I'm planning to go from 255/70/16s to 285/65/16s on my Screw 2WD. I read somewhere that some F-150 models can have their speedo correction done via computer and not by changing an actual gear.

Is the SuperCrew blessed with this capability or would I have to look for proper gear to make sure my speedo is as close to accurate?


Thanks!

 

The place I have seen this able to be done is with the Hypertech chip ( not condoning this for the perf vs another, TIRES ONLY ! ), if memory recalls correctly.

The programmign steps allow things to be changed on the setup, and I do believe that tires were one of them.

This is kind of an expensive route to take to correct a speedo change, I woudl rather just take it on the highway with a stop watch, and use the mile markers to figure out where it is at, it might not be off all that much at 60 mph, then again I did not do the math on the cir of the 2 sets of tires.

Other chip mfgrs might offer the ability to change tire size as well, I am not the expert in what each mfgr does and does not do, I only recall seeing this as an option on this mfgr.

[EDIT]

I just saw the truck type, should have looked before......
Last time I also looked at this mfgr, it was not available for the year / engine combo you have. This might have changed, so take all this with a grain of salt.

 

I am about to do the freeway stopwatch thingy.
Just making sure that I don't do it the hard way if there is way that the dealer can reprogram the setting.

Thanks!

 

Or - if you don't want to carry that stopwatch with you on long trips and recalculate whenever you speed up , my dealer only charged me $22.00 to recalibrate the speedo for my tire swap.

 

I wouldn't say blessed. I wish it was as easy as just a little cheap plastic gear to switch out in the tranny. It was a breeze in my mustang.

 

chesterc ...

Your 2001 will have to be reprogrammed at the dealer. Alot of guys have been charged over $100 for this service.

You would much rather swap the $9 speedo gear as in the 97-98's. Dang computers!!!!

 

I could use that $100 towards a purchase of a chip!

What do you call those gears and where do I get them? At the dealer?

I ended up getting only 265/70/16 since they had to order the 285s that I wanted.

Now... will the spedo be off with this small change?

 

you dont need the plastic gears....your truck is to new right?

 

I got the truck two years ago, brand new and loaded. But sure doesn't look like 2 years old

My stock General Grabber AW 255/70/16s are really low thread and I didn't want to over drive them and risk some problems.

So I should be fine with the speedo then.

Thanks 02XLT!

 

Hey guys,
I guess I'm one of the lucky ones who can make the correction with a $9 gear at the tranny in my '98 SC Lariat 5-speed, but I can't get anybody to tell me where I can find a 23-tooth gear to correct for a 3.55 to 4.10 change, going from a 20-tooth. Ford tells me there's no such thing for any F150 (21-tooth is their biggest, I think), and I'm told the gear for a Mustang won't fit. Anybody found one of these? I've posted quite a few times and had no luck so far. Believe it or not, I'm holding out on installing these gears until I can find the right speedo gear. Ford tells me that if a 23-toother can't be found, I'll have to crack open the tranny and change both the 'drive' and 'driven' speedo gears to compensate. I appreciate any assistance, and good luck to all others attempting the same project. Thanks.
P.S. I've been told that on '99's and up, unless you change the tire diameter, you don't have to make any speedo corrections, because the 'sensor' on those trucks is reading axle speed, not driveshaft speed. A gearing change would have no effect on axle speed at any given vehicle speed. Is this true? If it is, you guys are the 'blessed' ones in my book.

 

With just going to 265's I wouldn't worry about changing anything. I went from 255's to 33's (which is the same as 285's) and mine is only off 4 mph at 70mph. Not enough to worry about myself. Granted I am gonna get a programmer whenever Superchip comes out with one for the 5.4L, but I am getting it for performance and it just happens to fix my speedo. My Ford dealership here told me they could reprogram the computer up to 2 tire sizes bigger, which means you could go to 275's and they could do it. Just call and find out. Most only charge about $30.

Brad

 

chesterc

You are only going to off 1.8% at all speeds. Your factory tires were 30.06" tall and your new tires are 30.6" tall.

So if your speedo shows 100 mph, you actually going 101.8...

To find out the difference...

Factory 255/70R16

255mm wide/25.4mm per inch = 10.04" wide x 70% sidewall ratio = 7.028 x 2 = 14.056 + 16" rim = 30.06 tall

New tires 265/70R16

265mm wide/25.4mm per inch = 10.43" wide x 70% sidewall ratio = 7.3 x 2 = 14.6 = 16" rim = 30.6 tall

30.06/30.6 = 98.2% or 1.8% larger (taller) tire.

At 65 mph, you should be going 66.17, so basically, I wouldn't even worry about it.

Creole

 

Got the same with c=2*pi*r
I started to figure this in notepad between work things, and by time I got back here you had it done.

All in all, you can check it on the highway to verify, but I woudl not waste the time on trying to figure out how to correct this.

Now if you get a chip.....then you can adjust it yourself, if you'd like.

Attached is my math, did I get an A ? <g>

Given
255/70R16 with a ht ( diameter ) of 30.06 tall / radius of 15.03
vs
265/70R16 with a ht ( diameter ) of 30.6 tall / radius of 15.3

The formula c=2*pi*r
255/70R16 cir = 2*3.14159*15.03= 94.4361954
265/70R16 cir =2*3.14159*15.3= 96.132654

98.23 or Delta of 1.77 % between larger and smaller tire

 

I was trying to figure out the math too.. but I guess I was asleep when my professor was discussing this subject

Sure is very helpful to actually KNOW mathematically if it has an impact or not. Not just.. I think or maybe.

When these Duelers wear out.. I hope to get the 285/70/16. At that point I have all the math to make the calculations.

I like my Dueler ATs, they're just louder than I expected compared to the Generals that I had, but overall I am happy with them.

 

Yes, chesterc, I've noticed in my short tour with F150online that there are usually a couple of fart smellers, oh, I mean smart fella's in each of the forums.

I'm not calling myself smart by any means, but I was recently trying to get some bigger mud tires than my 265/75/17's and I just happened to know the metric conversion and just happened to have a calculator on my desk...

Anyway, I wouln't worry too much about your speedo difference. I checked my math with real world by using my GPS. They are very accurate for speed reference and the math worked out.

BTW, you know how to make your Duelers wear out quicker, huh?

Creole