Speedometer Error

[Cefalu]

This topic has been partially covered in previous posts but I thought I would share my findings.

When I bought my 56K miles 996 a couple of weeks ago, I had to have it smogged to transfer title. It passed smog easily, but I noticed that the speedometer was reading 4 to 5 MPH high compared to the smog machine it was being tested on. I asked the tech about it and he said that of course his machine was right on and the cars speedo had to be off. That's not hard to believe since the rollers that spin the tires could be easily designed and constructed to consistently duplicate a correct speed. And I assume the state is going to be pretty fussy about making sure the test equipment is accurate.

Wow, I thought maybe I have 50k mile car not a 56k mile car! So I performed a 64.3 mile test using a GPS that I use for sailing against the cars speedometer and odometer. All the driving was done on the freeway with the top down for optimum sattelite reception.

I found that the cars speedometer was indeed 4 to 5 mph high compared to my GPS. The variation was not a % variance, it was a consistent 4 to 5 mph overstating.

I also found that the cars odometer was dead on compared to the GPS track log. Thus, the car odometer was not reading high and in fact I feel comfortable that the miles on the odometer are reasonably accurate. The analog speedo is too hard to tell since the width of the needle is just about 4 to 5 mph anyway, and I didn't like staring at the analog guage while zipping down the freeway.

All that said, an earlier poster made comments that all speedos regardless of manufacturer read high. I also tested the speedo of my 2005 Toyota 4Runner, and the Toyota was dead on at all speeds. I prefer knowing what speed I am actually going without having to do the extra calculations

Edited April 1, 2010 by Cefalu

[drateam]

Thanks very much for going to the extra effort on this. As many others, I've been aware of the variance, but wondered if it is a % variance or a constant. Your post helps clear this up; allows us all to drive with a little less worry.

[number9ine]

This topic has been partially covered in previous posts but I thought I would share my findings.

When I bought my 56K miles 996 a couple of weeks ago, I had to have it smogged to transfer title. It passed smog easily, but I noticed that the speedometer was reading 4 to 5 MPH high compared to the smog machine it was being tested on. I asked the tech about it and he said that of course his machine was right on and the cars speedo had to be off. That's not hard to believe since the rollers that spin the tires could be easily designed and constructed to consistently duplicate a correct speed. And I assume the state is going to be pretty fussy about making sure the test equipment is accurate.

Wow, maybe I have 50k mile car not a 56k mile car! So I performed a 64.3 mile test using a GPS that I use for sailing against the cars speedometer and odometer. All the driving was done on the freeway with the top down for optimum sattelite reception.

I found that the cars speedometer was indeed 4 to 5 mph high compared to my GPS. The variation was not a % variance, it was a consistent 4 to 5 mph overstating.

I also found that the cars odometer was dead on compared to the GPS track log. Thus, the car odometer was not reading high and in fact I feel comfortable that the miles on the odometer are reasonably accurate. The analog speedo is too hard to tell since the width of the needle is just about 4 to 5 mph anyway, and I didn't like staring at the analog guage while zipping down the freeway.

All that said, some poster made comments that all speedos regardless of manufacturer read high. I also tested the speedo of my 2005 Toyota 4Runner, and the Toyota was dead on at all speeds. I prefer knowing what speed I am actuallt going without having to do the extra calculations

It's my understanding that Porsche calibrates the speedometers to a specific, standard wheel and tire size per model. This number could be thrown off by anything making a small change in tire diameter, a change to aftermarket or optional wheels, etc.

My car has N-spec (Porsche OEM approved) tires on the standard 19" wheels for its model spec and the speedo is dead-on as measured against an aftermarket GPS, which itself isn't 100% accurate. Is your car on stock wheels and N-spec tires of the proper size? Either way, our findings are anecdotal.

I've owned many cars with an optimistic speedo calibration. Every VW I've owned, for example, seems to have a 5-10% higher reading than GPS or radar (ugh) suggests.

Mark

[drateam]

This topic has been partially covered in previous posts but I thought I would share my findings.

When I bought my 56K miles 996 a couple of weeks ago, I had to have it smogged to transfer title. It passed smog easily, but I noticed that the speedometer was reading 4 to 5 MPH high compared to the smog machine it was being tested on. I asked the tech about it and he said that of course his machine was right on and the cars speedo had to be off. That's not hard to believe since the rollers that spin the tires could be easily designed and constructed to consistently duplicate a correct speed. And I assume the state is going to be pretty fussy about making sure the test equipment is accurate.

Wow, maybe I have 50k mile car not a 56k mile car! So I performed a 64.3 mile test using a GPS that I use for sailing against the cars speedometer and odometer. All the driving was done on the freeway with the top down for optimum sattelite reception.

I found that the cars speedometer was indeed 4 to 5 mph high compared to my GPS. The variation was not a % variance, it was a consistent 4 to 5 mph overstating.

I also found that the cars odometer was dead on compared to the GPS track log. Thus, the car odometer was not reading high and in fact I feel comfortable that the miles on the odometer are reasonably accurate. The analog speedo is too hard to tell since the width of the needle is just about 4 to 5 mph anyway, and I didn't like staring at the analog guage while zipping down the freeway.

All that said, some poster made comments that all speedos regardless of manufacturer read high. I also tested the speedo of my 2005 Toyota 4Runner, and the Toyota was dead on at all speeds. I prefer knowing what speed I am actuallt going without having to do the extra calculations

It's my understanding that Porsche calibrates the speedometers to a specific, standard wheel and tire size per model. This number could be thrown off by anything making a small change in tire diameter, a change to aftermarket or optional wheels, etc.

My car has N-spec (Porsche OEM approved) tires on the standard 19" wheels for its model spec and the speedo is dead-on as measured against an aftermarket GPS, which itself isn't 100% accurate. Is your car on stock wheels and N-spec tires of the proper size? Either way, our findings are anecdotal.

I've owned many cars with an optimistic speedo calibration. Every VW I've owned, for example, seems to have a 5-10% higher reading than GPS or radar (ugh) suggests.

Mark

Mark - when you say standard you must mean the base set, not the factory option. On my 02 4S, I've got the staggered factory set, and my speedometer reading is off, just as Cefalu says. Would have been good for the manual to speak to the variation between the two options, no?

[Cefalu]

My 996 came with the factory supplied optional 18" wheels, and I am using the stock tire size for the 18" rims.