larger tires cause trans. problems?

[sc98f150]

i just upgraded my stock tires to 285/75/16 on my 1998 ford f150 2wd standard cab. since i upgraded tires ive noticed my transmission wanting to shift out of over drive from time to time. any thoughts or information for me ??

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[MitchF150]

With the larger tires, you have changed your 'effective' gear ratio.... Meaning, if you have 3.55 gears, your trans thinks you now have around a 3.11 gear ratio...

So in OD, your engines rpms are a bit lower, taking it out of it's set range and that tells the trans to downshift.

If you put some 4.10 gears in your truck, you would be just about back to the way it was stock...

Mitch

[sc98f150]

what would be the cheapest way to correct this problem? any work i get done will have to be from a mechanic. i had a gmc truck prior and i didn't have these problems when i upgraded the tires on it ??? is this something that affects fords only ??

[MitchF150]

Maybe you have 3.08 gears to start with?? The bigger tires would make that gear ratio feel even worse!

Check your door jamb sticker and post what code is under 'AXLE'. Then we will know what you are really dealing with.

Mitch

[bricelittle]

Originally Posted by MitchF150

With the larger tires, you have changed your 'effective' gear ratio.... Meaning, if you have 3.55 gears, your trans thinks you now have around a 3.11 gear ratio...

So in OD, your engines rpms are a bit lower, taking it out of it's set range and that tells the trans to downshift.

If you put some 4.10 gears in your truck, you would be just about back to the way it was stock...

Mitch

i also put larger tires 30x9.5x15 on my 1993 f150 2wd 5 speed man w/a 4.9L I6
it has the masda trans w/a 2.73 rear end i want to upgrade to a 3.55-3.73 rear end any thoughts?? and how much would it cost??
my trany specs

Mazda:
1st 3.90:1
2nd 2.25:1
3rd 1.46:1
4th 1.00:1
5th 0.80:1

5th is useless with this setup but i get 20 mpg
iam greatful for any info

[97XL]

Originally Posted by MitchF150

Maybe you have 3.08 gears to start with?? The bigger tires would make that gear ratio feel even worse!

Check your door jamb sticker and post what code is under 'AXLE'. Then we will know what you are really dealing with.

Mitch

I've got 3.08 gears in mine, at least thats what the sticker says.
I went from 235/70r16s to 255/70r16s, would this make my ratios significantly off as well? I am not having the OD issue at all just wanted some info.

[glc]

To the OP: You don't HAVE a problem - the tranny is doing exactly what it's supposed to do - so if it ain't broke, don't fix it.

[1982_F150]

Originally Posted by sc98f150

i just upgraded my stock tires to 285/75/16 on my 1998 ford f150 2wd standard cab. since i upgraded tires ive noticed my transmission wanting to shift out of over drive from time to time. any thoughts or information for me ??

Everything your truck does as it goes down the road is based off the stock tire diameter and it's relation to the rear final drive ratio (gear ratio).

Your larger tires are sending the wrong signals to the computer: in your case you have to push harder on the gas to stay moving due to the larger diameter tires reducing your overall gear ratio. The engine has to work harder to overcome the added resistance / load, and your trans "thinks" you are passing or pulling a steep hill even though you are not.

All the signals your computer is getting point to increased load as the RPM's come down and the airflow / tps is telling the computer that you are in the gas more than usual. The response programmed into your computer for the conditions is to Disengage overdrive to bring the RPM's up and the load on the driveline down. So it downshifts early and at less throttle than it used to with the stock diameter tires.

This can also reek havok with your ABS brakes, speedometer, cruise control, fuel system, trans shift points, shift firmness and timing and so on.

If you are set on keeping the taller tires, you need to figure out what gear ratio you need to switch to in the rear end to get back to the same overall setup you had with the stock gears and the stock size tires. Not a cheap fix, but it's the nature of the beast. If you had 3.08 gears, you need to step up to at least 3.55 if not 3.73 to make up for the taller tires. If you already had 3.55 gears, 3.73 or 4.10 gears will be needed.

It's stupid how many systems on your truck / car can be affected with a simple tire size swap. Not like the old day where the carb could care less what was on the back

Welcome to the 21'st century!

[BrewScrew]

Can't systems like EDGE that allow you to adjust tire size help the computer to figure out that you have larger tires and the in turn 'compensate' for the tire size difference?

[glc]

Yes - that's one of its purposes. With larger tires and no gear ratio change, it is still going to be shifting out of OD more often because it NEEDS to.