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Old 11-27-2020, 08:38 AM   #1
dalek
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Default Understanding T5 gear ratios and their swapability

Just to make it clear, this is not a "which transmission should I put in my Volvo if I want to do X HP and Y torque using Z coffee" thread; I really just want to stick with the T5 in this discussion.

So I have had a GM 1988 2.8 V6 WC T5 off an F-body (tag 13-52-177). According to what I read, its ratios are 4.03(1) 2.37(2) 1.49(3) 1.00(4) 0.76(5) 3.76(R).
  1. If we ignore the first gear, how good are the other gears?
  2. Is the gear choice (excluding 1st) related to engine performance or just driver's habits (closed gear ratio for performance vs more further apart for economy)?
  3. If I want to change any of gears 2-4 (I noticed 5th gear is different), is this just a matter of swapping cogs or do I also need to change the (main?) shaft (which means I am stuck to whatever overall gear combination that is machined to the shaft)? https://garage-scene.com/t5-into-ag-...all_ratios.htm confuses me
  4. Let's now talk about the 1st gear. I did some looking and the available first gears for the T5s are

    [low torque] 4.03, 3.73, 3.50, 3.35, 3.27, 2.95 [high torque]

    Of those, some are available in the Chevy V6 WC T5 and others are not. Am I correct to assume that if I want to swap the first gear I need to change at least one shaft (main?) as one of its gears is machined to said shaft (the other is pressed into the other shaft)?
  5. I take the 4.03 1st gear is not only weak but also very granny gear
  6. My research (hence previous questions to support/clarify/contradict it), swapping gears means swapping all gears (ignoring input and output gears). Does that mean no matter the gears I chose the shafts still fit in the same housing?
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Old 11-28-2020, 03:48 AM   #2
Magnum TE
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It's my understanding that the desirable close ratio T5 gear-set is the 2.95 1st / .73OD
Like a -92 LB9

You have to swap everything, input shaft etc
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Old 11-28-2020, 08:31 AM   #3
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For the OP's understanding:

The 4.03 first gear is HIGH TORQUE (low speed)...... and the 2.95 Z spec is LOW TORQUE (but high speed).

Remember, the definition we learned in Physics is Power = Torque x Speed.

When your 200 lb.ft. engine (for example) sends that torque into your T5, the first gear multiplies the torque by that ratio. The torque increases, the speed reduces, and power is nearly constant (minus transmission losses). To get the complete torque picture from crankshaft to tires, your engine torque is first multiplied by trans gear, next by the final gear out back, and ultimately by your tire radius where "the rubber meets the road".

Years ago when shopping for a T5, I steered clear of the GM boxes because of that horrific 1st gear. Probably selected for an NA engine, pulling a carload of folks up a hill! Designed to accelerate a large mass with little torque, and speed was secondary.

What you need to consider is your application.... straight line car, corner carver, rally, DD. It has been proven over time that the Z spec T5 gearing along with a 4.10 final gear seems to be the clear winner when matched with a turbocharged 4 banger. The RPM drop between gear shifts makes the difference, and keeps the turbo in the right spot (in boost).

I think you can find anything you want (at Alice's Restaurant) in the T5 aftermarket support. However, you might be better off to sell that GM T5 and look for a Ford V6 version with the 4 banger friendlier 3.35 first gear as well as the 5th gear for cruising.

Years ago I wrote an Excel file which compares speed and rpm for various trans gearing ratios and gearboxes. If you would like a copy, shoot me a PM and I'll try to find the file and share it with you. There are online calculators which do the same thing, however...... but it's straight physics and torque/speed calculations, nothing magical.

Good luck!
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Old 11-28-2020, 08:32 AM   #4
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Thanks for the info. With that said, didn't ford go from 2.95 to 3.35 in their V8 transmissions somewhere in the 90s? Wouldn't that lead to a somewhat weaker transmission?

I have to say the more I read up on this transmission the more the belief I had that its gears could be swapped like legos is shattered. Yes, you can swap... the entire set as a whole: you can pick any gear ratio for any gear as long as it is available as a complete set.
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Old 11-28-2020, 12:54 PM   #5
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The higher/longer the first gear ratio (Numerically smaller), the stronger the transmission. This also has to do with the Input shaft gear and cluster/countershaft gear ratios as well.
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