I just got a 98 4X4 Supercab LB with the 5.4L.

It has the stock tire size on it 255/70R16 which is only a 30" tire.

On this truck, the tires look almost ridiculously undersized for some reason. Must be because the truck is so frickin long.

Since the tires are getting worn, I was thinking of going with 265/75R16 which is closer to 32".

My speedo will be off by about 5% and I know that a lot of people here go with a larger tire than this.

So does everyone here with bigger tires get their speedos recalibrated or do they just not worry about it? Any other potential problems to going with tires that are 5% larger?

I thought about going with 285/75R16 (33") but then I probably should get wider wheels and I don't want to spend that much right now. The stock wheels are only 16X7. Also, the speedo would be off by 9% with the 33" tires.

Thanks

 

I did the exact tire change you are considering. I didn't get the VSS gear changed right away, and noticed right away that it didn't shift the same anymore. It took longer to shift, because the speedo was going slower, so the trans thought the truck was going slower.

You can change the VSS gear on your 98. It's a simple procedure. You should have a 20 tooth gear in there now. You will need to get a 19 tooth gear to get your speedo back in sync.

Once I did mine, the shifting went back to normal and it's been fine for over 5 years now.

Mitch

 

I went from 30's to 33's and have never had a problem. My speedometer is off 5 mph at 60 mph however.

 

I just got that recalibrator from Ford Motorsport, it's coming along with some 285/70/17 Goodyear Wrangler silent armors. I don't see going backwards in tire size so I figure I'd buy this gadget and set everything for good. Plus, if I ever wanted different gears or some other stuff, it compensates for all that. My dealership said they don't recalibrate so that's what I got.

 

Hey GrnX
Welcome
I have 265/70 16's mounted on 16x8 wheels.

Mike

 

Pardon my ignorance here. This is the 1st truck I've had with an auto tranny so I'm a little confused here.

The auto tranny shifts at a certain RPM, correct?

So what you are saying is that at the same RPM the truck is now going slightly (5%) faster than before. Therefore it takes longer to shift because your truck is actually going slightly faster than before the tire change when it shifts. Also, you lose acceleration because of the larger tires, magnifying the problem, right?

So for example if it shifted at 20, 40, and 60mph before. Now it would shift at 21, 42, and 63mph instead and since it takes longer for your truck to get to 21mph than 20mph, the time before the 1st gear shift is longer. Is this correct?

So why does this really matter other than the speedo being off? If it is only a 5% difference in speed it seems like it would be barely noticable where the tranny shift points are? Is this likely to harm anything?

 

I think the trans shifts off of MPH and not the RPM???

At least that's what I determined was the case with mine..... Before I changed the tires, the truck shifted at certain RPMs and MPHs. Seemed to be smooth and how the factory meant for it to shift.

After I got the taller tires, the truck did not shift until it reached the same MPH as before, but since the speedo was now slower then actual MPH, it would shift at higher RPMs.. Causing it to shift later and it drove me nuts! I also didn't like my speedo and my odo being off as much as it was. For every 5 mile posts I passed, my odo only registered 4.8 miles. Over the course of time, that's a significant amount.

Since I use my truck for work every once in a while, I get paid mileage, so having the odo correct in my case was necessary for that as well.

Besides, once I put the new VSS (Vehicle Speed Sensor) gear in, the truck shifted like it did before and I've been a happy camper ever since..

I don't think it will 'harm' anything to not get the VSS changed, but just know that if/when you sell your truck and have to sign off on the odometer reading part and you say it's the 'actual' mileage, it really won't be true... (it'll have a lot LESS miles on the odo then actual)

Mitch

 

Where can I get the VSS gear? How much does it cost? So you have to take apart the tranny to put it in? So it's a real beeaatch to put it in?

How long does it take to do the job?

 

Ford parts counter.
$10-$12.
No.
No.
Took me < 5 minutes.

On top of the tail shaft of the trans, look for where a wire goes into it. Clean any dirt from around the area and remove the single screw. Carefully pull out the end. The gear is on the end of it. Remove the old gear, put on the new gear, tighten up the screw, done deal!

Mine had a 20 tooth gear on it. I replaced it with a 19 tooth gear. I've got 3.55 gears as well, so if you have a different ratio, then the number of teeth will be different...

Mitch

 

Confused? Has the FORD changed the transmission shift parameters from MPH to RPM? I have a newer F-150 with an EDGE programmer and my shift points are determined by RPM not MPH, right? I also have larger diameter tires and wheels and the shifting seems the same to me... I was however able to recalibrate my speedometer with the EDGE programmer.

I was not aware that on early FORDs the transmission monitored MPH to determine shifting... I always thought is was based on engine vacuum pressure which is developed by engine RPM's, right or wrong?

Just asking...

 

Thanks Mitch.

I looked up under there and I see the VSS now. It's actually pretty easy to get to. I have the 3.55 with the LSD so I will get the 19 tooth gear as well.

 

I have 265/75/16 and it is off by about 3 mph at 60 no biggie and you rack up a few less miles but i never changed anything on my truck

 

Wandell,
I have the same truck, including the tires and wheels. What kind of mileage are you getting?
slarson