Speedo correction question

[2Tight]

I'm in the market for new tires and have a question I can't find in a search. The PO installed 285-75/16 on the stock 16x7 wheels resulting in poor tire wear patterns, especially on the rears. I think my speedo is off but don't have a GPS to check against. I would like to go back to the 265's on the OE alloys and correct the speedo (rpm input) if necessary with a standard scan/diagnostic tool if possible. Is access to the rpm correction function a dealer only thing?

Thanks

[LOOSEWHEEL]

You can do it with Autoenginuity.

[SmokeyWren]

Originally Posted by 2Tight

Is access to the rpm correction function a dealer only thing?

No, anybody with a diag tool or scanner that can access and change parameters in the ABS (anti-lock brake system) module can do it. The 285s were probably 628 to 634 tire revs/mile. Most 265s are 650 to 655 revs/mile. You have to change the revs/mile parameter in the ABS module to calibrate the speedo. Rear axle ratio doesn't matter. Only tire revs/mile matter. Get the actual revs/mile from the tire specs section for your exact tire/tread on the tire maker's website.

But most cheap diag tools and scanners cannot access the ABS module. You need the gee-whiz Hickok NGS or Snap-On Tools or comparable scanner, and those cost around $2,000 or more, so usually only dealers or some big independent shops will have one. Or you can probably use AutoEnginuity software with the optional "enhanced Ford support" in a PC laptop to do it. If you don't count the cost of the laptop, the AutoEnginuity is a lot less than the Hickok NGS.
Hickok, Inc. - New Generation Star Mach II
AutoEnginuity® Scan Tool

Or one way to DIY is to buy yourself a calibrator. The most popular is TruSpeed™ Speed Sensor Recalibrator for about $200 street price.

With the truSpeed, you can play with it (change the revs/mile by only a few revs) until you get either the speedo or the odo/tripmenter to over 99 percent accuracy. But you have to choose whether you want the speedo or the tripmeter to be accurate, and allow the other one to fall where it may. In my case I want an accurate tripmeter, so it's set at 650 tire revs/mile for 99.7% accuracy with my tires. But my speedo is about one MPH slow at highway speeds.

If you don't plan to be changing tire sizes very often, your Ford dealer will probably charge you one hour labor rate to do it for you. My dealer has done mine three times over the last 10 years - to 295s, then to 285s, and finally back to stock-size LT235/85R16E.

[2Tight]

Thanks Guys. It looks like the dealer would be my best option as I won't be changing tire size after this purchase. Is there a tire rpm count PID my Insight could 'read only' to see what it's set at now or is ABS not part of OBD II?

[BTBBT]

If you are going back to the stock tire, most likely the PO didn't change the setting in the computer. I'd find someone with a gps and try it once you put the new tires back on. I'll guess you won't have to do anything.

[SmokeyWren]

The ABS module is not included in OBD-II. That's why most cheap scanners won't read it or allow you to play with the revs/mile parameter.

[mnpowerstroke99.5]

I have 285-75/16 tires on the stock 16x7 alumninum rims and they wear perfect.As far as the speedo being off with the bigger tires,my speedo reads 70mph when it actually is going 70.5mph on gps.I think the odometer could be off a little, but not too much.I never had it reprogrammed either.

[2Tight]

Originally Posted by mnpowerstroke99.5

I have 285-75/16 tires on the stock 16x7 alumninum rims and they wear perfect.As far as the speedo being off with the bigger tires,my speedo reads 70mph when it actually is going 70.5mph on gps.I think the odometer could be off a little, but not too much.I never had it reprogrammed either.

I wish I had your luck but the tires on mine came on it when I bought it. They are almost touching the wear bars across 3" of the centers on the rears but the outer edges are in great shape. I don't know if the PO ran them over inflated or what but I haven't been able to find the right pressure the stop the wear. I tow so having them too low isn't going to help me. Maybe it's just the tire (Dunlop Rover AT) plus I don't think they are that great a tire anyway. It's good to know the speedo will probably be fine. What tire are you running, I would like to stick with 285 as long as I can get them in an E rating?

[SmokeyWren]

Originally Posted by 2Tight

They are almost touching the wear bars across 3" of the centers on the rears but the outer edges are in great shape. I don't know if the PO ran them over inflated or what but I haven't been able to find the right pressure the stop the wear.

Sounds like the tires are inflated for towing but a big chunk of the miles are without trailer and its hitch weight.

You need to weigh the truck twice on a CAT scale that will give you seperate weights on the front and rear axles. One CAT scale ticket when loaded for bear with the trailer tied on and a full tank of fuel and driver and passenger(s) on board. Then another scale ticket when the truck is in its normal "unloaded" state with you and all your tools and junk in the truck but without the trailer. Then use the following load/inflation table and air the tires up for towing then let out the extra PSI when not towing.

The following load/inflation table is for each tire, so divide the axle weight by 2 to get the weight on each tire of SRW pickups:

Load/Inflation Table
LT285/75R16E
--------------------
PSI . Load (SRW)
--- . --------
35 . 2130
40 . 2340
45 . 2540
50 . 2755
55 . 2925
60 . 3110
65 . 3305
70
75
80 . 3750

You should NEVER need more than 65 PSI in your tires, else you would be dangerously overloaded. Until recently no one even offered LT285/75R16 in load range E, because no SRW pickup needed tires with more weight capacity than the 3,305 pounds per tire of load range D (65 PSI max). So I don't have a load/inflation table that includes 70 or 75 PSI. The 80 PSI weight capacity is on the sidewall of a load range E tire in that size. But customers like you demanded a load range E tire in size LT285/75R16, so if you really want a load range E, then the best is now available:
LTX A/T2: Light Truck Tires: Michelin Tires

Not cheap, but you're worth it.

Examples:

When you have only a driver and no trailer, your front axle weighs 4,400 and your rear axle weighs 3,200, for a GVW of 7,600 pounds. So that's 2200 on each front tire and 1,600 on each rear tire. So you need 40 PSI in the front tires and 35 PSI in the rear tires. More than that and you'll wear the center of the tread faster than the outside of the tread. Plus you'll have a harsher ride than necessary.

When loaded for bear with trailer and passenger(s) and junk for a long trip, with a full tank of fuel your front axle weighs 4,800 and your rear axle weighs 5,600 pounds for a GVW of 10,400 pounds. (Yes, that's overloaded for a 7.3L F-350 SRW, but it's common.) So that's 2,400 on each front tire and 2,800 on each rear tire. So 45 in the front tires and 55 in the rear tires. That's all you need, even though overloaded for an F-350 SRW and really overloaded for an F-250 7.3L. But if a lot of miles will be at over 70 MPH, then add another 10 PSI for a max of 55 front/65 rear.

So now you can see why the tires are worn in the middle. Those big tires have a lot more weight capacity than you need unless you severely overload your pickup, so they've probaly been overinflated all the time, loaded or not.

[2Tight]

Originally Posted by SmokeyWren

Sounds like the tires are inflated for towing but a big chunk of the miles are without trailer and its hitch weight.

You need to weigh the truck twice on a CAT scale that will give you seperate weights on the front and rear axles. One CAT scale ticket when loaded for bear with the trailer tied on and a full tank of fuel and driver and passenger(s) on board. Then another scale ticket when the truck is in its normal "unloaded" state with you and all your tools and junk in the truck but without the trailer. Then use the following load/inflation table and air the tires up for towing then let out the extra PSI when not towing.

The following load/inflation table is for each tire, so divide the axle weight by 2 to get the weight on each tire of SRW pickups:

Load/Inflation Table
LT285/75R16E
--------------------
PSI . Load (SRW)
--- . --------
35 . 2130
40 . 2340
45 . 2540
50 . 2755
55 . 2925
60 . 3110
65 . 3305
70
75
80 . 3750

You should NEVER need more than 65 PSI in your tires, else you would be dangerously overloaded. Until recently no one even offered LT285/75R16 in load range E, because no SRW pickup needed tires with more weight capacity than the 3,305 pounds per tire of load range D (65 PSI max). So I don't have a load/inflation table that includes 70 or 75 PSI. The 80 PSI weight capacity is on the sidewall of a load range E tire in that size. But customers like you demanded a load range E tire in size LT285/75R16, so if you really want a load range E, then the best is now available:
LTX A/T2: Light Truck Tires: Michelin Tires

Not cheap, but you're worth it.

Examples:

When you have only a driver and no trailer, your front axle weighs 4,400 and your rear axle weighs 3,200, for a GVW of 7,600 pounds. So that's 2200 on each front tire and 1,600 on each rear tire. So you need 40 PSI in the front tires and 35 PSI in the rear tires. More than that and you'll wear the center of the tread faster than the outside of the tread. Plus you'll have a harsher ride than necessary.

When loaded for bear with trailer and passenger(s) and junk for a long trip, with a full tank of fuel your front axle weighs 4,800 and your rear axle weighs 5,600 pounds for a GVW of 10,400 pounds. (Yes, that's overloaded for a 7.3L F-350 SRW, but it's common.) So that's 2,400 on each front tire and 2,800 on each rear tire. So 45 in the front tires and 55 in the rear tires. That's all you need, even though overloaded for an F-350 SRW and really overloaded for an F-250 7.3L. But if a lot of miles will be at over 70 MPH, then add another 10 PSI for a max of 55 front/65 rear.

So now you can see why the tires are worn in the middle. Those big tires have a lot more weight capacity than you need unless you severely overload your pickup, so they've probably been overinflated all the time, loaded or not.

Thanks for the great post I'll see if I can use find a truck scale in the Spring and get some weights as a guideline but I've winterized the trailer for now. I thought maybe the 7" rim width might be too narrow for the 285's and distorting the tire some since 7.5" is the recommended minimum. I've been running 55psi in the rears loaded or unloaded and am going to try the 40(F)/35(R) and see how it feels. The rear tires were already very badly worn when I bought it and I've only put 4K on them since. I recall 65psi was what I found on all 4 tires at that time. I think I'll stick to the D rating since there are more choices offered and my trailer isn't that heavy anyway.

Again thanks for the informative post; I learn something good every time I come here

[SmokeyWren]

Originally Posted by 2Tight

I thought maybe the 7" rim width might be too narrow for the 285's and distorting the tire some since 7.5" is the recommended minimum.

Posssible, but I doubt it. I wore out one set of LT285/75R16D on stock 7" wide wheels on the rear axle of my 4x2. Most of those miles were when towing and grossing around 16k GCW. The tires wore evenly across the tread, probably because I kept them inflated per the load/inflation table.

"Normal" tires, including those 285s, last only 25,000 miles on my rear axle. So I tried Michelin XPS and they last almost twice as long. So even though the XPS cost more, they are cost effective for me because they last so much longer than ordinary premiun-brand tires such as BFGoodrich AllTerrain. But the biggest size they come in is LT235/85R16E, so if you want 285s you'll have to look elsewhere.

If you don't like the "French-owned" Michelin brand, Goodyear makes a "real truck" tire equivalent to the XPS. But it's just as expensive, and also doesn't come in sizes bigger than LT235/85R16E.