My 2015 Vantage V8 has an annoying speedometer error. It reads about 3 mph fast at 40 mph and about 4 mph fast at 70 mph. This seems especially odd since it is not linear. I verified true speed using two accurate GPS receivers that agreed to better than 1mph at all times. I find this error to be oddly annoying when driving and would like to seek a remedy.

So far as I can make out the car has stock tires and rims, 285-30-R19 rear and 245-40-R19 front.

My Foxwell reader tells me that the car assumes 777 rev/mile and a final axle ratio of 4. Could someone verify if that appears to be the correct stock values on their car? It is conceivable that a previous owner had custom wheels corrected for speed by a dealer, but swapped them back to stock when sold. Of course such an adjustment would result in a linear error.

It was suggested in a moan I had with a car guy that the ABS wheel sensors may be flaky. Oddly, my Foxwell cannot see the brake module on either of the two reader ports. Does that only register when the engine is running, I wonder? My ignition was off for the extended period I was scrolling through hundreds of live data feeds. Needless to say, no ABS errors are throwing P codes so it cannot be totally broken.

Does anyone know if the AM calculates speed from an average of all four wheel sensors, or like the BMW M3, just one (rear left I think).

Many thanks

Philip

 

You should have 285/35/19 on the rear.

 

My typo. Indeed I do have 285/35/19 on the rear.

Philip

 

I thought most ,if not all, car speedos over-read by about 5%? I have a Snooper speed alert with GPS display and have used it on my last 4 new cars,3 Mercs and an Audi. Their speedos all over-read .I just assumed the Aston would be the same .I'll check it later.

 

In Germany the law says the speedometers cannot indicate less that the actual speed. To protect themselves the cars manufacturers in Germany usually have their speedometers calibrated to read faster than the speed limit. My Porsche reads 3 miles over at 60 MPH. It’s possible that your cars was destined for the German market?

 

All cars do this. I don't know why, but they do

 

If you had an actual issue with speedometer error or an ABS sensor fault it would be a C#### code for chassis. P#### codes are powertrain. Manufacturers are allowed 10% discrepancy and always error on the fast side. Helps achieve warranty miles quicker is what they used to say. IDK if that's actual truth because my phone lies to me more than 10% of the time. Besides I've never gotten a speeding ticket that I didn't deserve.

 

Is your speedometer error in the digital readout or the gage in the cluster? Or both?

 

Does this mean you car is registering more mileage than actual mileage?

 

Very typical, it’s very rare for a speedometer to read the exact speed. They’ll always read a little high, figure that as tires wear OD will change, OD differences between tires of same size but different manufacturer etc and indicated vs. actual speed will change.

Manufacturer must ensure that the speedometer doesn’t read low (for obvious reasons), but also must account for differences in actual/registered speed as tires wear/swapped for different brands.

Nothing wrong with your car, this will not be solved with replacement wheel speed sensors.

 

Interesting and annoying. I had heard that modern European cars (including but not strictly true the UK) tended to read fast. Back in the days of just analog dials it was hard to read to better than plus or minus 2mph. With modern GPS and digital readouts it is easy to reach the theoretical limit of plus or minus 1 count limit of a digital readout.

I guess I am just spoiled. All three of my last Subarus have been dead on. Absolutely correct. All the nasty US government cars manufactured in the US I drive for work are dead on when the digital display is activated. One thing they do correctly.

I guess I had just assumed that accuracy was the modern norm for all cars.

Now oddly, I thought more about the 777 revs a mile number the Foxwell reports. Several tire calculators state that the average count for the very slightly different front and rear sizes is 753 revs/mil. That then is actually the wrong way to explain the error with stock tires. It would lead to a 3% under read if taken at face value not the ~5% over read that I observe. That means either the Foxwell is reading a duff number, or there is a hidden fiddle factor in the cars speedometer calculation.

The answer one question asked , The false high reading is reflected in the analog gauge, the digital display next to the analog gauge and the SatNav speed. The car is convinced it is going faster than it is.

Philip

 

Is the car running the factory OEM tires designed for and supplied by AM?

Remember that the speed sensors are just counting pole changes on the hub as it rotates.

Now that I think about it, you may actually be able to have an AM dealership correct this with an AMDS. I am sure the capability exists for optional wheel/tire packages etc. If it bothers you this much may be worth a call/visit.

 

it absolutely does man that.

 

Most cars read fast AFAIK, due to that German law. The most extreme is my old Toyota MR2 Spyder speedo read I think 9% higher than the OBD2 speed. I noticed I was always significantly falling behind traffic if I drove at "65mph" since the speedo error was so much more than other cars. When I put on tires that were 5% taller, that phenomenon went away.

 

That's what I thought. I would be more concerned about that than what speed I am going if it were my car. Good news if he bought used, car has less mileage than was assumed.