I was driving by a local city radar and compared my speedometer to the radar. I noticed that my speedometer was reading way more mph than the radar was displaying. If I recall it was at least 5mph different.

I went home and grabbed another car (Infiniti) and the speedometer registered similar. I grabbed another 911 and the speedometer registered similar as well.

Is there a way to correct this mph difference? (1) I would like to accurately now my speed. (2) I would like to keep the miles true.

Has anyone else had this issue?

 

mine turbo had the same issue, it was exactly 5 mph fast. After much research, it seems normal

 

NOOOOOO!!! I CANT BE!!!!!

that would mean that the miles on the car over estimated too. there must be a fix for that.

My Mercedes ML's overestimated it too. I hated that.

There must be some sort of fix. maybe 24 inch DUBs

 

Mine over-reads by 7km/h. It seems 4-5mph over-read is normal. (if you can call it that).

 

It's been well documented in earlier threads. There is a weird way to get the radio to show actual speed. My experience is closer to 3mph. Something Porsche seems to think we need.

I understand the odometer is calculated accurately though.

 

I put slightly taller tires on mine. Almost completely corrected...

 

I have 245/40. 315/30 18s and im just a mile off

 

Is this one of those radar-in-a-box things that they set out to let drivers know their speed with a large illuminated display? I don't think those have the same precise calibration as an actual law enforcement radar gun. Unless they're using that unit to write tickets, I would not assume that its accurate. It's there to make people aware of their speed.

 

The calibration of those units is always suspect.The best way to ck your speedo is the old mile marker/time split-set cruise @ 60 & time section from 1 marker to the next.I believe there is a federal law which states that speedos are required to read above actual,but most manufacturers have a tolerance of +/- 5%

 

Mine is also off by 5mph and confirmed against several cars and two GPS Nav units. Odd-

 

I agree with both of you on calibration issue.

But, I have used both radar and GPS to measure speed vs. speedo indicated speed. The speedos are off to the positive side about 2-3%.

 

My speedo in my last car (Subaru) read high too but I have heard that the odometer measurements are accurate and independent from one's speedometer.

 

Just to confirm guys........

Your speedo IS meant to read high/fast

The reason for this is to save you from receiving tickets all day long

ie,

If you're driving on a road where the limit is 30mph, and your speedo is reading 30mph......you will actually be driving at ~25mph

You will therefore be slightly under the limit, and will not receive a ticket should you be caught by radar

Now, if the speedo was accurate....

You'd be driving at 30mph, the speedo would say 30mph......you get caught on radar doing maybe ~32mph and therefore get a ticket!

Your speedo and the radars cannot be calibarated using the same equipment at the same time, meaning there will always be a variance

By making the speedo read slightly higher you will be 'protected' from this variance



BTW - your speedo and odometer are completely separate from each other, and your odometer will be accurate (assuming the fitment of stock tyres)



Also - this is not particular only to Porsche
Ever car manufacturer in recent history follows this very practice

 

Mine reads 5 mph fast as well.

 

I have heard that the law says a manufacturer can have up to a 10% error on the too fast reading side and a 0% error on the too slow reading side. This means that by law your car can never read that you are going slower than you actually are told by the radar cop. For the manufacturer to avoid costly fines, they build in a fudge factor on the too fast side.