rear gear ratio
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Navigator
Joined: Mar 2004
Posts: 9
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From: Hammond IN
rear gear ratio
Can someone help me? I want to change my 95 over to a 3:54 ratio I can get all the parts needed but when I purchased the hypertech programer the only option that it has is 3:45 . If I used the hypertech programer and change my settings to 3:54 instead of 3:45 what can of effect will that do to my car, if any? :confused:
Race Director
Joined: May 2001
Posts: 14,165
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Re: rear gear ratio (gary95)
You can use this little spreadsheet I wrote to see what effects you'd have.
http://www.wku.edu/~nathan.plemons/calc.xls
Just at a glance, I'm assuming it's an auto tranny and you have the standard size 285/40/17's on the rear.
If you have the computer programmed for 3.45 gears you will be register a given speed but will actually be going slower. If you register 70 MPH in 4th gear which would be 2200 RPM's, you would actually be going 69 MPH.
That's probably close enough huh?
http://www.wku.edu/~nathan.plemons/calc.xls
Just at a glance, I'm assuming it's an auto tranny and you have the standard size 285/40/17's on the rear.
If you have the computer programmed for 3.45 gears you will be register a given speed but will actually be going slower. If you register 70 MPH in 4th gear which would be 2200 RPM's, you would actually be going 69 MPH.
That's probably close enough huh?
Drifting
Joined: Jun 2002
Posts: 1,305
Likes: 11
From: Savannah Ga.
Re: rear gear ratio (Paul G)
JUST ...change the tire size on your computer with the Hypertoy....to about 25.5 length or 26....can remenber how ...but play with the tie sizes on the computer and .......oh hell I'll do do a search on the topic and post the right information .... please stand by :chill:
25.5 tire height that's it folks
[Modified by redcorvettedave, 10:57 PM 4/21/2004]
Le Mans Master
Joined: Feb 2002
Posts: 7,858
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Re: rear gear ratio (gary95)
Your PCM will still be using the MPH shift points for whatever your stock gear ratio was, unless you recalculate the tables and allow for the new ratio (a math situation to think about for a while...). So to get your car optimized for the new gears there is more to it than just adjusting the speedometer related settings (which are in the PCM constants table).
Here is what the MPH shift table look like for my '95 which has the 2.59 rear gears:
And here is the same MPH shift table for a '94 vette w/ factory 3.07 rear gears (the performance axle ratio):
As you can see, the PCM will never allow a shift into 4th at or near WOT for a car that came with 2.59's from the factory. The lower speed normal driving shift points are slightly different as well.
I do not know if the Hypertech unit adjsuts these tables or just adjusts the speedometer calibration entries in the constants table.
[Modified by Lone Ranger, 5:13 PM 4/21/2004]
Here is what the MPH shift table look like for my '95 which has the 2.59 rear gears:
And here is the same MPH shift table for a '94 vette w/ factory 3.07 rear gears (the performance axle ratio):
As you can see, the PCM will never allow a shift into 4th at or near WOT for a car that came with 2.59's from the factory. The lower speed normal driving shift points are slightly different as well.
I do not know if the Hypertech unit adjsuts these tables or just adjusts the speedometer calibration entries in the constants table.
[Modified by Lone Ranger, 5:13 PM 4/21/2004]
