Da Wiz

My current running gear is the stock 3.15 from GM, stock automatic tranny, 3200 stall, and a heads/cam/headers/Callaway DD setup running in the neighborhood of 420 rear wheel horsepower and torque. I run a B&M tranny cooler as well.

I'll be installing a Level 5 tranny from RPM with the 1st and 2nd gear ratio changed, Stage 3 differential, probably a 3k Vigilante stall, FAST 92 intake and an LS2 throttle body. (The purpose for this project is to replace a marginal aged Fuddle Stall. Since everything is out to begin with, well, it's a perfect excuse to upgrade.)

I drive the car daily, and throw it around at local autocross events on the weekends to become a better driver mostly. I have no aspirations of becoming an SCCA champion, I just want to know how to handle the car better.

I have frequent travel in my job, so when possible, I drive the Vette.

Finally, the question. I'm changing the differential anyway, and have a choice to make regarding gear ratio. What will I notice going from a 3.15 to a 3.42 in acceleration when I play, and efficiency when I cruise? For you folks that have done this, what were the plus and minus effects?

Thanks in advance for your observations.

Caveboy

Blow Torch

You will get different opinions, but for me, the 3.42 is a perfect ratio. It is not a big change, but you WILL notice the difference between the two. It wont change it a lot, but If you're doing H/C and have a stall, mpg is not your biggest priority. I believe the 3.42 will make it fill a little more responsive coming out of the corners and away form redlights

Da Wiz

Thanks Blow Torch. You are absolutely right about not having great mileage concerns. The largest concern for me was spinning the motor at significantly higher rpms on 500 mile road trips. That is a significant portion of the way I use the car. To "use up the motor" cruising along the highway didn't seem wise. It seems to me the 3.42 is a fair compromise. I expect a small bump in performance (that you characterized very well) without a high rpm penalty over my current 3.15. Thanks for a well thought out response.

martysauto

with blow torch. You can google rpm mph calculators. Double check, but i think OD in the a4 is.7 to 1. A 3.42 gear would give you a 2.39 final drive. Your 3.15's give you a 2.20 final. 10.73 in 1st compared to 9.89 you have now. Go for it! I switched from 3.42's to 4.10's with my m6 and it still cruises fine at 75-85mph.

rt1970ls6

My 2003 50th Anniversary edition has 3.42 rear gears and an M6 transmission. The transmission 4th gear ratio is 1:1, 5th is 0.74:1 & 6t is 0.50:1. At 70 MPH in 6th gear, the engine is turning 1,600 RPM. At 70 MPH in 5th gear (0.74:1 ratio, about the same as the O/D in an automatic), the engine is turning 2,350. FYI

Da Wiz

Really appreciate the info. Im sold on the 3.42 myself, and hearing the personal accounts really closes the deal.

martysauto

That doesn't sound right. What size rear tires are on your car? I had a m6/3.42 and my rpm's at 70mph were 1500

Da Wiz

There is a pretty good formula that I use for calculating RPM at specific speeds. It has checked out perfectly with my current 3.15 rear gear. On almost all of my long cruises, I drive 78 mph. That puts me just under 2k rpm. This formula works out to 1992 rpm. shifting to a 3.42 will result in 2163 rpm, less than 200 rpm shift. By the way, I get my tire height measurements from Tire Rack.

The formula:

336 x Gear Ratio (3.42) x MPH / (divided by) tire height (my Michelin PS2's are 29 inches).
Multiply this number by your final drive ratio and that gives you the RPM. My final drive in an Automatic is .70.

336 x 3.42 x 78 = 89631.36
89631.36 / 29 = 3090.73
3090.73 x .70 = 2163.51 RPM

0Shane@RPMtransmissions

Moving to the 3.42 from a 3.15 will cause an rpm increase of about 250 RPM

QCVette

I saw a pretty good chart for comparisons.

rt1970ls6

That statement is not exactly correct, the rpm difference is dependent upon speed. It is a percentage, not a specific RPM. The correct way to determine rpm after a rear gear change (with no other changes, such as tires) is as follows:

Existing RPM X Proposed Gear Ratio/Existing Gear Ratio = Proposed RPM.

Therefore, if the car is equipped with 3.15 tires and it turns 1,500 RPM at a given speed, if you change the ratio to 3.42 gears, the new RPM would be 1,628.57, an RPM increase of 128.57 RPM @ the 1,500 RPM cruising speed.

rt1970ls6

The "loaded" radius of the tie is about 13", for an overall diameter of about 26". I determined the rpm mathematically, not using speedometer info, which is sometimes off a percent or two.

0Shane@RPMtransmissions

Correct it will be a percentage of difference and my 250 rpm was calculated with factory height tires and a cruising speed of 75mph

Da Wiz

I think we did the same thing, only stated differently. Since I'm familiar with my car on cruise at 78, that is the speed I used, along with the height measurements from my specific tire. My rears are 19's, which contributes some to the differences in our numbers.

Your point about being a percentage difference at a given speed is very helpful. That will prevent folks from assuming a consistent rpm change. Good stuff.

By the way, a 250 rpm increase is easy to live with. I'm really looking forwrd to this conversion.

Shane, looks like you folks are going to build me a Level 5 transmission with changes in the first and second gear ratios. What a great product !!!

rt1970ls6

Shane,
Thanks for the comments. My 2003 50th Anniversary Corvette has P275/45ZR18 tires on the rear, which have a loaded radius (distance from center of wheel to ground) of 12.875". If you double this number, you get the "effective" diameter of the wheel/tire combo for calculation purposes. Using a spreadsheet I prepared (which I will be happy to email to anyone who sends their email address) when I plug in this size tire and the gear ratios for my M6 transmission, at 75 MPH with 3.42 gears, you get a cruise RPM of 1,672.52 RPM. With 3.15 gears, you get a cruise RPM of 1,543.86 RPM. This is a difference of 128.66 RPM, in 6th gear. Although this is theoretical and discounts factors such as tire growth due to centripetal force (see www.diffen.com for the difference in centrifugal force and centripetal force) and tire slip due to pushing the car through the wind, these are pretty much negligible. Also, since tire growth is purely a function of RPM, it would be the same each time. Not trying to be argumentative, just providing correct data as much as I am able. Thanks for the comments.

Robert T. Tolbert, P.E., BSME, MSCE, MSSE

C5Natie

I use to love my 3.42's and thought they were a great middle ground between enough gear and not too much. Recently though, as I'm near 700rwhp, i find 1st gear useless even when I had drag radials. I'm working in getting as close to 1000rwhp as possible so I know this will only get worse. So, I've been thinking of going with taller gears like 3.23s or 3.15s. I was even thinking of seeing if someone on the forum would wanna do a straight swap. Anyway, i guess it's all in the combo you have. I have a built 4L65E with a circle d triple clutch 3200 stall converter.

rt1970ls6

My son has a 2006 Z06 with a ground up engine (aftermarket block, crank, heads, intake, cam, etc.) with all top shelf parts & twin turbos. The car, based on the fuel map and other data, produces in excess of 1,000 HP, and is virtually useless for street driving at any real level of boost. Under 60 or 70 MPH the car smokes the tires, even with drag radials; way too much power. When you exceed 600 - 800 HP, that level of power really needs to be in a purpose built car, in my humble personal opinion. After all, how much power is really "streetable"? And no, I'm not trying to initiate a debate, just some realistic thought.

In your case, with the traction difficulty you have with 700 HP, drag radials, gear ratios, etc., adding another 200-300 HP to the mix will not improve it; it will probably make things worse and could potentially slow the car down at the drag strip due to traction problems (although really increase top end on an open track). At that level of power, one really needs to look at optimizing suspension & much larger racing slicks. It is a juggle to hook up that much horsepower. Good luck, and always be safe!

C5Natie

Thanks, i try to be as safe as i can be. I understand what you're saying. The good thing about my setup is I'm not in boost at all really unless I get on it so it's quite manageable on the street. But when I do drag race, or do top speed events, a slightly taller gear can help get the power down as well as bring up the mph. At these power levels there will always be traction issues unless youre on full slicks all the time. Just looking for something to help it a bit.

rt1970ls6

Oh! By the way, my son's Z06 has an aftermarket high end six speed manual, making driving even more tricky. Good luck!