Speedo accuracy

G'day dudes,

I have correct dimension yokohama advan sports on my 550, by my calculations the speedometer reads about 7.5% faster than a gps based speedometer app that i measured it against.

If anyone else has done this sort of test, what sort of results do you get. ?

Cheers
M

 

My 550 is about the same, speedo reads faster than GPS actual. I haven't done the calculations, but the speedo does read between 7-10% faster than the GPS.

 

Marcel- Seems to be a constant with Ferraris of 5-10% fast on the speedo, but the odometers are usually pretty accurate. Was the same for the ones I owned, all the way back to the Dino in 1975.

 

Agree. I watch the gps to determine cruising speed then keep it steady using the tach.

 

If your really fussy there are a number of different GPS interface units that correct the speedo, this is one.
SENSOR MODULE, GPS SPEEDOMETER INTERFACE, 16 FT. CABLE, INCL. GPS ANTENNA

Interesting my 456 in Ks over reads 5%.

 

Works great with one of their speedometers, but not sure it is adaptable to a Ferrari.

 

There is an interface unit that connects to the speedo wiring directly.
Classic Instruments Store / Sky Drive

 

Graeme, not sure if you use this skill set of yours outside the forum, but you're a bit like a google!

 

I've calculated the gearing, tire size, etc. to translate to 26.2mph for every 1000 rpm in sixth gear. This seems close to the 26.18mph figure I seem to remember reading somewhere. Interestingly, and assuming my tach is correct, my speedo is dead-nuts accurate up to about 80mph. I mean perfectly accurate. After about 85, the speedo starts immediately reading about 5-6% fast (or perhaps the tach becomes inaccurate). For example, by 4,000 rpm, I should be going a tick under 105mph, but the speedo shows about 110-112. I know there is some "tire growth" and perhaps some aerodynamic changes that occur at speed but this transition seems too abrupt and too large to by accounted for by those types of things. I also seem to remember reading that Ferrari (and others) build in some speedo inaccuracy at higher speeds. I don't know if that's true or not.

 

That's funny, It just so happens I have another project car and have resently been reasurching
Speedos and found there are a variety of GPS speedo drives. One in particular has a programable stepper motor and will drive an analogue gauge. Wish I had known about it for my Maserati. Only down side is loss of signal in cities and tunnels.

Also I recently dismantled a Alfa Jeager speedo that was running slow, only needed lubing with very fine oil. And talking to a friend he has calibrated his own by increasing or deacresing the magnetism of the rotating magnet, bit fiddly. I wouldn't suggest it here for Modern Ferrari's as I think most are digitally driven.

I cannot help but wonder is the calibration of modern V12 is the same for Ks and Miles and the marking on the speedos isnt as accurate for milage speedos.

 

My 550 MPH speedometer seems very accurate when comparing the reading to stationary radar at "legal" speeds. But, then again...it is analog...

 

550 mph is really humming. I make that to be about mach 0.72 at sea level.

 

Meant to say the "MPH" speedo in the 550... It does zip along nicely though!

 

Is this not to do with Construction and Use or Type Approval Regilations?

The UK (European?) regulations include the following paragraphs:

2. For all true speeds up to the maximum design speed of the vehicle, the true speed shall not exceed the indicated speed.

3. For all true speeds of between 25 mph and 70 mph (or the maximum design speed if lower), the difference between the indicated speed and the true speed shall not exceed—

V / 10 + 6.25mph

where V = the true speed of the vehicle in mph.

Presumably manufacturers er on the high side to compensate for the fitment of different tyres, temperatues etc.

And, of course, GPS readings may not be 100% accurate, dependant on gradients and satelite visibility.

 

Domestic GPS units don't to the pin point accuracy of a Military unit, having said that If you run a GPS Speedo app like Ulyssees it will tell you the number of Satellites it has identified.

 

My 456 reads 10kmh high, consistently...

 

John- So at 10 kph it reads 20 kph?

 

Correct. It reads 10kmh when it's stopped.

Far as my gps reports, it is accurate it's whole travel, less 10kmh

 

Whenever I see one of those speed trackers (supposedly slows people down), I just go by at 20, 30, 40, etc., mph to calibrate my speedometer. Speedometers are always about 5% to 7% fast.

 

You can do anything in Maranello!

Mine reads about 575 at 550. ;-)

 

When I put the larger diameter (by one inch) tires on my 550, it reduced the revolutions and now my speedo reads accurately.

 

The speedometer in my 1995 456 seems to read about 10% too high based on comparison with a roadside radar-based speed sign. I assume this is due to the regs mentioned earlier that require it to never read low, despite alternate wheel and tire fitment.

 

When I wear my Speedo, oh never mind.