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Discussion Starter · #1 ·
I recently put a 3.90 gear differential in my 02 z. When I purchased the gears from DTE they told me that no calibration was necessary when tuning the car for the new gear ratio. I then traveled to Cleveland to get the car tuned (I also added some LG longtubes, borla stingers and a vararam). During the tune I was told that I did need to calibrate the speedometer to 3.90 gears. On my way back to Pittsburgh I was traveling 68-70 mph (according to the speedometer), but it sure seems like I was going a heck of alot faster than that. I don't think I got passed by one car during the entire 2.5 hour trip home. This has me worried. I watched the tuner type '3.90' into the program (hp tuners) but I have a bad feeling my speedomoter is wrong. Can anyone let me know how or if there speedometer was corrected when they put 3.90's in their car. Any help would be appreciated.

Thanks
Ted
 

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This topic just came up on the other board, no you do not need to recalibrate the speedo on a Corvette.
 

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Oops. it was this very board, not the other one....

390's
 

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Just a couple of suggestions to help you verify that your speedometer is accurate or not:

(1) Go past a portable radar unit that you might find on one of your local roads. If you are in FL, you can find one every couple of miles.

(2) If you have one or can borrow one, try comparing your speedo to a portable GPS unit like a Garmin hand-held unit.
 

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Listen to DTE they know there stuff

C5s speed is measured by the rotation of the output shaft. LS1 Edit Menu specifically says recal is not required when changing gear ratios. However, it is required when changing tire size. :cheers:
 

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I checked out the HPtuners table. Edit > Speedometer > Speedometer > Gear Ratio. The VSS ticks per mile with 3.42 gears and a 26.0 inch tire height is 31028. When I put in 3.90 gears, it came out to 35383. Since it should be the same, you are running 14% faster...aka 70 mph is now 80. When the speedo shows 61, you are doing 70...

:)
 

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DELLZ06 said:
I checked out the HPtuners table. Edit > Speedometer > Speedometer > Gear Ratio. The VSS ticks per mile with 3.42 gears and a 26.0 inch tire height is 31028. When I put in 3.90 gears, it came out to 35383. Since it should be the same, you are running 14% faster...aka 70 mph is now 80. When the speedo shows 61, you are doing 70...

:)
The gears wont change the speedometer readings since the reluctor ring on the ring gear is the same size as all the other gears. Yea if the speedometer sensor was mounted in the tranny then u would have to recalibrate since the pinion gear is spinning faster.
 

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If it's possible for you to get a GPS for a new hours you could tell for sure it there is an error. My stock Z16 is off 1.7 mph. Good luck.
 

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Hey ITLGO,

If he changed the setting in HPTuners, it would be off. Sorry, should have made that clearer.

:)

John
 

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Nothing needs to be done when changing rear end gears with the M6/M12 C5 6 speed transmissions.

HOWEVER, changing rear end gears on C5's with automatic transmissions DOES require a PCM change.

:cheers:
 

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01ZEE06 said:
Nothing needs to be done when changing rear end gears with the M6/M12 C5 6 speed transmissions.

HOWEVER, changing rear end gears on C5's with automatic transmissions DOES require a PCM change.


:cheers:
True, to adjust the shift points not the speedo :z:
 

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Discussion Starter · #13 ·
wow, I guess I should scan the site better next time. The guy who made the similar post a few days back is actually my good friend who went out to get tunned with me! I guess he was curious to.

Anyway, thank you all for your replies. DelZ06's number of 61 on the speedo equaling an actual 70mph seems about right. I am going to make an attempt at seeing how far off it is, but with your imput from these two treads it sure looks like I need to get the computer set back to where it was.

So, if I understand correctly I should have hp tuners program my computer for 3.42 gears since the speedo does not know the difference between gear ratios and my tire size is still stock?
Thanks again guys for helping me out
Ted
 

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RBKiller said:
wow, I guess I should scan the site better next time. The guy who made the similar post a few days back is actually my good friend who went out to get tunned with me! I guess he was curious to.

Anyway, thank you all for your replies. DelZ06's number of 61 on the speedo equaling an actual 70mph seems about right. I am going to make an attempt at seeing how far off it is, but with your imput from these two treads it sure looks like I need to get the computer set back to where it was.

So, if I understand correctly I should have hp tuners program my computer for 3.42 gears since the speedo does not know the difference between gear ratios and my tire size is still stock?
Thanks again guys for helping me out
Ted
No that's not correct, You need to read the thread again :jawdrop:
 

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Discussion Starter · #15 ·
Reading through the treads it sounds like everyone is telling me that the computer should not be changed if you install a different gear ratio as long as you keep your stock tire size. This is is what I did with my car, but the car's computer was changed. So If the car shouldn't have been changed, but was, it should be changed back, right?

Sorry If I'm overlooking something here.

ted
 

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RBKiller said:
Reading through the treads it sounds like everyone is telling me that the computer should not be changed if you install a different gear ratio as long as you keep your stock tire size. This is is what I did with my car, but the car's computer was changed. So If the car shouldn't have been changed, but was, it should be changed back, right?



ted
Correct! Maybe I need to reread it: :eek:o: :lol:
 

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I'm a newbie and considering 3.90 gears. Why did you change to that gearing instead of some other? How does the change improve performance? Can the improvement be measured?
How do you like the new gears?
 

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Discussion Starter · #18 ·
I was hoping I wasn't missing something. Some times us polocks are known to look over some pretty simple stuff! I appreciate your knowledge and all of your replies! :jammin:

As for my liking of the gears.... I couldn't be more happy with the way the car feels. I olnly added about 25 rwhp to the car (longtubes,catback,vararam) but the car feels like a whole new ride now. The car just seems to explode into the powerband alot faster than it did before in first and second gear. It also seems to pull much better in third and fourth. I can't give you any numbers since I haven't been to the track with the car yet, but I can tell you that with minimal horsepower gains the 3.90's along with a good set of bfg drag radials has the car planting me further into the seat.

I chose the 3.90's because it seemed like the most logical gear ratio for my intentions with this car. I wanted gears that I could notice, but still street friendly. I have plans to put a cam into my car in the near future as well. A steeper gear ratio will definetly enhance the performance of that cam.Doing research on this site, as well as some others, I felt that the 390's would be the right ratio for the cam and for feeling a much improved take off, with out turning the car into a to much of a drag set up.
I hope this explanation helps.
Ted
 

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Stumbo said:
I'm a newbie and considering 3.90 gears. Why did you change to that gearing instead of some other? How does the change improve performance? Can the improvement be measured?
How do you like the new gears?
The Z06 has the MN12 tranny versus the MN6 tranny which is in the regular C5's. Most consider 4.10's to short of gear for the Z06. 3.90's or 3.73's are a better match. :cheers:
 

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HMMM: I just had 3.90s installed and the shop charged me $25 to reprogram the ECU for the new gear ratio. I even mentioned it and they cofirmed it had to be reprogrammed when changing the gear ratio. Did I get ripped off, or is the whole world crazy? :screwy:
 
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