Posted this in ECMPCM programming section, but didn't know how many of you went in there and how often so I'm asking in here also:

I changed the axle ratio from 2.59 to 3.54 in my 1994 LT1 automatic. Bought the Hypertech III mainly so I could recalibrate my speedometer. It only has 3.45 gear ratio available (closest to my 3.54), question is: How can I get the 3.54 ratio put into my car? I'm a little miffed as I thought the Hypertech III would let me do this, may have just wasted $330!!! Your input greatly appreciated as always!

 

I remember correctly, I believe you can set the tire size in inches with the HPP. If so, tell us what is specified now (stock) and we can change that value to correct from the 3.45 to the 3.54. The difference is 2.609%. Adjust the tire size by that amount.

Tom Melton
82 CE
86 Z51 Coupe

 

thanks for the reply, I have the tire size programmed at 24.75" currently. Should I decrease it by the specified percentage?

 

Nickd on the forum, also a good friend of mine did this last year. He swapped for to the 3.54 but just used the 3.45 gearing in the HPP III
If you look up the speedo gears for the 89 with the 3.54 and the 1991 with the 3.45 the speedo gears are the same.. the difference for the gearing is negligible. he did this after he went through and changed the tire sizing ETC. but found the 3.45 programming to work much better

Hope this helps..

 

What Jeff said.
Actually someone else on the board here suggested I adjust the tire size to compensate. I ran the car at the track and found that using the 3.45 worked just as good. In fact to me it seemed like the shift points were slightly off by using the 3.45 combined with the tire size adjustment.
Like Jeff said, GM used the same spedo gear for both the 3.45 & 3.54.
I would just use the 3.45.
Otherwise do like Tom said above. If you have trouble I can probably find my calculations and tell you what I used for the tire height adjustment.


[Modified by nickd, 11:57 AM 3/1/2002]

 

I found my old notes and here is what I came up with.
Normally you would divide the old gear ratio by the new one to get the correction, so
2.59/3.54=.731 (73%)
In this case since we are starting with the 3.45 ratio in the programmer since that is what you will select, your spedo will only be off by:
3.45/3.54=.974 (97.4%)
so you need to correct for 2.5% like Tom Said

Next we use the tire height which is 26.0"
Here is how I arrived at that, lets take the 255/45R17
255 is the width in mm and 45 is aspect ratio

First take the width 255/25.4 to convert into inches: 10.3"
take 10.3" x 0.45 = 4.51" sidewall height
Then take the sidewall height x 2 (4.51*2=9.027) and add the diameter which is 17" and you get
26.0"
When you do the same with the rear tires (285/40R17) you'll come up with the same.

Now all you need to do is take your 26.0" and multiply x 2.5%
=0.65"
You'll decrease the tire size by .75", the programmer only allows 1/4 inch incriments
You want to decrease the tire height, what youre doing is increasing the effective gear ratio, which is what you want to do to get from the 3.45 to the 3.54.
If you added tire height you would be doing the opposite, reducing the effective gear ratio, which you don't want to do.
I hope this makes sense.

[Modified by nickd, 6:45 PM 3/1/2002]


[Modified by nickd, 6:52 PM 3/1/2002]

 

Thanks for taking the time to explain! I really appreciate the help. Sometimes when a 'certain' type of black & white vehicle gets behind me I really like to know EXACTLY how fast I'm goin, hehehehe. Although it's only off now by 2.5%, it could make a difference in THAT circumstance.

 

I wouldn't sweat it.
@40mph it would be off by 1 mph
@60 it would be off by only 1.5
@150mph it would be off by 3.75 and even that I would not sweat -lol