Stupid Tire Question for Those in the Know

[EasyRhino]

Vehicle: 97 XLT 4 dr 5.0L AWD, no lift, no offroading.

OEM tire/wheels: P235/75R15 on factory alum. wheels.

Factory Goodyear WIlderness A/Ts went 55K before GY replaced due to cord separation. Prorated at 50% worn.

Replacement Goodyears now at 26K and are out of round, driving me nuts. Probably 25% worn.

What about this:

Replace with LT tires in the largest diameter size allowed for the factory wheels/ride height/street use that don't produce excessive road noise.

If my math is right, P235/75R15s are 29" tall and 9.4" wide. Also, they do not fill up the tire wells.

Also, if may math is right, if I can use a 31" tire, it will effectively reduce the axle ratios, improving gas mileage and reducing drivetrain wear.

(29/31)(3.73) = 3.49, or a factor of approx. 6.5%

The question is: What is the biggest tire I can use, and how do you correct the speedo for the error?

I know this is review to you guys that have experience with this stuff, thanks for bearing with me.

 

[Stephen S]

Couple of things. The Wilderness ATs were made by Firestone, not Goodyear. They were replaced new Goodyears that you are not happy with or with new Firestones?

The biggest tire you can fit on a new model with no lift or rubbing is a 31x10.50. The larger tire will effectively reduce the final drive ratio, but it does not reduce drivetrain wear. In fact, it accelerates it. The larger tires are harder to turn, and the lower final drive ratio makes the truck harder to move for the engine (like it's starting in a higher transmission gear). The lower drive ratio causes the truck to use more gas when accelerating, due to the increased strain. But once you get up to speed, your gas mileage would likely increase due to the engine running at a lower RPM to keep up the same speed than it did with stock tires. The effects are likely to balance out if you drive conservatively, and your overall gas mileage will change very little.

To correct the speedo, (not %100 sure) but I think all you have to do is go to Ford and let them reprogram your computer to account for the larger tires. The older trucks have a speedo gear that needs to be changed to account for the tire change.

 

[EasyRhino]

Originally posted by Stephen S
Couple of things. The Wilderness ATs were made by Firestone, not Goodyear. They were replaced new Goodyears that you are not happy with or with new Firestones?

You got me! The original tires that were failing were Goodyear Wranglers, as are the replacement tires.

 

[addkev]

The biggest tire you can fit on a new model with no lift or rubbing is a 31x10.50

You can fit a 31, but most people have minor rubbing. It seems that every X is a little different. 30x9.5 will guarantee that you have no rubbing. I have 31's and I had to trim about 2 " off of my front bumper just to turn my wheels.

The larger tire will effectively reduce the final drive ratio, but it does not reduce drivetrain wear. In fact, it accelerates it. The larger tires are harder to turn, and the lower final drive ratio makes the truck harder to move for the engine

But, in your equation you used 3.73, so I assume you have 3.73's. X came from the factory with anthing from 3.23, 3.55, 3.73, and 4.10. By going up to 31's you are still looking at a 3.58 gear ratio. You might barely notice it, but add a KKM and that will make up for it.

To correct the speedo, (not %100 sure) but I think all you have to do is go to Ford and let them reprogram your computer to account for the larger tires. The older trucks have a speedo gear that needs to be changed to account for the tire change.

That is correct, Ford can reprogram your computer to recognize the new ratio. It runs about $75. I was going to do mine, but then I realize how little it affected it. CBoug76, is a moderator, in his signature he has a link to a calculator where you can plug in the info and it will give you the correct speedo.

 

[addkev]

Actually, since you have a 97, you can just by a new gear. $20 fix. 98+ have to reprogram the computer.

 

[Robert]

The new speedometer gear is $7.62 at Ford and takes 5 minutes to replace. I didn't go through the calculations, but if the 6% is accurate then get a new speedometer gear with one less tooth than your current one. It will make up about 5% of the 6% difference.

 

[hornz2000]

do you guys think I need to have my 98 computer reprogrammed then. I went from 235/75/15 to 285/60/17 which comes out to approximately 30.9"

 

[Robert]

Only you can decide if the difference is great enough for you to want to spend $75-$100 to have your speedometer reprogrammed. Your 285 tires actually work out to 30.5". Your 235 tires work out to 28.9". The difference is 5.5%. Multiply your speedometer by 1.055 and that is how fast you are actually going. If it were mine, it would be worth it.

 

[hornz2000]

aight thanks for the input man.

 

[01EB5.0]

If your havin so much trouble with goodyears, get a 31X10.50 set of Michelin LTX M/S or Cross Terrain tires. They are expensive, but will last for 60,000 or more. Also I run 31's on my truck and have not had any rubbing except in the upper part ofthe wheel wells in front at full compression offroad. Never any rubbing on road.
Good luck!

 

[EasyRhino]

Originally posted by 95XLT
If your havin so much trouble with goodyears, get a 31X10.50 set of Michelin LTX M/S or Cross Terrain tires. They are expensive, but will last for 60,000 or more. Also I run 31's on my truck and have not had any rubbing except in the upper part ofthe wheel wells in front at full compression offroad. Never any rubbing on road.
Good luck!

That's what I'm thinking. Of all the tires I've had on all the cars I've had (I know it sounds like a bad Willy Nelson song ), the tires I have never had a problem with are Michelins. But, I'm the kind of guy who can get 80-90K miles out of a set of Goodyears, so the cost is not the biggest deal. I've actually had Michelins go over 110K miles (not on the X).

I also like this size recommendation. with the 235s going just under 29", I have over 2" clearance from the tire to the closest obstruction, so I'm thinking 31" will do okay, as they only add 1" radius.

Okay if my math is right on this:

(29/31)(3.73) = 3.49, or a 6.5% difference. (yes, mine has 3.73s), so 65 mph indicated, would be 6.08 actual. In this case, I will change out the sppedo gear.

Does anybody here know if the spedo gear change-out is like doing the same job on a Mustang??

Again, thanks for all the help, guys.

 

[Robert]

I've never replaced the gear on a Mustang, but I have on my '97 Explorer. All it involves is removing one bolt through a clip that holds in the vehicle speed sensor (VSS). The VSS will pull out of the transfer case (transmission tailshaft if 2WD). At the end of the VSS is the speedometer gear. It slides off the end. Slide the new one back on and put everything back together.

 

[01EB5.0]

Yeah the speedo gear replacement is easy. i have the part number for th eone that fits a 4X4 if you want it. I swapped from 29's to 31's so if you need it let me know.

 

[EasyRhino]

Originally posted by 95XLT
Yeah the speedo gear replacement is easy. i have the part number for th eone that fits a 4X4 if you want it. I swapped from 29's to 31's so if you need it let me know.

From Robert's post above, I believe this is the same job as on my Cobra, when I swapped the rear gears, cool.

Yes, please shoot me the part number, if it is convenient.

And thanks to all you dudes in the know!!

 

[01EB5.0]

ford number: C0DD-17271-B
Good luck! need any other help....just post...

 

[Robert]

Remember that the part number posted above will only apply if you have the same year Explorer, same transmission, same engine, same axle ratio and the same size tires. All of these items dictate which speedometer gear Ford installs.

The only way to know for sure which gear you need is by pulling your current one and taking it down to Ford and have them match a new one up to it that has one or two less teeth. The number of teeth to go with is dependant on how much larger or smaller your new tire size is in relation to your original size.

 

[01EB5.0]

The year isn't as much as a factor as the gearing...if you pull out a tan or pinkish gear, then that part number is the correct one. also just to let you know, my ford dealer said this gear was in their book as being for a manual transmission ranger, so the gears are all confused anyway. If you have a 3.73 rear end and a 4WD and you pull out a pinkish gear that says 19 RH on it, then thats the correct number.

 

[01EB5.0]

Hmmmm...just noticed you got a V-8.....well, it may be different in your case...sorry I didn't think about that before. I would just pull out your current gear and see what color it is because ford does them by color. also it'll see the number of teeth near the base of the gear. just get one tooth less then what you have now and it should be pretty darn close.