Ok, Looking for some info here. I WAS thinking about 3.15's but it sounds like 3.42's might be the way to go, sooooooo

1) How bad would gas mileage suffer on the highway. I know thats not why I bought the car, but with gas approaching $4/gallon it's nice to have a car that when I'm not on it, it actually gets decent mileage. Anybody have an estimate? My cam dropped it by about 2 or 3 MPG

2) How far would top speed be reduced with 3.42's. I know, I'm not driving 180, but I'm just curious......

3) Interior noise?

4) How horrible would traction be on the low end (I don't have Z06 rears and the budget wouldn't allow me to do gears and new rims/tires). Would a different TC help?

Any constructive input is welcome......

 

I think Spin said RPM's were about 2000 at 70mph? Drive down the road in a gear that will put you at that RPM near 70 and see what your computer says for instant mileage. SHould give you a rough idea. I'm in the same boat, wondering what gear to go with. I take my car on lots of long trips so mileage is a consideration for me.

http://forums.corvetteforum.com/show....php?t=1973550

 

Ugh, 2k at only 70mph?
I regularly cruise the highways at around 75-80mph and I really do love my 31mpg but then again I'd love QUICKER ETs even more.

 

Such is our delema.... One of the guys told me with his 315's he's turning 2200 at 80 so that's only 400 RPM more than I am now with 273's

 

I went with 3:15's for my A6. I lost 2 MPG on the highway and about 3-4 mpg in the city. You should expect an additional loss of 1 to 2 mpg with the 3:42.

 

I don't think I'd go that steep. Habits are hard to break and a 3.42 sounds like a middle of the road rearend ratio, but remember, the A6 has a 1.53 third gear instead of a 1.00 like the A4 (4L65). So a 3.42 is equivalent to an A4 with a 5.23 gear. We would never hear of an A4 guy running a 5.23 gear. Remember, your first gear is going to be 13.8 to one, even steeper than a M6 with 4.10's (12.2 to 1). You better run drag radials on the street. Heck I have trouble getting my stock rearend A6 to hook up on the street.

It appears that people keep trying to turn the A6 into an M6, that is make it run the 1/4 mile with 4 gears instead of just 3. The A6 has a torque converter that multiplies torque at lower engine speeds. That's a big advantage of an auto and it allows it to make fewer shifts and still be as fast or faster than the manual. I just think Chevy got it right with the 2.56 or 2.73 for the A6. The 2.56 is still equivalent to a 3.92 A4. That's plenty steep for me.

 

I love My 3.15's that I've had for over 6 months now!

 

For the average guy on the street....................315s rock. Dont know about the strip since I am no one to go there.

 

Hey,,,,,,,,,,,,,,,,,,are You calling Me "an average Guy"!!??
j/k Hey Bro,,,I'm gonna go to the track soon,,why not You?

 

5.23(A4)

 

Your car is far from average and deserves to go to the track. Wish I was there to see this.

 

I know that Chuck Cow has a 3:42 package for the A6, maybe talk to him. Also check out this thread

http://forums.corvetteforum.com/show....php?t=1974788

 

You keep comparing the A6's first gear to al these other scnearios and I to this day own a 6.13 geared car with 700+rwhp that says youre wrong. It dead hooks in to a wheelie so the gearing isnt hurting the deal. It wouldnt hook with 4.88's. GM didnt do a good job because of the rate of change for the final gear ratio in the rest of the gears. Really fast cars with power, gears and traction from good tires need a 4th gear. If you run with 3 gears, you are slow to begin with.

Having been in a car with 3.15's and feeling it was a bit lacking and having been in a 3.42 A6 car with 400rwhp, I can say first hand that the car hooks all on its own shortly after spinning the tires on the take-off with the car floored. In my opinion having actually experienced it, the 3.42's are an absolute minimum. Your observations with the numbers crunch is bench racing and no more accurate than dyno numbers are in predicting 1/4 mile performance. The most misunderstood aspect of gearing is that the steeper you go the faster it rehooks. Yes it breaks loose more often but it doesnt spin forever like an overpowered car does with taller gears (smaller number).

The 3.42 A6 car did 70mph at 1975rpms and it did 80 with less rpms than my Z51 with 4.10's at 2250 vs my 2350.

The 1/4 mile guys will be happy to know the trap speed for a stock redline will be 139mph in 4th gear with 3.42's.

There is a reason drag racers go with extreme gearing and its 100% fasle that they hurt traction. The wheels rotate sloer and that gives you the best 60's and it hooks quickly off the line even though they do break loose more often. If you break loose with 2.73's on your A6 and it spins for 30 feet with your power level, it will break loose easier with 3.42's but may spin only 15 feet when it rehooks since it hooks when it reaches the speed the tires are rotating at. If the car has tsteeper gears they spin slower and hence the car doesnt have to be moving as fast for it to rehook.

If you have no traction its because you have too much power for the tire used and handicapping gears is the wrong way to go. Choose the tire you will run and gear for the application. Stop adding power when you cant get it to the ground anymore. Max gearing and the minimum power to still have traction is always going to beat over powered, under geared, and poor tire choices. The proof is simple; Dennis50NJ has 400rwhp and runs 10.92 with the max gearing that he can have for his trap speed. He has 3.73's and a 3800 coverter and the only limiting factor keeping him from putting 4.10's on his auto is that he cant extend the redline anymore with his stock internals motor. He said he wold do it. Countless cars have 500rwhp and run high 11's. I have seen 700rwhp run a 11.8 @138 bragging he had so much power that he couldnt hook. He claims he cant have steeper gears because he has no traction. Great combo.

Mileage will fall 2-3mph. If its going ot keep you up at night, dont mod the car. Expect a stroker with A6 and 3.42's to get 22mpg. My 1999 6cly Honda accord should have been so effiecient.

Guys that have less mileage after a cam swap can recoup much if not all of the lost efficiency by at least bumping the compression if not head porting. When you swapped your cam and left the heads alone, you dropped the dynamic compression and hence lost efficiency. Most head cam swaps together will not see a drop in mileage unless you use the new found power which of course has to burn more gas to have it.

Top speed with the A6 I dont know but the redline in 6th gear on a Z51 manual with 4.10's if you had enough power to do it would be in the 230mph range. I seriously doubt there is a mechanical limit under 200mph. Given any gear set the top speed will always go to the gear set that allows you to be at the HP peak in 6th gear. On head cam cars that is ussually in the 6300 range and 4.10's do that better than 3.90's on any transmision car. Geared cars see the top speed in 6th while 3.42 manuals see the top speed in 5th because the 3.42's and even 3.73's are way too far from the HP peak to punch through the air creating a drag limited top speed. A Z51 with 4.10's M6 will be at 200mph at 6000rpm. 3.90's would be at 5850rpms further from the HP peak. Stock cars dont have enough power to run 200mph but they do have a HP peak at 5800 or so.

None of this is a bash on your opinion but I beg you or anyone else to go for a ride in a 3.42 geared A6 and dispell the myths yourself. You can ask yourself why it works so well later.

 

It would with 3.42's. Your stock gears are spinning the tires at a higher mph so the car cant hook until it gets to the speed the tire is rotating at. It will happen 20% sooner with 3.42's.

 

here's a link to A6 trans. (dont pay attention to the heading, ratios are for A6)
http://www.xse.com/leres/bin/gearrat...mference=84.04
6th gear @100mph, 2,880 rpms

 

Spin,
Thanks for your input, it's interesting.

What first gear are you running in your racecar? If you are running a close ratio box then first gear is probably 2.2 giving you a 13.5, about the same as an A6 with 3.42's. I'm betting it works great with your slicks, not sure about runflats.


You are probably racing at a better prepared track, but where I race (Firebird), once the runflats spin, there is no chance of rehooking. I agree that it will definitely rehook quicker with higher ratio rearends since the wheel speed is lower, but once it spins your run is ruined anyway. I also autocross and I really don't want a steeper first gear since it forces me to shift more often in the tight Solo 2 events we have here in the Phoenix area.


Everyone with 4.10's always quote rpms at 70. Always amuses me since on the Freeway here we always cruise around 80. The A6 is a great tranny because of the tall overdrive in 6th gear allows us to get such great fuel mileage.


This assumes that the converter is locked up on the top end. I haven't figured out if it locks up under WOT or not. Do you know? Best I could find is it's probably slipping around 3 to 5%, so lower your 139 mark down to around 133.


Excellent information and it makes perfect sense to me. I just don't want to spin at all since it ruins the run.


Dennis is my hero. This thread is advocating 3.42's for the A6 which would be like Dennis running 5.23's. You admit that he can't do this with his tire/power combo. And I agree with you. So why will it work so well with the A6 if it won't work with Dennis' A4? Dennis could always run the steeper gears and let his A4 shift to 4th before the traps.


I had a cam only car with my 01 Z28. Nice car, made decent power and the 6 speed manual was fun to drive. However, it would run 12.7's at 114mph here in Phoenix. My rule with modding my C6 was to not touch streetability (it's my daily driver). LG Long tubes, Xcelerator CAI, 160 stat, ported intake, and a tune has given me a best of 12.03 at 118 mph. And all the mods I've done has increased my fuel mileage. Gotta love Chevy, this is a great car!

Top speed is not a concern while living in the States. If I lived in Germany, now that would be different. Spin, I have always valued your opinion and your knowledge. Thanks for the input.

 

Any idea what MPH it would be at redline in 3rd gear in an A6 with the 3.42s?
In a bolt ons only application I would love for it to not go into 4th before the line as to save that extra shifting time though if it's still pulling hard in 4th (as it does with a manual and 4.10s) as well I guess it wouldn't matter.

 

I restate it again shorter but here it goes:

1-Choose the tire you will run.
2-Choose the gear for your application (you said autocross)
3-stop adding power when you cant get it to the ground.

If anyone follows what i said, the above should be recognized and you passed it over. If you choose runflats and autocross, you have made a decision. Choose your gear and power accordingly.

Since you stated runflats. The car I was in had an air box and a tune with 3.42's in the A6, and therefore my observations are exactly about the tire you selected. If you have runflats and go forced induction dont whine that runflats dont work with gears. The problem wouldnt be there if you were honest up front about what tire you were going to run. On the street with Hoosier R6 road race tires VetteJoel on this forum had 550rwhp on his Z06 with a 100 shot so we can assume there exists a street set-up with handling that supports the power levels and more gear on a car should you choose to mod in that direction. Joel isnt a regular at the track but he ran a 10.9 with the car in street trim. The tires work fine and are an option for street traction withhandling.

My Dart doesnt have a close ratio box and makes up the big end with a solid roller 10,500 rpm trap rev. The 6.13's are as wild as it gets. The point wasnt what the fianl gear ratio was. The point made was that with 760 rwhp I was not hooking with 4.88's and hooked with 6.13's.

Having been in a 2008 A6 car with 3.42's and having driven an exactly modded A6 with 3.15's your observations about what works and deosnt work based on math calculations are way off and anyone going for a ride in a 2008 A6 with 3.42's and the car is floored will not spin more than 10 feet. I noted it spun substantialy longer with 3.15's. 1975rpms at 70 mph is good and the traction issues werent there on base runflats.

Experience is always going to be more believable than the calculator. I'm sorry but you need to drive in a car with 3.15's or 3.42's before you give advice on them. The calculations dont hold true about traction because your belief that gears hurt traction is wrong to begon with. 3.73's would hook in less distance than the 3.42's. I've regeared so many cars at this point that I just go for the max that fits now. If there was a 4.88 gear set for the C6 I would get it and build an LS7 388 with a stock stroke whick would rev out to 7500rpms to make up the big end because it works and I've done the max gearing deal too many times to know that no 8 or 9 second all motor car runs hiway gears.

The A6 running 3.42's is not the same as 5.30's for any other gear except first and even if it did the car in fact hooks in 10 feet with 400rwhp ON STOCK RUNFLATS because I was in the car when it did it. I have tuned 6 A6's that were geared and all the 3.42's hook sooner. If it had 3,73's it would hook sooner than a 3.42 car because the tire rotation speed is slower and a car whooks when you hit the speed that the tires are rotating at. Higher numerical gears slow the rotation speed for any given rpm. Yes? No?

Your observations about stock converters and stock runflats are all variables. Where a converter locks is clearly dependant on which one you get. Not everyone stay with a stock anything. Rev limiters go up, stalls go up, tires get swapped to what is needed for the application.

Spinning doesnt ruin a run unless you have to come off the gas to rehook. It is more often than not that the car continues to acceleratr when the spinning is with steeper gears. Taller gears like your and runflats will never hook because the car will never get to the rediculous speed they are rotating at.

Seriously find someone with an A6 with 3.42's and then make a statement of the experience. No one will slam you for saying, "Oh, I was wrong....wow." Stop reading numbers on your calculaotr and interpreting them wrong. If a A6 had 5.88's the car would move forward so fast that the rehook would be almost 3 feet and you would need to shift immediately. There is less inertia.

Why does it amuse you that people quote 70mph for the hiway. Is it that hard to see the 80mph rev number? Come on. The hiway rpms for 80mph is 2250 with 3.42's and thats lower than the M6 Z51 with 4.10's which many people have. Mine is at 2350 at 80. If someone has OCD thinking that 80 mph with 2250 rpms is a lot dont get them. If you need 28mpg dont get them, but those arent performance considerations. They are honda civic specs.

Last point: you find me a 9 second car that lets you floor the car in first gear. Spining isnt ruining the fun. A driver's lack of skill from keeping it from spinning is. That power in any car with any diff gear will not allow you to nail it in first gear. It isnt a characteristic of fast cars that you be able to floor your car and it not spin the tires. All fast cars do.

EDIT: I just noticed you have a Z28 but you also have a C6? What car are you talking about? The Z28 on runflats tells you nothing about a C6 with 3.42's.

 

The A6 shifts so quickly and seamlessly that its tough to tell when youre in 3rd gear. Using all 4 gears doesnt slow you down and any real power level will require it. No human is going to shift better with the consistency of that transmission especially after its tuned.

 

True and I hate to admit it but as much as I'd LOVE to reap the 1/4 mile benefits of those 3.42s it's that loss of my current 30mpg highway capabilities that has me in a dilemma.
I just came back from Easter dinner at Dennis's house in my C6 and was experimenting a bit on the NJ Turnpike with gears and rpms. I put it in 5th and brought it to 70, 75 then 80mph and checked both the rpms and the instant average 'miles per gallon'...at 70 in 5th it was turning roughly 1900 and getting about 24mpg, not too bad. But at 75-80 it jumped up a lot (well relatively speaking) to like 2200-2300 and mpg dropped to roughly 19-21mpg and that hurt LOL.
Performance over fuel economy? A tough question indeed.
But considering that 1) this low 12 second/185mph car really had NO BUSINESS even getting 30mpg (that fact still AMAZES me to this day) in the first place LOL and 2) that many of us only drive the cars on a semi limited basis anyway I don't think it's too much of a sacrifice.