Speedometer Calibration

[IowaFoSho]

I think my speedometer might be miscalibrated. When I pass one of those unmanned radar displays, it appears my speedometer reads 3mph faster than the radar. Does this sound like I have the wrong tire size configured in the car's computer?

 

[steve142857]

I think my speedometer might be miscalibrated. When I pass one of those unmanned radar displays, it appears my speedometer reads 3mph faster than the radar. Does this sound like I have the wrong tire size configured in the car's computer?

What size of tires are you running with?

 

[nynd]

Sounds fine.. I believe the spec is for it to be +2.5MPH, -0MPH, so 3 is about the limit.

 

[steve142857]

The wheels need to be of a size which the speed is within 3% of real speed, if not there could be troubles with brakes... Now if he says he is 3mph faster than the radar, supposedly the radar is accurate, than he his wheels are probably too small. Normally they install those radars in school zone or residential zones where the speed limit is around 30mph maximum, so 3mph on 30 is 10%!!!, in a 50 zone it is still 6% too fast... So 3 things :
1 ) radar is not accurate and just wants the drivers to slow down;
2 ) wheels are too small;
3 ) speedometer is not accurate...

Good luck!

 

[LJRuddy]

Those radars parked on the side of the street are almost always not calibrated properly/recently. You have a smart phone (android/iPhone)? Find a GPS app and open it while cruising around. I'll see a 2-6mph difference in speed coming from those stationary radars but on my GPS app, my speed is within 2mph up to about 90mph.

 

[EcoBrick Bob]

You need to check it with a GPS.... Many of the radar units beside the road are not accurate.

 

[Showgun]

I think my speedometer might be miscalibrated. When I pass one of those unmanned radar displays, it appears my speedometer reads 3mph faster than the radar. Does this sound like I have the wrong tire size configured in the car's computer?

Borrow a friend's GPS.
My Garmin and Lowrance show speed and I'm assuming that they are the most accurate.

My SHO and my 'vette have minimal miles on them yet and are within 1kph at 100 kph or about 0.6mph at 60mph.

I've also read that speedos can be less or more accurate at different speeds, so an inaccuracy at say 60 may not hold true or be the same as an inaccuracy at say 30 or 90.

As tires wear, tho, it affects the speed shown on the speedo due to the smaller diameter of rotating wheel/tire.
I would hazard a guess that the difference between the speed shown by a new tire would be about 5% slower than one that is worn out and reading faster. (I wouldn't be surprised to learn that it's more than that.)

 

[steve142857]

Borrow a friend's GPS.
My Garmin and Lowrance show speed and I'm assuming that they are the most accurate.

My SHO and my 'vette have minimal miles on them yet and are within 1kph at 100 kph or about 0.6mph at 60mph.

I've also read that speedos can be less or more accurate at different speeds, so an inaccuracy at say 60 may not hold true or be the same as an inaccuracy at say 30 or 90.

As tires wear, tho, it affects the speed shown on the speedo due to the smaller diameter of rotating wheel/tire.
I would hazard a guess that the difference between the speed shown by a new tire would be about 5% slower than one that is worn out and reading faster. (I wouldn't be surprised to learn that it's more than that.)

Here is my number analysis lol! Worn tires would show faster speed of 1-2% at most. If you take a 20 inches wheel (245-45-20) and use 1.6cm all around, you would end up the equivalent of a 245-40-20 and then it would run 3.4% too fast... But I don't think the thread of a new tire is 1.6 cm deep... at least not an all-season tire.

Here is a website very useful for tire equivalences :

http://www.miata.net/garage/tirecalc.html

Sorry for the maths, can't help it, love it,

Cheers!

Steve, the number guy

 

[EcoBrick Bob]

If you have a SCT Tuner.... you can get it perfect. Just look up the Revolutions per mile on TireRack's website for your particular tire. Plug that into the tuner. I have to change mine from my summer tires to my A/S tires.

 

[Showgun]

Here is my number analysis lol! Worn tires would show faster speed of 1-2% at most. If you take a 20 inches wheel (245-45-20) and use 1.6cm all around, you would end up the equivalent of a 245-40-20 and then it would run 3.4% too fast... But I don't think the thread of a new tire is 1.6 cm deep... at least not an all-season tire.

Here is a website very useful for tire equivalences :

http://www.miata.net/garage/tirecalc.html

Sorry for the maths, can't help it, love it,

Cheers!

Steve, the number guy

Thanks for the clarification, Steve. Your numbers seem sound. Pardon my errors.

The typical car tire tread these days (that I've measured) is about 9/32nds or 10/32nds of an inch which equates to about 8mm -- so all else being accurate in this equation then we shouldn't see more than about 1% increase in indicated speed of a worn out tire.

I would have guessed more but now I can see that it isn't much at all.

Hope the numbers are good at your next track visit.

Cheers,
Colin.

 

[Showgun]

If you have a SCT Tuner.... you can get it perfect. Just look up the Revolutions per mile on TireRack's website for your particular tire. Plug that into the tuner. I have to change mine from my summer tires to my A/S tires.

Cool stuff Bob. Does the tuner actually change the program of the car's computer to accommodate different tire diameters? -- and if so to what extent? any limitations?

TNX for sharing. I wasn't aware that our tuner was this versatile. (magical.)

 

[IowaFoSho]

I used a GPS to compare my speed, and it appears that my speedometer is 3mph faster than actual. I do have significant tread wear, and the OE tires are at the end of their life. I will wait till I get some new rubber before I take any action.

I work for Bridgestone and get a significant discount on Bridgestone/Firestone tires. Any recommendations?

 

[Showgun]

I used a GPS to compare my speed, and it appears that my speedometer is 3mph faster than actual. I do have significant tread wear, and the OE tires are at the end of their life. I will wait till I get some new rubber before I take any action.

I work for Bridgestone and get a significant discount on Bridgestone/Firestone tires. Any recommendations?

That 3% would seem to fall within normal indicated speed differences for worn out tires. (at least at 60mph)

Can you share your discount should any of us decide on that brand?

 

[IowaFoSho]

I just checked out Tire Rack. The only high performance all season Bridgestone/Firestone tires that have a size of 255/45R19 are Dueler truck tires. The OE are the Goodyear Eagle. They are the cheapest, but have the worst ratings.

I think my best bet is to lay down $1G on actual winter tires, then buy some new 20" wheels in the spring. I can get a set of Firehawks for less than $800.

Then I can just swap them out in the summer. I will probably need a tuner to change the wheel size with each change.

 

[steve142857]

That 3% would seem to fall within normal indicated speed differences for worn out tires. (at least at 60mph)

Can you share your discount should any of us decide on that brand?

3mph on 60mph is 5% and beyond acceptable difference of 3%.
BTW, IowaFoSho, what size are your current tires?
As I mentioned, a worn tire shouldn't account for more than 1%
I personally found that the Hankook were the best bang for the buck (245-45-20) at 152$ each... unbeatable.

 

[Showgun]

3mph on 60mph is 5% and beyond acceptable difference of 3%.
BTW, IowaFoSho, what size are your current tires?
As I mentioned, a worn tire shouldn't account for more than 1%
I personally found that the Hankook were the best bang for the buck (245-45-20) at 152$ each... unbeatable.

LOL, yes a simple math error -- bad boy, bad boy .... --- Good thing someone's awake today. zzzzzzzzzz

As far as sizes, I was assuming he was running the proper sized tire (OEM) but yeah if he's running different than OEM he'll get a error in his speedometer reading.

 

[EcoBrick Bob]

I think my speedometer might be miscalibrated. When I pass one of those unmanned radar displays, it appears my speedometer reads 3mph faster than the radar. Does this sound like I have the wrong tire size configured in the car's computer?

I just remembered... was down in Bettendorf for the G8 Natl Event a couple weeks ago... and noticed those radar displays was way off compared to my GPS & my G8 Digital Speedo readout. The unmanned radar display was at least 3 mph off in a 35 MPH speed zone.