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Tire Size
Just 2 quick questions. One on tire size . I have 275 x 65 x 18 tires on my truck at this time . I used the formula in the Gryphon manual to calculate the tire size and came up with the number 2482 with a 3% squish .
My truck came originally came with 235 x 75 x 17 tires . Tire size in the Gryphon was 2356 - If I remember correctly . I have changed the size in the Gryphon a couple times and have got different results each time . Can anyone fix me up with the correct tire size number for my tires and maybe how you did it . I am waiting on my custom tunes . Should I go ahead and download the Pegasus software now while I am waiting . Thanks in advance Paw Paw |
The best way to get your tire size correct is to use the odometers of the truck and a gps. First program your truck to the 2482 so you can be 100% sure of your tire size. If you dont have a gps you can use the mile markers between cities or get the distance between two intersections using google maps. If you have a gps give it time to properly locate and then clear the odometer for the gps and truck. Then drive about 10-20 miles and check the two odometers. To calculate the new tire size divide the truck's odometer by the gps odometer, then multiply that number by the tire size you programmed the truck to. This final value is what you enter into the programmer. Do not recalculate for squish or anything.
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Thanks Longshot270 , I don't have a GPS , so I will try the mile marker method.
Paw Paw |
I would try doing two intersections or addresses on maps.google.com. That is accurate to more than 1/100th of a mile. :thumbsup:
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Thanks Longshot . I will give it a try.
Paw Paw |
Paw Paw - Your 2482 is going to be VERY close in my opinion. That 3% reduction works very well unless you have some extremely low profile tires, and you don't. Longshot's technique will "dial in" any small changes needed to correct for "slight" errors in the "squish" value.
I just returned from a trip to California and on the interstate, in a 70 mile stretch (using the roadmarkers), I found I had an approximate 0.1 mile error in my odometer (reading low). To put this in perspective, my "entered" TS was 2481mm. Now, 70/69.9 x 2481 = 2484.55, or 2485mm. (Longshot, I think you need to divide the GPS (actual) miles by the truck's miles, not the other way around). If I INCREASE TS, I show a longer distance on the ODO. So, my 3% squish value that seemed to give me correct speed, is actually a bit too much and each tire rotation actually moves me a whole 3.55mm more, and that works out to a whopping 7.6 ft more each mile! Wow! :hehe: So, in 70 miles I actually covered about 530.6 more feet and that is about a tenth of a mile. I'm kind of having fun with all this, because if I were to let my tire pressure drop, I might easily get back to the 3% value. Everything is a bit of an approximation. Don't lose sleep trying to get things "exact". - Jack |
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Another way, personal favorite lazy-way, is to chalk a tire on the edge near the tire tread and on the ground, and then roll one complete tire rotation (making sure your tire chalk mark is exactly where it was before the rotation - a plum bob works well). Then straight-line measure the distance. Just make sure all of your tires are air'd up correctly and evenly.
milehigh |
Hey guys ! My actual calculation on tire size came up to 2559 - not subtracting anything for tire squish. So, I decided to set the tire size to 2550 as a starting point.
Did the Longshot270 suggestion an got a milage distance from Mapquest on a route that I was going to travel on Sunday evening . Zeroed the trip odometer on the truck . Mapquest mildage 11.03 Truck 10.9 11.03/10.8=1.011926 2550x1.011926 = 2580 This is where I am right now . Should I leave it at 2550 or bump it up to 2580? I grew up working on the ole FLATHEAD V8'S , and 8N Ford tractors (which I still have ) and this whole digital thing is just too much fun and facinating . Gotta love it . THANKS FOR ALL THE REPLIES PAW PAW |
Paw Paw, I don't trust the mapquest mileage in your case. If you DON'T factor in "squish", your PCM's going to record a LONGER distance than you actually traveled.
No matter what Tire Size you use, the tires rotate a "fixed amount" in a fixed distance. I think mine rotate 647 turns in a mile. So, in 11.3 miles, they turn 7311.1 times. With a TS of 2485, I think the PCM will "see" 11.3 miles for that many rotations. But, if the entered TS is bigger, at 2550, you get: (2550 x 7311.1)/25.4 = 733988.39 inches, or 733988.39/12 = 61165.7 ft and that is 11.58 miles on the odometer.By the way, you have the same size tires as I have (275/65-18). I am now using 2485mm for TS in the old Gryphon or a 31.1" diameter in the CTS, at 40 psi inflation. Tire rack says my Goodyear Wrangler SilentArmor tires spin 651 revs per mile. They expect more "squish" than I experience, possibly because they assume a lower inflation. If they were a particular Michelin, they say 648 revs. A Yokohama tire is 645. If you tell me the exact make and model of your tires, I can point you to the page that will show you the manufacturer's "rev" number, and from that, it's an easy calculation to TS. But, I am CERTAIN that 2550 is TOO LARGE! - Jack |
I know, aren't these numbers fun? :notallthere:
I used pegasus and inputted the same amount of squish as my truck and I got about 2380. Your tires are slightly larger than mine (235/70-17) and mine come pretty close to 2332. I think that number is about 1/10 of a mile off at 25 miles. FYI, I used about 5% tire squish on pegasus. That program is set up a bit different compared to the hand calculations. I have also had issues with mapquest, google has been right on target when I zoom in and manually pick my two points and the route to them. If you type in two addresses there is sometimes error from the points not matching up to the gps they used to map it. |
Thanks JackandJanet . I have Michelin LTX M/S 2 tires . Tire Rack says the diameter is 32.1 " and Revs per mile is 648 .
Thanks for all the help. PAW PAW |
Great! You found the page I was referring to. The 32.1" diameter is with no weight on the tire (and you need this value to check for clearance at the top of the wheel well or at the sides when the wheels are cornering). The revs/mile figure IS with weight on the tire and works out to a TS of 2482mm. I think that is going to be VERY close to the number that would be dead accurate.
- Jack |
Thanks JackandJanet . I am at work right now and do not have my figures in front of me . I will check my numbers when I get home and get back with you .
THANKS AGAIN FOR ALL THE HELP !!!! Paw Paw |
OK , JackandJanet. I made it home and found my work sheet that had the figures on it.
WHOO HOO ! JackandJanet, you are exactly right . I found the conversion factor for miles to mm and did the math . I divided the mm per mile by the revolutions per mile and came up with 2483. I did not know that the revolutions per mile in the chart already had the tire squish calculated in. Thanks for the information . THANKS AGAIN - JACKANDJANET PAWPAW |
You're most welcome! Again though, this is really a starting place (that is pretty close). Your truck may weigh more or less than the weight the tire manufacturer assumes for their table. You may also inflate your tires higher or lower than the cold inflation pressure they assume.
And, even speed has an effect! At 75 mph, the speed limit on the interstates here in AZ, the centrifugal force (and heat) on the tire will cause it to "expand" and increase its "rolling radius" compared to the same tire driven at a more sedate speed of say 55 mph. You can see this "radius expansion" quite easily when a drag racer does a "burnout" to condition his tires. Of course that is an extreme rotational speed, but there will be an effect like this even at lower speeds. This is one reason I don't care for the "measure one revolution of the tire on the driveway" method. I think the result will be low. So, you are probably within about 2 mph of an "exact" value for speed and distance at 2483mm. If you can do a LONG run like I just did (for 70 miles), you can make it a bit more accurate for YOUR particular weight/speed/pressure setup. - Jack |
There must be an easier way than all this math.
I just did a 400 mile round trip and with my programmer in the canned towing mode. I was 2 MPH off from my GPS. So I decided to try all this math and change the tire size. I have 275/70/18 tires....I came up with 2646. Well that didn't work....Now my Speedo and programmer are 4-5 MPH off from the GPS. |
2646 is much too much of a change for a 2 mph error (which is about 2-3%).
You did not say if the error was fast or slow. What I'm asking here is is the speedometer saying you are faster than the GPS, or is it saying you are slower? And, are we talking about the dashboard speedometer or the Gryphon speedometer? If the Gryphon speed was 2 mph slow, try 2533mm. If the Gryphon speed was 2 mph fast, try 2433mm. But if the dash speed was 2 mph fast, you were probably dead on. And, according to your FIRST post in this thread, your tires were 275/65-18s. Now they've "morphed" into 275/70-18s? The 70-series tires are a bit over 1 inch larger than the 65s. - Jack |
My Speedo and my Gryphon seemed to be sync'd pretty well, but they were both 2 MPH slower than the GPS. Once I changed it to 4626, none of them were sync'd.
No "morphing" here, my tires are 256/70/18 and always have been. |
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If it's the former, they are 33.1" tires. If the latter, they are 32.1". That's an unloaded diameter of course. I would use a value of 2560mm if you have the bigger tires. If they're the ones you said you have in your last post though, try 2483mm. In either case though, 2646mm sounds way too big (that's the larger tire with NO "squish" effect). You'd have to be driving a VERY light truck! :giggle: - Jack |
Sorry for the typo, they are 275/70/18.
Long weekend, my fingers won't cooperate this morning. |
FREE SOFTWARE...
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Just in case, here's an Excel spreadsheet that eliminates all that math.
Well, not exactly "eliminate", but at least automates... |
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My guess is though, you were not using 2560 earlier, but had entered something smaller? AND, when you enter the TS, also make sure the Gear Ratio (GR) is correct. The older Gryphon uses "placeholder" values for both TS and GR in the Custom Options menu, and BOTH of them can be wrong. - Jack |
I would ball park it in the mid to upper 2500s by using the gear ratio calculator in pegasus. You just type in the tire size and double the amount of squish. Anything that is more than 50-100 from there is probably incorrect.
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When I make the change...should it be a permanent part of that tune? When I changed it before, I used the 87 tow tune and changed the tire size to 2646. Then I changed it back to the #1 tune...and all was OK, If I go back to #2 will the 2646 tire size be there again?
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Try programming to the tow tune without making any custom changes. Then the next time you program, after you select "yes" for the custom options menu, go directly to "load previous settings" and see what value is in the tire size. I dont think it puts a placeholder value when you do that...at least it doesn't seem like it on mine.
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So, by bypassing the Custom Options choice, you actually got a pretty accurate value, the one that's held in the PCM's VID area. However, there's no way for you to find out what that is! Seems weird, but that's just the way things are. Here's what I'd do then. The Gear Ratio Calculator in Pegasus gives the 2560mm value assuming a 7% rubber squish (deformation due to weight on the tire). That's about the same as a 3% overall reduction in the unloaded circumference of 2646mm. I think 2560mm is a good starting point and is the value I'd put in for TS when you reload the Level 2 tune. Make sure the Gear Ratio is correct too. Now, when you let the Gryphon program your PCM with those values, they will be the ones saved in your "previous settings file". Whenever you change your tune in the future, you can select Custom Options and then "Reload Previous Settings" as the FIRST step! This will replace the placeholder values with your saved TS and GR values. If you discover the speed is still slow, increase the TS a bit. If the Gryphon is registering fast, you'll need to reduce the TS a bit. To give you an idea how much, a 1mph error at 70mph would call for about a 37mm change. - Jack |
How did you guys get so smart? I'm sure glad I found this forum.
I guess my next step is to figure out what my rear gear ratio is. |
That should be listed in the door jamb where the latch is.
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What Longshot is pointing you to is the "VIN sticker". You'll see a code under the word "Axle". Mine is B6, which is a 3.73 Limited Slip differential. These are the OEM codes:
19 = 3.55 openAftermarket axles can have different ratios. 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! :hehe: It's terrible when you find out everything you learned about vehicles back in the 60s is all meaningless! :rolleyes: - Jack |
JackandJanet, I know what you mean about everything that you learned about in the automotive world from the 60's is meaningless.
Heck , I cut my teeth on a 51 model ford FLATHEAD V8 . I still love the sound of the old FLATTIES. PAW PAW |
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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????
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:thumbs up yellow: Welcome to the unofficial (but highly popular) TS Calculation Club. :woot: Membership don't cost nothing, but it's hard to get! |
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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! :hehe: - Jack |
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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". :hehe: But, it sure is fun! - Jack |
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I'm going to bump it up to 2575 today and see what I get and will then continue to adjust as necessary. |
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). :hehe:
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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. :happy-dancing:
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. |
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 |
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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. :shrug: Any advice on this??? Thanks in advance :) |
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. |
My TomTom has speed. Show I should compare speed on the GPS with speed on the Gryphon, right? Not speed on the analog gauge.
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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 |
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My favorite method is the "3 roll" method. This has always proven to be extremely accurate and takes into full account for tire pressure, tread depth, and any tire squish. :2thumbs:
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According to my GPS, I'm over on my odometer approximately 4 miles over a 140 mile ride.....and the speed on the GPS was consistently 1 to 2 mph fast on the Gryphon compared to the GPS. HOWEVER, the mile markers on the interstate made it seems like I was a little under on the odometer!!! I'm going to trust my GPS on this one and scale the TS back I think.
BTW Jack...regarding my shifting issue we spoke about in the other thread, I've confirmed that it's only when there is load on the truck. I had to come up some hills to get to my parents house tonight. On an incline, every shift from 1 to 2 was rough and there was a physical "bump" present from the rear end. I'm not sure if I want to play with the settings on the Gryphon to fix or let my mechanic feel it. Appreciate your write-up though in case I decide to play with it myself. |
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You shouldn't be in any danger playing with the part throttle shifting yourself. I think I'd REDUCE the 1-2 upshift and downshift by the max (-2) and then maybe add to the Torque Converter unlock/lock speeds by the max for both 1st and 2nd gear. (I'm trying to keep the TC from locking right away so I think values of +2 in 1st and +3 in 2nd might work. Anyway, it's worth a try. - Jack |
4 miles over at 140 miles? I'd say that is really close. Personally I would leave it. I'm currently in the process of dialing my tire size in aswell. Just got a set of cooper discoverer atrs and I'm loving them. :woot:
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Hey Guys,
I had to buy a new set of tires so I have to reset my TS in the Gryphon. I went back to the stock tire size of 275/65/18. Right now with the Gryphon set at 2560, My GPS is showing 2-3 MPH slower than the Speedo. The Gryphon is showing in between the GPS and the Speedo. Should I increase the MM size and how much? |
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And remember, the dash speedo is always going to read a bit faster than you're traveling, because Ford builds them that way. - Jack |
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