Why Does My 986 Boxster’s Speedometer Seem Off at Higher Speeds?

[Abdelkader Helvia]

Hey everyone, I’ve been noticing something weird with my 2002 Boxster S, and I’d love to get your input on this. While driving at moderate speeds, everything seems fine, but as I push past 60-70 mph, it feels like my speedometer isn’t giving me an accurate reading. I first started suspecting this when I was pacing a friend’s car on the highway, and we both noticed a discrepancy between our speed readings. It’s not a drastic difference, but enough to make me question whether something is slightly off.

Now, before I jump to conclusions, let me give you some background. The car is running stock wheels and tires, properly inflated, and I haven’t done any modifications that could throw off the speed sensors. The transmission shifts smoothly, and I don’t feel any hesitation in power delivery. I checked for any ABS warning lights, but nothing seems unusual there either. However, I did recently rotate the wheels, and I wonder if that could have had any indirect effect.

A speedometer is a device that measures and displays the speed of a vehicle in real-time. It typically works by detecting the rotation of the wheels or transmission and converting that into a speed reading. Some newer systems use GPS-based tracking for improved accuracy. While most people associate speedometers with cars, they are also widely used in public transportation, such as in an instant speed measurement tool for buses and trains to ensure precise speed control. Given this, I’m wondering if my issue could be due to sensor calibration, wiring problems, or just a normal variation that I hadn’t noticed before.

Beyond the speedometer issue, I’ve also noticed that at higher speeds, my steering feels slightly off. It’s not a dramatic pull, but it seems like I have to make more small corrections than I used to. The car has about 85k miles, and I don’t think any major suspension components have been replaced. Could this be related to worn bushings or an alignment issue? I don’t feel any vibration in the steering wheel, but something just feels a bit different.

Another thing I’ve been considering is whether temperature fluctuations could be affecting the handling. I live in an area where the mornings are much cooler than the afternoons, and I know that tire pressure changes with temperature. Could that be affecting how stable the car feels on the highway? I always check my tire pressure when they’re cold, but I haven’t monitored how much it changes throughout the day.

Also, could the aerodynamic setup of the car be playing a role? I don’t have any aftermarket modifications, but I wonder if something as simple as an undertray being slightly loose could be causing a bit of front-end lift. Has anyone experienced something similar?

[JFP in PA]

Welcome to RennTech 

Nearly all car manufacturers purposely set their speedometers to read slightly fast, usually about 5 MPH, in an attempt to reduce any liability for potential speeding tickets.  Some sages have also noted that with the speedo running fast on purpose, your warranty also runs out faster, but there is no evidence supporting that conjecture.

 

As for your "at speed' questions; alignment, uneven tire wear, and worn or loose suspension components' are all candidates.  A decent alignment shop should be able to tell you what's up in that department.

[JE 17]

As posted in another forum, it is a European requirement that speedometers read fast. It dates back to the 70s!

My Porsches run about 2 mph, my Chevy truck about 4, and the BMWs about 3. That's according to Illinois' numerous radar speed displays.