Has anybody changed the rear end gear ratio in their C6 yet? If so I'd like to hear all about it. Are upgrades even available yet? And is there a way to correct the speedo and odometer if you do it?

 

I have a mag-red coupe Z51, 6 speed. I picked up my C6 at Kerbecks on 2-7-05 and drove to Cartek in Garwood, N.J. Cartek installed their package as described in the March issue of Popular Hot Rodding magazine including 3.90 gears. I averaged 24 MPG from Garwood to my home in Appomattox, Va. Contact Dave Busch at www.Cartek.net with any questions. I plan on having Cartek install their 2x head & cam package in the near future. Dave can give you estimated 0..60 and quarter mile times with his different set-ups. My experience with Cartek was very positive and even though they are 400 miles from my home, I plan on dealing exclusively with them. My next step is to order their clutch package and install that myself.

 

Is your speedo corrected? What are your rpms at freeway cruising speed?

 

tluck....i believe we met the day you had your corvette taken to cartek...it was great to meet you...(I am the guy with the towtruck driver friend). anyway how do you like your new c6?

 

VA Beach may be doing business with them as well. Keep us posted on your mods.

 

Changing gears will not affect your speedo.

 

It does on other cars, is that special to the C6?

 

The speedo gets its information from the ABS wheel speed sensors. There is no cable connected to the transmission tailshaft as in ancient days. The only time you'd need to correct the speedo is if you fitted tires of a different diameter than stock.

 

Actually it gets it's info from a VSS (vehicle speed sensor) which is mounted to the differential in such a way that it reads the rpm's of the ring gear, since all ring gears have the same number of teeth on the sensors wheel it does not affect the speedometer when the ring gear is changed.

It's the same way for the C5's

 

You are an absolute treasure trove of information. Are you Dave Hill’s half brother? You know your stuff Mr Dog. Thanks for the input. I owe you a beer

 

IM going to put 18x10 1/2 wheel with a 305x35 tire on the back of the C6. Will that affect anything? mph, speed.....

 

I'm interested in the same size wheel and tire. The current Z51 tires don't give me the traction I need from a launch, especially with my 3.90 gears.

 

MTI installed my 3.90's last week and they made a big difference. The gear shifts ALWAYS land in the sweet spot of the next gear and the shifts just flow. Highly recommend it.

 

Changing the rear gear set won't have any affect
on where shifts "land" in the rpm range of the next
gear.

If you shift from 1-2 at 6500 rpm, you'll hit second
at the same rpm no matter what rear gear set you
have. You'll just be going slower when you hit the
next gear, with a 3.90 gear.

To change where you land, you'd need to change the
transmission ratios.

Pat

 

I think I understand what you are saying but it sure doesn't feel like that. Maybe it's the engine work that makes it feel smoother. Good point though, thanks for the clarification.

 

sorry to correct you but where your rpms land is affected by the rear end ratio as the tranny ratio works in conjunction with the rear ratio and as the final drive ratio is constant each gears ratio in the tranny is influenced by the rear end change. that is why when you change your gears your cruising rpm at 60 mph is usually higher

 

I thought for the automatics you still need to adjust the shift points. i am sure this is how it was in the C5's but with a new computer set up in the C6, this might not be the case any more.

 

shift points dont change thats controlled by ecu. but whereyour rpms land after each shift does the car will be at higher rpms after each shift

 

Let's construct a hypothetical case.

Assume 1st gear has a ratio of 2.97:1 and 2nd has a ratio
of 2.07:1. Further, for now, assume a rear gear set of 1:1.
We can deal with a 3.42 or 3.9 ratio in a minute.

The total gear reduction in 1st is therefore 2.97:1 and in
2nd is 2.07:1. This means when you shift from 1st to 2nd
at 6500rpm, you'll be doing 149 mph. In second gear, at
149 mph, your engine speed will be 4500 rpm.

4500/6500 is 69%. In other words, there is 30% drop in
engine rpm going from 1st to 2nd. This is because 1st
gear offers 30% more torque multiplication and 30% less
speed than 2nd.

Now let's change to a 3.42 rear gear set. Here, when you
shift from 1st to 2nd at 6500 rpm, you'll be doing about
44 mph. In second gear, at 44 mph, your engine speed
will be.... 4500 rpm!

In other words, no matter what rear gear ratio your car
has, the drop from 1st to 2nd when you shift will be
from 6500 rpm to 4500 rpm, assuming the 1st and 2nd
ratios I mentioned above.

I didn't include all the arithmitic because it's repetitive
and not that interesting but I'll be happy to show you the
math if you want.

From this, you should be able to see that rear gear ratio
has absolutely no effect on the rpm on where shifts "land"
in the rpm range of the next gear. In this hypothetical
example, we varied the rear end ratio from 1:1 to 3.42:1
and saw zero difference.

Pat

 

So after a few weeks of driving with the 3.90s, how
do you like them? Are you shifting a lot more? Worse
mileage? Better kick in the pants? Higher oil temp?

I'm very interested in this mod and I'd like to hear
more about experiences with 3.90s.

Pat