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Re-Calibrating VSSB due to tire upsize

A load range tire (LT) should have the circumference stamped in the sidewall of the tire. Usually there are two numbers one is with the tire at rest and the other is at speed or low end and high end.

The difference of the tire circumference of being on the vehicle and being off the vehicle is shorter. In other words the rolling surface is larger off the vehicle.
 
Further driving with new skins and proper speedo calibration seems my shift points are quicker also. She gets herself right out of first gear in a hurry... Mileage gains are inevitable with proper calibration.
 
Someone PM'd me about this, so This bump is for that reason.

After months of driving, when my Speedo reads 65, I'm actually doing 66. So Damn close.
 
the worst part of this is, when you have way too much gear ratio and way too small a tire and your calculation is off the chart... but then somehow you guess at the correct random position of the switches and it works! and then 2 weeks later you change to a transmission with a different pulse sensor doh!
 
I picked up a few VSSB's on the last U-Pick-It hunt & converted a couple for my use & a friend's. Have an extra core, if anybody wants to solder in the dip switches on a core, instead of the one currently in their truck.

Be happy to send it for the cost of postage & a padded envelope if anybody wants it?
 
I never thought I was stupid until I read this post and couldn't grasp any of the math. My speedometer and odometer are way off. I have gone from a 225/75 stock tire up to 235/85, then up again to 255/85. Mods have gained me back lost power, however I do have trouble pulling on the longer hills, which are not the 6.5s strong suit, and the tire height has only made it worse. I also just mentioned elsewhere about my highway speed, my speedo stops at around 60-62, no matter how fast Iget going. I don't think I can go much over 80 now. Before the ATT I couldn't even step on the pedal at highway speed or it would fall flat on its face, bucking and stumbling. No power at all. But 98% of my driving is 35-50 mph, and it is very happy there. And pulls extremely hard 0-60. I feel like this vssb issue is my problem. I replaced the speed sensors when I was fixing the tranny because I thought they may be bad and screwing up my OD, but they seemed fine.
I'm afraid to attempt this because it sounds way too complicated. I just don't want to do the math, it makes my head pound.
 
Detroit Dan said:
I never thought I was stupid until I read this post and couldn't grasp any of the math. My speedometer and odometer are way off. I have gone from a 225/75 stock tire up to 235/85, then up again to 255/85. Mods have gained me back lost power, however I do have trouble pulling on the longer hills, which are not the 6.5s strong suit, and the tire height has only made it worse. I also just mentioned elsewhere about my highway speed, my speedo stops at around 60-62, no matter how fast Iget going. I don't think I can go much over 80 now. Before the ATT I couldn't even step on the pedal at highway speed or it would fall flat on its face, bucking and stumbling. No power at all. But 98% of my driving is 35-50 mph, and it is very happy there. And pulls extremely hard 0-60. I feel like this vssb issue is my problem. I replaced the speed sensors when I was fixing the tranny because I thought they may be bad and screwing up my OD, but they seemed fine.
I'm afraid to attempt this because it sounds way too complicated. I just don't want to do the math, it makes my head pound.
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You will be PLEASANTLY surprised how your truck runs when its shifting when it should. Do it. Start your own thread, and we'll help. Then start by finding the circumference of your tire with a piece of chalk make a mark on the tire and the ground, then roll the truck forward until that line on the tire hits the ground again. Make another mark on the ground there. Measure the distance between the 2 chalk lines on the ground.
 
I dont think I have any problems shifting now. Only trans related problem I have noticed is if I'm at highway speed and boot it, it will downshift and go no faster. Same thing happens towing heavy up a hill. Theres nothing there for me if I downshift or try to hold a higher gear. I think a low revving diesel with an auto trans is just that way. With a gasser you generally can downshift it and still have 3000 rpms to go.
 
Detroit Dan said:
I dont think I have any problems shifting now. Only trans related problem I have noticed is if I'm at highway speed and boot it, it will downshift and go no faster. Same thing happens towing heavy up a hill. Theres nothing there for me if I downshift or try to hold a higher gear. I think a low revving diesel with an auto trans is just that way. With a gasser you generally can downshift it and still have 3000 rpms to go.
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When I changed my VSS I noticed a VAST improvement in shifting, it felt like it shifted 1-2 almost instantaneous, and being in the right spot at the right speed is HUGELY important, especially in a low revving, not super powerful diesel. Don't cheat yourself out of making this right.

If you downshift and nothing is there, I suspect you have other issues, lack of fuel it seems? You should get a fuel pressure gauge... You're on the right track anyhow with the KOJO... I'll be going that route within months.
 
Dan, do you have a tuned PCM now? If not it will be a dog of course.

The taller the tire you obviously are losing power to the ground for any given RPM, because you are effectively raising the gear ratio and putting more lever force working against the engine.

Having the speedo off is going to confuse the PCM, because it will believe you are under a lot more load, higher RPMs at lower speeds. And shifting and TCC is all dependent on MPH. So not only regular shifts, but kickdown and TCC are going to be screwy. tranny also determines drive ratio which could throw it off.
 
I really think the conflicting info from the vssb is causing your ecm to defuel, you should have more power and top end than that, those tires are not that much of a gearing change.
 
That tire change effectively brings him from 4.10 to a 3.63 gearing, assuming both are 16" wheels. If you went up in wheel size its worse.

So lost all advantage of having a 4.10, and when speedo says 80, proabably almost 90mph. stock PCM cuts out somewhere around there, 97mph maybe. Stock PCM also cuts fuel as RPM rises, and peaks at 1800rpm, so even through foot is in the floor will be giving you less and less fuel as RPMs go up.
 
now thats starting to make sense. Like I said, its really strong up to highway cruising speed, there's just no more. At all. I used to have problems with overboost and limp mode on the highway, I could barely drive it on the highway without tripping the SES light then it would be on for a day, and presumably in limp mode. Switching to the turbomaster and keeping it turned down within reason helped that. Now with the ATT it's not a problem at all. But, I thought limp mode or defueling would result in a light, and I thought limp mode lasted for a period of time, reset by a certain number of start run cycles or something like that. If I am defueling at speed (and I think I am) it goes right back to normal when I get off the highway.
Honestly, it hasn't been much of a problem for me, because I don't even really need or want to go wide open on the highway. But if it is sneakily affecting me elsewhere, like pulling heavy, then it will be good to have it corrected. I'm getting the vssb recalibrated, and I just pulled the trigger on the Kojo tune yesterday. Just hope my truck has enough usable life left in it to enjoy. She's getting old.
 
:iagree: with Buddy

Not only have you changed your gearing 12%, you also have a much heavier tire to rotate. This is much worse than having a higher gear and a smaller tire.
 
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