Car Clinic: Jetta VR6 Underperforming

Car Clinic: Jetta VR6 Underperforming

Question: My 1994 Volkswagen Jetta 3 VR6 came with automatic transmission, but the gearbox was giving me problems, so I had it replaced with a manual gearbox. Now, when I push the car to its maximum speed, it only reaches 185km/h. Could this drop in top speed be due to the transmission transplant?


Answer: Michael, the transmission transplant would not have affected the power produced by the engine at the flywheel. The fact that the speedometer now shows a lower top speed can only be due to one of two things:

-The speedometer reading may have become inaccurate, or at any rate more inaccurate than it used to be. I don’t know how the Jetta speedometer senses the road speed, but if it takes its signals from the gearbox, something like a different speedo gear ratio in the replacement box might cause systematic under-reading. I believe you can now download a “speedo app” onto your phone (if the phone is smart enough) that uses GPS technology to display true road speed. I suggest that, as a first step, you check your speedometer accuracy in this way or by more traditional methods.

-The gear ratios in the manual box will probably differ from those of the auto box, and this may mean that the engine speed (rpm) is higher for a given road speed (km/h) than was the case with the auto box. At a certain engine speed, you will reach the equilibrium point, where the engine’s propulsive force (determined by torque output and wheel diameter) equals the opposing forces of air resistance and friction. That will be the engine speed at which maximum road speed is reached. While this engine speed might not have changed much with the gearbox transplant, the corresponding road speed might have changed significantly. To alleviate matters, you could consider a different final drive ratio or wheels of a bigger diameter. But in view of the cost, you might choose to live with the present setup.

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