Tire Size

[BigSur]

Originally Posted by JackandJanet

Your truck speedometer is an analog device. It receives digital data from the PCM, the same that is fed to the Gryphon and the digital odometer. In the conversion to analog, mechanical gear/spring errors cause the difference. I suspect that Ford insured the speedometer would read slightly fast so they could never be sued for someone getting a speeding ticket due to a speedometer reading slow. That's a guess, of course, but if there has to be an error, I'd expect Ford to "bias" it in that direction. Because mechanical devices can't really be dead accurate, speedometers in different vehicles are going to have slightly different errors.

But, there's no "conversion" for the odometer and the Gryphon speedometer. You should be able to get them to tell you something that is very close to the truth.

However, all this is a bit like counting angels on the head of a pin. Variations in tire construction, tire height/width profiles, tire pressure, vehicle loading, tire "slippage", and vehicle speed will ALL effect the accuracy of both the digital and analog reading. The PCM essentially counts axle revolutions, through a sensor. It then "calculates" speed and distance based on the number of revolutions it expects the axle to make in a mile (which the Gryphon inputs after calculating it from your tire circumference (or tire diameter if you have a CS/CTS). You should be able to get close, probably within 0.1%, but much more than that is just wishful thinking. (0.1% is 1 mile in 1000 miles).

As shotgun said: "Welcome to the unofficial (but highly popular) TS Calculation Club." I'll add, "No matter what you enter, you'll be wrong". But, it sure is fun!

- Jack

Well, as always......thanks very much for the help. Without the forums and members like you guys who are willing to help those in need, many of us would still be sitting around confused w/ tons of unanswered questions. With that said, thanks again.

I'm going to bump it up to 2575 today and see what I get and will then continue to adjust as necessary.

[Longshot270]

You could always go flying through a school or construction zone with the cruise control set to about 95. Then a police officer can give you your actual speed in a fraction such as 95/30(if your tire size is dead on).

[BigSur]

I asked a buddy of mine to borrow his GPS to test this over the weekend. He turned around and bought me one for my b-day yesterday.

How accurate can I expect the GPS to be. I haven't had much time to test yet...or driving distance rather, but I tested it over a 5 mile commute to work this morning, and it's almost dead on with my TS set to 2575. The analog speedometer gauge was consistently over approx 2mph in comparison to speed on the GPS.

Anyways, just wondering if you guys think that using a GPS is one the more accurate ways of checking distance....in comparison to mile markers, google maps, etc.

[Jackpine]

Wow! Sounds like you have a good buddy!

I think the GPS will be VERY accurate. I'd say the figures you're describing now show you have the TS nailed. The analog gauge is reading right where I'd expect it to be, and if the Gryphon and the GPS match, that's as good as you're going to get it.

- Jack

[BigSur]

Originally Posted by JackandJanet

Wow! Sounds like you have a good buddy!

I think the GPS will be VERY accurate. I'd say the figures you're describing now show you have the TS nailed. The analog gauge is reading right where I'd expect it to be, and if the Gryphon and the GPS match, that's as good as you're going to get it.

- Jack

Jack, did you use a GPS to confirm accuracy? I've run into an issue and am not sure how to best ensure accuracy.

I realized that my TomTom GPS does not have a standard odometer. The only way I can confirm distance is to plan a route and see what the total distance is to the destination. For instance, if my route says it's going to be 25 miles from A to B, my odometer is accurate if it is showing 25 miles when I hit my "exact" destination, or when the remaining mileage on the GPS hits 0. However, I've been told there is an approximate 5% leeway with the GPS, as the mileage adjusts and may vary over the entire route distance.

The other night, I ran 22.8 miles (according to the GPS). When I hit 0 on the GPS, my truck odometer was at 23.3, a .5 mile discrepancy. This tells me that with a TS of 2275, I am slightly over. However, it was weird when watching the GPS, becuase it seemed to be dead on, i.e every click down on the GPS, there was a simultaneous click up on my odometer. A few times during the ride, this sequence must have varied, but visually, it was unnoticeable without keeping my eyes on it the entire time. I did however watch both readings for over 2 miles straight with no variability.

Any advice on this??? Thanks in advance

[88Racing]

I use a garmine like the one Jack's got.
It has a speedometer in it which has proven to be a good approximation of the trucks mph.

[BigSur]

My TomTom has speed. Show I should compare speed on the GPS with speed on the Gryphon, right? Not speed on the analog gauge.

[Jackpine]

I can't comment on your Tom Tom, because I don't have one. Mine's a Garmin hiking GPS and it measures both current speed and elapsed distance. I just sat it up on the dashboard and confirmed the speed. The position accuracy is within 10-16 feet (WAAS enabled) and the steady state velocity accuracy is 0.05 meters/sec which is about 0.1 MPH if I converted properly.

Regardless, I suspect your GPS measurement is pretty close too. When I apply the error to your TS figure: 22.8/23.3 X 2575 = 2520, which is VERY close to the original 2522 figure that you started out with, and which I still think is closer to the right value.

Honestly, I'd put in 2522 and call it good enough until you get some data that PROVES otherwise.

Edit: Guess I was slow in giving you an answer. Compare your Tom Tom's speed with the Gryphon's speed.

- Jack

[BigSur]

Originally Posted by JackandJanet

I can't comment on your Tom Tom, because I don't have one. Mine's a Garmin hiking GPS and it measures both current speed and elapsed distance. I just sat it up on the dashboard and confirmed the speed. The position accuracy is within 10-16 feet (WAAS enabled) and the steady state velocity accuracy is 0.05 meters/sec which is about 0.1 MPH if I converted properly.

Regardless, I suspect your GPS measurement is pretty close too. When I apply the error to your TS figure: 22.8/23.3 X 2575 = 2520, which is VERY close to the original 2522 figure that you started out with, and which I still think is closer to the right value.

Honestly, I'd put in 2522 and call it good enough until you get some data that PROVES otherwise.

Edit: Guess I was slow in giving you an answer. Compare your Tom Tom's speed with the Gryphon's speed.

- Jack

Thanks and will do on the speed. The original TS of 2522 however proved to be too low according to the Camry, which we "suspect" to be accurate. I'm going to set at about 2550 and see what I get. I'm about to depart on a 180 mile journey as we speak. I'll let you know how it turns out. Thanks again.