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Author Topic: R25/R28 gearbox ratios  (Read 398 times)

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Stev0

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R25/R28 gearbox ratios
« on: 13 July 2021, 09:19:09 »

My 5 speed manual omega has the code R25/R28 on the gearbox, i can find the ratios for R25 and for R28, but not sure what it means when both codes are on the box..

Does anyone know what ratios this gearbox has?
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Re: R25/R28 gearbox ratios
« Reply #1 on: 13 July 2021, 12:43:01 »

The number refers to the torque rating of the gearbox.

The exact ratios depend on the engine and market. A GM or Opel dealer might be able to supply you with the build data from the VIN. Failing that, an email to Opel at Russelheim  :-\
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omegod

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Re: R25/R28 gearbox ratios
« Reply #2 on: 13 July 2021, 19:34:48 »

My 5 speed manual omega has the code R25/R28 on the gearbox, i can find the ratios for R25 and for R28, but not sure what it means when both codes are on the box..

Does anyone know what ratios this gearbox has?

As far as I have been able to find out over the years is possibly there was an R-25 gearbox in low power 2.0 early on in production but I've been breaking manual Omegas for years purely for the gearbox's and never seen one with just R25 on,  R25-28 boxes are near identical and have both numbers cast into them, possibly the dti has a different 5th ratio.   There was allegedly an R28 box that has the same internals but they were shot peened so a little stronger, again I've never seen one or spoke to anyone that has.

I'm guessing you may be doing some form of transplant ?
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Stev0

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Re: R25/R28 gearbox ratios
« Reply #3 on: 14 July 2021, 09:26:42 »

Correct.. I'm considering the gearbox for a manta A build, but i need to know if the ratios are going to suite the final drive that I have for the car..

The exact code for the gearbox is:
90 523 122
0822680

Can anyone assist with ratio info?




« Last Edit: 14 July 2021, 09:46:21 by Stev0 »
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Nick W

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Re: R25/R28 gearbox ratios
« Reply #4 on: 14 July 2021, 09:45:32 »

Correct.. I'm considering the gearbox for a manta A build, but i need to know if the ratios are going to suite the final drive that I have for the car..

The exact code for the gearbox is:
90 523 122
0822680

Can anyone assist with ratio info?


changing the final drive is usually easier and more effective than finding 'perfect' ratios in a gearbox. For instance a 2.0l Pinto in a RWD Escort needs a higher diff ratio than is fitted to the smaller engine for first gear to be usable.


My friend's X30 engined MGB(which also has the 'better' 3.0l gearbox) isn't quite that bad, but the diff from a V8 car would improve drivability. Torquey engines with low gearing are a waste: 3.0l Capris with the 2.0l diff are slower than with the correct diff; put a bigger K-series in a Metro with the 1.1 gearbox and it just revs through the power with the wheels spinning
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Stev0

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Re: R25/R28 gearbox ratios
« Reply #5 on: 14 July 2021, 09:48:27 »

Thanks for the input Nick, but obtaining ratio info off a forum is far easier than changing out a final drive ;)

Better to do the planning upfront and determine a good ratio before the conversion, than to retrofit parts only to have to swap them out later..
« Last Edit: 14 July 2021, 09:51:56 by Stev0 »
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Enceladus

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Re: R25/R28 gearbox ratios
« Reply #6 on: 14 July 2021, 13:16:27 »

R25-R28 is a gearbox type or family as opposed to a specific designation. The "GK" at the start of the serial is the specific designation and tells us this box is an R25 intended for a Y26SE since it was manufactured in March 2002.

The ratios are:
1st 3.95:1
2nd 2.19:1
3rd 1.39:1
4th 1.00:1
5th 0.85:1
Reverse 3.53:1
Differential 3.90:1

 
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