Jump to content

Welcome to RennTech.org Community, Guest

There are many great features available to you once you register at RennTech.org
You are free to view posts here, but you must log in to reply to existing posts, or to start your own new topic. Like most online communities, there are costs involved to maintain a site like this - so we encourage our members to donate. All donations go to the costs operating and maintaining this site. We prefer that guests take part in our community and we offer a lot in return to those willing to join our corner of the Porsche world. This site is 99 percent member supported (less than 1 percent comes from advertising) - so please consider an annual donation to keep this site running.

Here are some of the features available - once you register at RennTech.org

  • View Classified Ads
  • DIY Tutorials
  • Porsche TSB Listings (limited)
  • VIN Decoder
  • Special Offers
  • OBD II P-Codes
  • Paint Codes
  • Registry
  • Videos System
  • View Reviews
  • and get rid of this welcome message

It takes just a few minutes to register, and it's FREE

Contributing Members also get these additional benefits:
(you become a Contributing Member by donating money to the operation of this site)

  • No ads - advertisements are removed
  • Access the Contributors Only Forum
  • Contributing Members Only Downloads
  • Send attachments with PMs
  • All image/file storage limits are substantially increased for all Contributing Members
  • Option Codes Lookup
  • VIN Option Lookups (limited)

Speedometer Error


Recommended Posts

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 by Cefalu
Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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

  • Upvote 1
Link to comment
Share on other sites

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?

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.