Tire Size

[Longshot270]

Originally Posted by JackandJanet

Some of us, like me, got "smart" by haunting these forums and then reading everything I could find on the new vehicle designs. When I got my truck, I decided it was running on "magic" since there was no distributor, plug wires or throttle cable! It's terrible when you find out everything you learned about vehicles back in the 60s is all meaningless!

Dont worry Jack, we are in the same boat. A year ago I didn't know what a PCM was and look at me now.

[BigSur]

I wanted to use this thread to clarify TS for myself. I have 285-65 R18's. The manual calculation puts at about 2522 (w/ 3% squish). I tested it today by pacing w/ my wife in her car. Speed seemed to be fairly accurate but a little fast at times. However, Over an approximate 4.8 mile ride (according to the Camry), my odometer was .1 mile low. SO, I need to increase TS, correct. I'm thinking 2575????

[shotgun]

Originally Posted by BigSur

I wanted to use this thread to clarify TS for myself. I have 285-65 R18's. The manual calculation puts at about 2522 (w/ 3% squish). I tested it today by pacing w/ my wife in her car. Speed seemed to be fairly accurate but a little fast at times. However, Over an approximate 4.8 mile ride (according to the Camry), my odometer was .1 mile low. SO, I need to increase TS, correct. I'm thinking 2575????

You got it!



Welcome to the unofficial (but highly popular) TS Calculation Club.



Membership don't cost nothing, but it's hard to get!

[Jackpine]

Originally Posted by BigSur

I wanted to use this thread to clarify TS for myself. I have 285-65 R18's. The manual calculation puts at about 2522 (w/ 3% squish). I tested it today by pacing w/ my wife in her car. Speed seemed to be fairly accurate but a little fast at times. However, Over an approximate 4.8 mile ride (according to the Camry), my odometer was .1 mile low. SO, I need to increase TS, correct. I'm thinking 2575????

You would increase TS if your truck's odometer reads lower than actual. However, if the Gryphon speed PID is reading high, then you should REDUCE your TS! As has been said before, the dash speedo will read a bit fast (about 2mph) when the odometer and Gryphon are accurate.

The Camry's odometer can be off too. Everything is fairly approximate. I personally like to use interstate mileage markers (the longer the run, the better) or a GPS. I have a Garmin hiking GPS that shows current speed as well as distance.

Your 2575 figure is correct, however, for a 0.1 mile error in 4.8 miles (About 2%). And, your initial figure of 2522 is what I would have started at too. Personally, I suspect it is VERY close to the right number, Camry or no Camry!

- Jack

[BigSur]

Originally Posted by JackandJanet

You would increase TS if your truck's odometer reads lower than actual. However, if the Gryphon speed PID is reading high, then you should REDUCE your TS! As has been said before, the dash speedo will read a bit fast (about 2mph) when the odometer and Gryphon are accurate.

The Camry's odometer can be off too. Everything is fairly approximate. I personally like to use interstate mileage markers (the longer the run, the better) or a GPS. I have a Garmin hiking GPS that shows current speed as well as distance.

Your 2575 figure is correct, however, for a 0.1 mile error in 4.8 miles (About 2%). And, your initial figure of 2522 is what I would have started at too. Personally, I suspect it is VERY close to the right number, Camry or no Camry!

- Jack

I was looking at the truck speed gauge not Gryphon, so maybe that's right. Why is there a discrepancy b/w the Gryphon and Truck speedometers? I thought the Gryphon was supposed to correct the truck gauge when TS is accounted for. But yes, I'm definitely off .1 mile w/in a very short distance, so I think I need to crank it up a bit. Thanks guys!!!!

[Jackpine]

Originally Posted by BigSur

I was looking at the truck speed gauge not Gryphon, so maybe that's right. Why is there a discrepancy b/w the Gryphon and Truck speedometers? I thought the Gryphon was supposed to correct the truck gauge when TS is accounted for. But yes, I'm definitely off .1 mile w/in a very short distance, so I think I need to crank it up a bit. Thanks guys!!!!

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

[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).