bilbob2

O.k.

I made a thread earlier this week about my gear install and the speedo being off. For this new problem I will tell the story over again.

I had 3.55's and went in for a 4.10 install.
After the install on the drive home (long drive), I noticed my speedo was off because the person that came with me was in a different car and when we both set our cruise to 70, their car started to pass mine.

I got home and read that with my size tire (275/40/18) my RPM's at 70MPH and 5th gear should be at 2442.
My RPMs were just over 2k at 70 which was wrong.
I looked at my tune on the SCT and both the axle ratio and revs per mile said stock so I could not see the numbers. (I was told later the reason being was that they were set the the STOCK setting of the TUNE that was loaded by the custom tuner)

I then adjusted my revs per mile and it was still wrong.
So... I went back and put in 4.10 for the gear ratio.
After that the RPM's seemed as CLOSE to 2442 as could be...
given that you cannot get an exact number because the RPM odometer is not digital.

I called the shop and they had accidentally put the gear ratio at 3.73 on the tune.
He also said the revs per mile were off and switched them.
I asked him if they were SURE they put 4.10's in the car and not 3.73's and he said yes.

So he sent me a corrected tune with the axle ratio set for 4.10's and the revs per mile corrected.
The speed to RPM in 5th was exactly at where it was when I CORRECTED the first tune. All seemed correct.

So I wondered....how would I know FOR SURE that 4.10's were put in and not 3.73s?
The shop told me that if it was for 3.73's that the speedo would be off.

Well... that means the RPM to SPEED ratio should be off and not read what it should read... not only for 4.10's but also for 3.73's. EVEN if the car is programmed for 3.73's because there are 4.10's in the car PHYSICALLY.

(Meaning if the car has 4.10's in physically... and is programmed for 3.73's... the RPMs should NOT READ. 2222RPMs at 70MPH, which is the correct RPM to SPEED ratio for the car with 3.73's and my revs per mi)

So I went in for curiosity's sake and set the gear axle to 3.73.
I then set the revs per mile to what they should be.
I was amazed to see this. The RPM's were from what I could tell exactly what they should be at 70MPH in 5th gear for 3.73's !!!! 2222 RPM'S!

How can this be? I am judging by the odometer and not actual numbers but they BOTH look almost right on when I set the right parameters in.

This would mean that there is NO WAY to tell whether I do have 4.10's or 3.73's in!!

It seems the car WILL seem to have the right gears in as long as the right ratio is entered NO MATTER WHAT MECHANICAL GEARING REALLY IS INSTALLED.

What I mean is if you have the car programmed for 3.73's it will read the correct RPM to SPEED for them.

If you have the car programmed for 4.10's it will read the correct RPM to SPEED for them.

I also want to tell yall that the difference is so minute that it makes it hard to see if each one is EXACT.

ex: 3.73 RPM should be 2222 at 70.
It is easy to see the marking for 2250 and 2500 on the dial. But since 2222 is in between them and there is SUCH little room in between the gap... it is hard to tell JUST where the needle is setting.

I do not understand why the RPM reading would not be CLEARLY off if I had car was programmed for 4.10's..... but the actual gears in the car were 3.73's.

How can both the RPM to SPEED ratio's be the correct readings for their gear ratio's if I obviously only have one physical set of gears installed?

I am so confused and do not know what to do.
Especially since BOTH RPMs seem to be where they are supposed to be.
When the car is programmed for 3.73s, the RPM to SPEED ratio seems to be where it should for 3.73's.

When it is programmed for 4.10's, the RPM to SPEED ratio also seems to be correct for 4.10's.

I dont know how the hell I am going to distinguish which I have and make sure I have the correct ones installed.
Can someone please help me with this?
I have no clue about the mechanics but is there somewhere I could see the gear ratio marked if I lifted the car?
(Might be a stupid question but just asking.)

Also. I have an SCT X3.
I read you could data log with these on the instructions but
on the site I cannot figure out how the hell to do it.
If anyone has done this... so I can look at DIGITAL RPM numbers... I would appreciate it.

Any other help is also appreciated.
Thanks!

Boozshey

You'll have to remove the rear housing...

onederful100

the reason why they read correctly when you program it for 3.73's or 4.10's is bc you are reading the info from inside your own car.
you need to use something you know is correct for reference, like another car right along side you.
or you could put your car on jackstands, turn your rear wheels 1 turn, and if the DS spins 4.1 times then you got 4.10's, if it turns 3.73 times you got 3.73's.

bilbob2

lol I have no clue how to remove the rear housing or what it even consists of.
After you did that what would you look for to tell which you had?
I am by no means a mechanic. Does that mean that would be kinda disassembling the gears again?
Could that mess up the gears?

onederful100: By DS do you mean driveshaft?

Should I take it to a local shop and just have them pop it up on the lift real fast and toss the guy some cash to tell me what I have?
Like a friggin Firestone or something and have them lift it and do what onederful said?

That shop is a pretty long drive.

Torch Red GT

Yeah do that, there's a way to check by spinning the tires on a lift

bilbob2

Errr.... that didn't tell me much.


DS= Driveshaft?
Oh and is there a tool to measure how much it has spinned or is this gonna be an eyeball job someone does?
Will they need to mark a point on the DS and Mark the Tire to know complete turns?

Would'nt this be easy to misread with the eye? That is not much of a difference in rotating.

And someone post some info on the other questions in my last posting in this thread? (rear housing, etc.)

Hehehe. Thanks.

Diabolical!

Jack up the rear. Paint a small white dot on the highest point of one of your rear tires. Paint a small white dot on the lowest point on your driveshaft near the pumpkin. Lay under the car and have someone rotate the tire EXACTLY one time. The driveshaft should spin a hair over 4 times if you have 4.10s.

bilbob2

K thanks.

Also..
Please don't laugh at this question.

If the car is programmed to 3.73's.... will it still rotate 4.1 times with 4.10 gears?
(I.E. nothing stops it from rotating the full 4.1 times when the car is programmed for 3.73's)

Also... should I make a mark not only on those points but perhaps make a removable mark
on something that is not moving close to the tire so that the two points meet up exactly when the tire has rotated one time?

And the same thing with the driveshaft?
Make another mark on something stationary so you can see when the full rotation has occured?

BruceH

275/40/18's should have 775 revs per mile. What are you putting in?

siggyfreud

The paint the tire/paint the driveshaft message can not be affected by the tuner. It's mechanical. Do it and put the issue to rest