Gear ratio’s and a possible transmission swap

When starting an electric conversion it’s tough to know exactly what gear ratio will perform the best and feel the best when driving.  I was happy that my car had the turbo transmission installed since it would handle the extra torque from the 11″ motor, however I really didn’t know what the ideal gear ratio would be, but having a free box with 5 to choose from is a good start.

After driving the car for awhile on 300v I’ve come to the conclusion that my ideal ratio wasn’t included.  The car does drive beautifully and it’s quite quick, however I wouldn’t mind a few slightly “shorter” gears to choose from.

First gear is too short (too high a ratio) and 2nd is pretty close to ideal but could be a bit shorter for low speed driving and fun acceleration.  3rd gear is also close to ideal but is a bit tall for a 0-60mph run or spirited but not high speed driving.  What I feel might be ideal is a 2.5 gear, with a ratio about halfway in between my existing 2nd and 3rd gear.  As it turns out Porsche did make that transmission, it’s has the tough internals like the turbo transmission to handle the hp and torque that I have, but has my desired 2.5 gear.  It came in the later 944’s namely the 944S and S2.  It also has a 2nd gear that’s shorter than my 2nd gear and would provide slightly better acceleration without being too short like my existing 1st gear which is too far in that extreme.

944 Turbo
1st – 11.81    34.5mph @ 5500rpm
2nd – 6.949 58.5mph @ 5500rpm
3rd – 4.725 60mph @ 3830rpm
4th – 3.490 60mph @ 2830rpm
5th – 2.798 60mph @ 2268rpm (top speed 5500rpm 145.5mph)

As you can see from the above speed vs rpm vs gear ratio my existing 2nd gear is almost perfect for a 0-60 run, of course I would accelerate through 60 for the best time which puts the motor slightly above the recommended maximum RPM.  So for a safe 0-60 acceleration I need to use 3rd gear, which is a bit tall.

944 S2

1st – 13.56    30mph @ 5500rpm
2nd – 7.979 51mph @ 5500rpm
3rd – 5.425  60mph @ 4400rpm
4th – 4.007 60mph @ 3250rpm
5th – 3.015 60mph @ 2445rpm (top speed 5500rpm 135mph)


The S2 info is above, and as you can see a 0-60 run in 3rd gear would take me to 4400rpm which happens to be around where the power starts to taper off anyway.  4th gear would also provide a nice RPM for highway cruising keeping the fan speed fairly high for good cooling.

I have many other things to work on first, but I’ll keep my eyes open for a good used S2 transmission for a future swap.
In the mean time back to suspension and motor updates!

4 Replies to “Gear ratio’s and a possible transmission swap”

  1. If I remember correctly, these cars have transmissions that are interchangeable with some of the FWD Audis of the same era. You might look in to that and see if you can find the ratios you want in one of those. It might be easier to track down (and cheaper) than a Porsche one.

    1. Simply let off the throttle and let the syncro’s do the work while moving the shifter. No super fast shifting, however it does work well enough. An auto would be in the wrong RPM range for this type of motor as well as less efficient in this type of car. (newer auto’s would be much closer in efficiency to a manual)

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