Speedometer recalibration

[krabman]

Been busy with the holidays, I will have it in Thursday unless I screw something up.

 

[Duck]

I kind of enjoy the speedo being off a little.



1) The 10% calculation is easy enough to do. And when travelling, the wife wakes up and sees I'm only doing 75-80 (speed limitish), she smiles and goes back to sleep. Little does she know I'm actually ~82-88 You guys know what I'm talkin about.



2) This might sound shifty, but I kind of like the odometer showing ~10% less than actual too. Overall miles are always 10% lower, which may help one day with the warranty cut-off since I will run out of miles before months. Also might help a little with the resell value.



Still interested to know how easy/cheap a fix would be. If the price is right and it's simple enough, then why not. But for now, I have the wife and D/C living in ignorant bliss

 

[krabman]

The TruSpeed turned into a bust. When the wire/pin diagram didnt match I took it into a shop and no one wanting to smoke an ABS controller ended up calling superlift who said they didnt offer em for an 03 and had no idea what might happen as they had not started the process on testing it on an 03. Called Rickson where I purchased it and they admitted they had never installed one on an 03 either and had no idea as to how or if it would work. Im going to include a letter asking them to reimburse me for my cost for shipping it back to them as I spoke to them on their tech line and they recommended the unit for an 03.

 

[David Muench]

Bummer. This weekend I am going to try to accurately measure the revs/mile of my 315's. BFG claims 601, but when I used dieselman's math I came up with closer to 550. If it's 601-ish, I may just have the dealer reprogram to 636 and see how close that is.

 

[krabman]

I was wondering if you might be able to do the entire thing at the dealer. If their program allows you to put in a tire number in that set range is that also true of the diff number? In other words if the diff number can be set to any number within a range would it be possible to do the math and put in the number needed to get the speedo corrected? Would that hurt anything?

 

[David Muench]

Sounds like a question for sag2 maybe. . Although, where is the speed sensor? Is it sensing off the ABS sensors on the brakes, or is there still a driveshaft/t-case sensor? If it's on the brakes, the diff makes no difference.

 

[Mathew Clausen]

I think you will be very close to what BFG shows for rev's/mile, but probably a higher number. If you look at the BFG chart - it shows rev's/mile @ 45 mph. This is because they actually measure the tire at speed which will give slightly different results than slow roll (effectively "taller" I am guessing due to centrifugal forces), and definately different results than just measuring OD or static radius or calculating based on 315/70-17.



Also - if you are going to measure with the chalk and roll - I would go 5 or 10 revs of the tire to measure so that your accuracy is better. I am very curious how close you will be to the BFG numbers!



Mathew

 

[DIESELMAN]

Originally posted by David Muench

Bummer. This weekend I am going to try to accurately measure the revs/mile of my 315's. BFG claims 601, but when I used dieselman's math I came up with closer to 550. If it's 601-ish, I may just have the dealer reprogram to 636 and see how close that is.

Don't blame me, Blame BFG. Measure the tires and you will see they are actually 305X73R17, or 305mm wide ( 12 inches ) and 34. 5 iinches high which is 73% of the section width.



So, using my stupid formula yields 584 revs per mile which is also different than the manufacturer shows.



And, measuring the circumference with a chalk mark gives me 108 inches and that is 587 Revs per mile.



So, take my formula and throw it out the window because the diameters just don't always match the sizes which are rounded off marketing numbers for idiots like me who like big numbers, the simpler formula is.....



Sharpen your pencils...



Diameter * 3. 14 / 63360



And This gives, 585 Revs per mile, close enough.



Class Dismissed, we all pass.

 

[Prairie Dog]

Maybe the only reason the formula does'nt work out is it doesn't take into account the flat area on the bottom of the tire (caused of course by the weight of the truck) and corresponding reduction in working diameter.

Whaddya think?

 

[DIESELMAN]

Thats the difference with for instance the 265X75R16, but the 315X70R17 its primarily the sales pitch sizing used.



The flatenning actually only changes the height as a measurement across the tire, less in height, but more front to rear.



The tire has the same rolling surface until you reach a certain speed where centrifical force causes the sidewalls to rise and the height of the tire, and thus the diameter / circumference increases causing the turns per mile to go down, not up. The sizing formula actually shows a turns per mile less, not more, than the tire at its 45MPH rating.



The larger the tire and the lower the pressure, the softer the sidewall, the more dramatic the effect. But the result is that the sizing of the tire would give the correct turns per mile above that speed where the tire is more round again, if the sizing acurately reflected the true tire dimensions. 305X73R17 is the closer to truth size for the tire.



The effect is like on a drag racing slick, just much less dramatic.



The 315X70R17 is actually a 305X73R17 on an 8. 5" wheel, if the ratio is slipped to either 70 or 75 the section width never really matches what it actually is so the numbers are fudged. And you always fudge the bigger is better number. You can sell more 315X70 tires than you can 300X75's I would think. And the aspect ratio is always a round lot number, 50-55-60-55-70-75-80 etc.



Anyway, its just academic, like taliking about syth lubes vs dino juice, not that I would do that though.

 

[scbroder]

Has anyone with 315's successfully corrected their speedometer? I would like to purchase 315s but not unless I can correct the speedometer. An accurate speedometer is a must for me.

 

[Hammerhead]

Is this a stupid idea?

I have a theory that I think I read someone elude to in another post that I would like to run by you guy's. My truck came factory with 3. 73 gears and 265/70/R17 tires. The revs per mile are approx. 657. Now, I put 315/70/R17 tires which turn 601 rev's per mile. I am going to trade axles with a guy with 4. 10 axles. OK, after all that here's my question:if I multiply 3. 73 x 657 revs /mile =2450 turns of the driveshaft per mile. Also if I multiply 4. 10 x 601 revs/mile = 2464 turns of the driveshaft per mile.



In my tiny brain this tells me that changing the gears and leaving the computer as it is will get my speedo close to accurate. Is this bone-headed or is this sound thinking?



To recap:

3. 73 (Gear ratio) x 657 ( tire revs per mile) = 2450 driveshaft revs

4. 10 (Gear ratio) x 601 (tire revs per mile) = 2464 driveshaft revs



That's less than 1% difference in revs. I think the speedo will be correct. If anyone can tell me if this is right or wrong please post it or PM me.



Thanks

Eddie

 

[David Muench]

I'm afraid it won't work - gearing has nothing to do with it. These new trucks pick up the speedo from the ABS sensors, which is either in the hubs or in the pumpkin (I think in the hubs). . Either way, it means the sensors pick up axleshaft speed, not driveshaft speed. It's not in the transmission or tailshaft housing like it was.



Sorry, but it's been thought of.

 

[GSatler]

u all talking about speed.

i need a correction in miles. i'm putting on more miles than i'm driving. i checked it with gps and mile markers. DC customer care

says it has to be over 5% before thay will work on it. when i replied i would like them to send me a extended warrenty,

they said no & hungup. what customer care??? i'm about 3% high

3% @ 36000 miles is 1080 miles. what a savings on warrenty for DC anybody out there with the same problem??

 

[JARichard]

OK I have 315's on my truck and had the dealer calibrate the speedo. With stock tires at 30,000miles I was below my GPS by 1. 8 mph. When i took the truck to the dealer I brought the spec sheet from BFG with the revs per mile. It was over by 2. 1 mph. I figured that was cool b/c as the tires wear I would be more accurate. and wouldnt be too far off when thet get real bald. I do not know what number they plugged in But i do know they thanked me for bringing the sheet in so they did not have to look it up.

 

[Udaloy]

I find it hard to believe the Superlift Tru-Speed won't work. I installed it on my '02 without issue. If you have the service manual, you should be able to confirm you are tapping into the correct wires.

 

[Yo Hoot]

There is a sensor on the pumkin.

 

[Mercam]

as always TDR has timely threads. I just put 35x12. 5x17 Procomp MT's on my rig and was wondering how to get the speedo calibrated since i am about 10% off on my speedo ( i verified it with my Garmin GPS) i have been told one of the hypertech programmers can recalibrate . but i do not want to spend 350. 00 just to re-cal the speedo