 3.42.