Recalibrating Speedometer?
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You can change sizes In a menu on the mfd when looking at tire pressures. It has various sizes. As long as you aren’t putting a tire size on that is way out of spec
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It assumes you are sticking with the factory recommended diameter.
At worse, you can build a comparison table using an independent GPS and observed speedometer speed. Of course, depending on the GPS accuracy, there may be some variation too, but I wouldn't think it would be much. I once got my wife out of a ticket versus a officer with a certified speedometer by presenting a judge with time/distance table showing my wife's speedometer was showing 2 mph faster than what she believed she was doing. Apparently, miles posted on U.S. federal highways are actual traveled mileage (versus as the crow flies).
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Considering that Porsche has painted in a corner, anyone who wants to buy All Season tires (that means anyone that lives around the Med or near the Gulf and we can not change winter/summer tires every three days as the temperature rises/drops) they could allow for + 1, even + 2 tire size/speedo calibration... but no!....
I am wondering if there is an impact on the PSM (Stability Management) programming, if you put +1 in the rear...
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Food for thought:
As a tire wears its tread from 12/32 to 2/32 (where the wear bars typically are) the diameter goes down by nearly two-thirds of an inch, and the circumference becomes more than 2 inches shorter.
At 65 mph, a worn tire makes a speedometer read faster by almost 3 mph.
@MikePhDPE , this is why self-service calibration isn't offered -- and also part of the reason why most manufacturers calibrate speedos to read slower than actual speed.
Back in the day (the 1970s, I believe), several class-action lawsuits were floated around that placed blame for speeding citations on improperly calibrated OEM speedometers. As part of a deal to avoid government involvement, manufacturers agreed to build 'padding' into a speedometer's 55mph reading to minimize the effect.
As a tire wears its tread from 12/32 to 2/32 (where the wear bars typically are) the diameter goes down by nearly two-thirds of an inch, and the circumference becomes more than 2 inches shorter.
At 65 mph, a worn tire makes a speedometer read faster by almost 3 mph.
@MikePhDPE , this is why self-service calibration isn't offered -- and also part of the reason why most manufacturers calibrate speedos to read slower than actual speed.
Back in the day (the 1970s, I believe), several class-action lawsuits were floated around that placed blame for speeding citations on improperly calibrated OEM speedometers. As part of a deal to avoid government involvement, manufacturers agreed to build 'padding' into a speedometer's 55mph reading to minimize the effect.
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I had done these calcs some time ago trying to figure out what the effect of the +1 and +2 sizes would be. I concluded that at least for the +1 (this was done for higher profile tires) I was playing within the range of wear.
My concern (then and now) is how the change in circumference affects the various stability programs and my assumption was that the manufacturer had programmed the stability programs assuming the mid wear point (7/32").
I haven't re-visited these calcs for the 718 tire dimensions, maybe I should...
I am always in favor of anyone else calculating anything!I had done these calcs some time ago trying to figure out what the effect of the +1 and +2 sizes would be. I concluded that at least for the +1 (this was done for higher profile tires) I was playing within the range of wear.
My concern (then and now) is how the change in circumference affects the various stability programs and my assumption was that the manufacturer had programmed the stability programs assuming the mid wear point (7/32").
I haven't re-visited these calcs for the 718 tire dimensions, maybe I should...
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Not meaning to quibble here but here's the math. C = 2 PI * r, so delta C = 2 PI * delta r.
Thus delta C = 2 PI * (10/32) inches = 1.96 inches, which is a huge, monstrously large, gigantic difference from 2, now isn't is?
Okay, not to an engineer but to a logician. Anyway, the fractional change is the same whether calculated from circumference or radius, and that's what affects the speed. I've never measured a tire diameter or circumference so I'll take your word for it. Dang, now I have to go out to the garage with a tape measure. Hmm, the effective radius is probably the axle height which would be less than the radius of a tire unloaded. Which means tire pressure affects the speed for any given rotation rate. Never mind, it's too much trouble.I recall that hubbub about speedometers, didn't pay much attention though. The thing is, if manufacturers calibrate the speedometer to read high, it means you are going slower for any given reading. So the only reason you'd get a ticket from that is if you tried to compensate by driving to a higher reading and overdid it. Or if your speedometer really wasn't so inaccurate after all. Similarly, as the tire wears you will go slower for any given reading. So manufacturers might calibrate it to read a bit low at first so that it would gain accuracy as it wears, become correct with medium tire wear, and then eventually read high. Driver's following the speedometer closely would drive a bit fast at first but gradually slow down. Even so, 1 or 2 mph over is not sufficient by itself to get a ticket in most situations.
My recollection, or at least the impression I remember getting, is that older cars' speedometers, i.e. cars from the 50's and 60's, were further from accurate than later, 70's onward. The error was always to read too high. It might have been manufacturers' way of making drivers feel better, or to protect themselves from a lawsuit that a ticket was caused by a speedometer reading too low. Or it might have been to compsate for known manufacturing variability by skewing everything downward. In any case, they seem to have gotten more accurate at some point, probably because of those class-action lawsuits.
You might be right on the timeline there Jim. The case I cited was with my wife's 1969 Thunderbird four-door (with suicide rear doors) and 429 Thunderjet engine. In her case, my calculations equated for the judge that the speed she said she was going was just as credible as what the officer said his speed was. Funny aside, the first day she came home from work after driving the Thunderbird, she said, "It is really weird. When I pull away from a red light, all the other cars get really, really small really, really quickly!"
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Sorry Jim, not the math I had in mind. I want to test whether for low profiles, the +1 size (or more) affects the Δr more (or less) than half the wear range. But I am in a hurry to catch a plane, so I'll leave it for later...
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Overspeed error shortens the new car warranty period, something a number of manufacturers have been accused of over the years. Conspiracy...perhaps.
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True ... and I suppose that's a byproduct of the policy -- the 'silver lining' for manufacturers, eh?
Bottom line is that manufacturers can't control what either tire wear or governmental attitude effects; they can just engineer to consider and cover for it. (And in this day and age of ultra-accurate GPS, an accurate speedo is kind of optional, anyway.)
To wit: most LEOs adhere to the 'five percent rule' when clocking vehicle speeds to account for speedometer error. That rule: build a five-percent cushion into any reading, which would be 3 mph for a 60-mph reading, for instance. Also, most LEOs generally won't cite for speeding for readings at or under 5 MPH over the posted limit because it's not worth the costs involved for issuing the citation. Therefore, it's generally OK to speed by 8-9 MPH at highway speeds.
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Always took an extra 9 or 10 highway mph during my first 45 years behind the wheel...and never received a citation (for that doing 10 or less).most LEOs generally won't cite for speeding for readings at or under 5 MPH over the posted limit because it's not worth the costs involved for issuing the citation. Therefore, it's generally OK to speed by 8-9 MPH at highway speeds.
Then, I got pulled over and ticketed in a very rural location...on my bicycle in Kendall County for “running“ a stop sign...at an LEO claimed 2 mph.
I waived as I passed the stationary LEO ~100 yards prior to my “transgression” and knew he was following very close behind. I even slowed and signaled for him to pass ~75’ before the stop sign.
I’m a very seasoned rider that can safely clip-out and track stand (road/mtb) for several seconds pretty much anywhere. Once the LEO failed to pass, I signaled and executed a left turn, then went up an extremely bumpy 15 degree gradient next to the stop sign.
The squad car was so close behind (per helmet mirror) that iI feared it would have run over me, had I fumbled foot placement on the bumpy incline. So, I slowed and brought the bicycle to what I thought was a complete, albeit very brief stop...wheels stopped, but body still moving forward, then released brakes and proceeded through the intersection.
Sargent Stadanko (Officer Daily) flips on the lights, hits the siren, and shouts over the megaphone “you on the bike...pull over....NOW!”
I did, but seriously thought he was kidding. Clipped out and dismounted in a safe spot...on an I-10 overpass. LEO asks why I “ran the stop sign.” He effectively said my explanation was bogus and asked for my DL. I said he could see my health insurance card, but I don’t carry a DL on the bike.
The LEO snidely stated that was another violation of the law. I knew that was a load of crap and called him on it...well, that’s when everything went sideways.
At that point I had been riding 4.5 hours, it’s literally 104°, my water bottles are empty, and the LEO says “wait here while I run your information.” 20 minutes goes by before I start waiving my arms hoping he will exit his air conditioned squad car and release me.
Five minutes later a Texas state trooper boxes me in...seriously, it was like something out of Alice’s Restaurant. 10 more baking minutes passed, the citation was issued, and I was on my merry way.
Long story short ? the citation had open check boxes for a variety of other high crimes and misdemeanors, including speeding. The old speeding options began with “10 mph” over the posted limit. The first speeding related check box on this citation was for 10% over.
The moral of the story...in a 30 mph zone, 33 mph warrants a citation, etc. Not sure most LEOs would issue one. Just don’t run into Sargent Stadanko having a bad day.
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Anywhere near Hempstead?
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