# Tesla Model S Ratio Change



## 4G63T (Jan 9, 2019)

Hi all,

The Model S large rear Drive unit has a gear reduction ratio of 9.73:1 . I’m contemplating on different installation options of this drive unit as I plan to keep my Mitsubishi Evolution 8 all wheel drive. 

Options I’ve been thinking of:

1. Install large rear drive unit as rear axle and add small front drive unit at the front. Dual motor awd. Not 100% sure if large rear drive unit will fit the back. So below are my other secondary plan B or C options.

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(EDIT) Option 2 won’t work as I incorrectly calculated multiple ratios . Should be multiplied not added. 

2. Keep oem rear differential with 3.307 ratio and mount another rear differential at the front of the car. I will install a gear reduction 4.5:1 inside the Tesla drive unit. This would give me a total ratio of 7.807:1 . 

Then I will rotate the motor orientation horizontally so the left axle is facing towards the rear and right axle is facing the front of the car. The drive unit will spin both “rear diff’s” at rear and front of the car. Essentially creating a single Tesla motor awd setup. 
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3. Again install oem rear differential in the rear and one more in the front of the car. This time completely remove the Tesla transmission and directly connect the motor to the rear diff. I’ll do the same at the front with a second drive unit. But doing this will result in each motors having to spin a ratio of 3.307 : 1 . 

Now here is my concerns . As the ratio is reduced the effort the motor will need to propel the car is increased. I don’t want to have a drastic decrease in performance . 

Option 3, I will have two motors rated at 700-800 combined awd hp (in oem ratio) but now they will try to spin a ratio of 3.307 : 1 . 

What do you guys think about option 3 ? Its mainly the ratio and how much it will effect power output that I’m concerned about . Is the ratio too low to be useful or outside of power curve of the motor(s) . Other issues I missed ? I don’t care about increased top speed, I just don’t want to lose on acceleration 0-60 etc..


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## kennybobby (Aug 10, 2012)

You got me curious, how would you change the gear ratio of the tesla inverter/motor/gearbox/differential?

How would you control the motor if you separated it from the inverter and gearbox? What about routing for the coolant ports. How would you couple the motor shaft? The motor "wiring" is internal to the drivetrain, the external wiring runs to the inverter.

What are you planning to use for a battery pack? The battery pack is one of the primary factors that determines how fast you can accelerate (voltage, current capacity, weight).


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## 4G63T (Jan 9, 2019)

kennybobby said:


> You got me curious, how would you change the gear ratio of the tesla inverter/motor/gearbox/differential?
> 
> How would you control the motor if you separated it from the inverter and gearbox? What about routing for the coolant ports. How would you couple the motor shaft? The motor "wiring" is internal to the drivetrain, the external wiring runs to the inverter.
> 
> What are you planning to use for a battery pack? The battery pack is one of the primary factors that determines how fast you can accelerate (voltage, current capacity, weight).


ZeroEV sells reduction gears to 4.5:1 . Check out this clip . 
https://m.youtube.com/watch?v=_GigD0s47RI

For option 2 , I would separate and install it elsewhere. I play to source a broken Gear case of the inverter side. Install the inverter there, modify the casing to connect coolant feed and return lines . And regarding the 3 Inverter leads/connectors/terminals , I would just run extended wiring to the motor. 

As for the Cooling of the other half where the motor and trans are, I would modify the inverter ports on the case so the coolant flow continued back to area that cools the gears and return. 

For option 3, I would separate the inverter but keep attached to that half of the case. Again modify the fluid ports and use the same 3 inverter to motor leads but with extended wires.

Coupling the motor Option 3 : After watching videos of disassembly and removal of the gears, I believe I can get/have made couplers to fit the motor splines and U joint on the other end to bolt to the rear differential . 

Haven’t decided battery pack yet . Leaning towards LG chem. Range is not as important as power, 70 miles would be awesome but it might be less at 40 miles, I can live with that.


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## electro wrks (Mar 5, 2012)

4G63T said:


> 2. Keep oem rear differential with 3.307 ratio and mount another rear differential at the front of the car. I will install a gear reduction 4.5:1 inside the Tesla drive unit. This would give me a total ratio of 7.807:1 .


For the 4.5:1 Tesla ratio change for option 2 are you referring to something like these Quaife supplied parts?: https://www.youtube.com/watch?v=_GigD0s47RI Last I heard, Quaife was having durability problems with this gear set. Does anyone know if this has been resolved?
Also, when you calculate the overall ratio of gear boxes or gear trains in series, you don't add the separate ratios, you multiply them. 4.5:1 Quaife set-up Tesla X 3.307:1 stock axle ratio = ~14.88:1 overall ratio.


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## 4G63T (Jan 9, 2019)

electro wrks said:


> 4G63T said:
> 
> 
> > 2. Keep oem rear differential with 3.307 ratio and mount another rear differential at the front of the car. I will install a gear reduction 4.5:1 inside the Tesla drive unit. This would give me a total ratio of 7.807:1 .
> ...


Your right overall ratio is calculated by multiplying . I was wrong. 

I don’t think there are differential ratios of 2:1 and doing something custom to accomplish that is out of my capabilities . So option 2 won’t work. 


I guess then I just need to know is 3.307 ratio too “tall” for each motor ? Tesla designed the drive unit to have 9.X:1 ratio for a ~4800 lbs car. I estimate mine to be around 3300 lbs. Perhaps the power curve of the motor will still allow it to be adequate for tall ratio?


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## remy_martian (Feb 4, 2019)

Option 4. Ditch the idea of using a Tesla drive unit


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## 4G63T (Jan 9, 2019)

remy_martian said:


> Option 4. Ditch the idea of using a Tesla drive unit


So then what do you recommend to use ?


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## ev-swap (Jan 31, 2020)

Hi 4G63T,

I think its a great idea to keep your evo 8 all wheel drive! And in my opinion, Tesla drive units are the way to go. With over 500,000 Model 3's produced so far, the junk yards of the world are going to fill up with these modern ev drives. And Damien Maguire is so close to cracking the code on these newer drives, its just a matter of time before they become the defacto standard.

So, back to your design. There is a guy named Sam that converted his AWD Audi by splitting a Tesla Large Drive unit (like you propose), putting in a gear reducer to make it more like an ICE (rpm wise) and putting the electric motor where the engine was; connecting it directly to the front transaxle. 

Its a slick build. With a lot of custom fabrication (especially since he indicates that he built the reducer from scratch). 

https://www.youtube.com/watch?v=IOYY_AlRWQA

Go for it!


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