Ok, when I bought my truck, it had 265/75R16s on it. So I assumed (I know...) that they corrected the speedo by switching out the VSS gear, although I knew it was a possibility they had not. But for the most part, when I am going 70, Im pretty much keeping up with everybody, so I never had any issues with it.

The other night I was following a friend and I was going about 65 flat, and I know they dont usually drive slow.

So, my question: can someone tell me what RPMs I would be running at when traveling 70 MPH. Here are the specs on my truck:
3.55 gears
255/70R16 stock size tire, I now have 265/75R16s
5.4L engine (if that matters)
4R100 tranny.

As it is, I currently run right at 2000 RPM when I am going 70 MPH.
Is my speedo off?

 

I have no idea how to calculate using RPMs, not even sure its possible but the difference in tires is 5.294% too slow meaning if you're speedo says 70MPH you're actually only going 66.5ish.

 

i had my gps's with me the other day and both said that my speed was about 3-4 mph faster than that of the truck running the same set up as u
and at 70 mph i run about 2000 rpms

 

70 mph with your 255/70R16's, = 66.47mph with the 265/75R16

 

I can go online and plug those numbers in to get the differences, but that doesnt tell me if my speedo is adjusted properly or not.

I guess one of these days Ill just have to have a buddy get on the highway with me and drive 70 and Ill follow along.

 

I used the mile markers on the highway and the sweep second hand on my watch to calibrate my speedo for a new VSS gear.
At 60 mph you will travel one mile per minute.
Any variation from that can easily be converted into a % and applied to # of teeth on the vss driven gear.
You best pull the driven gear you have out and count before you start if you think it may have beenn changed. LOL

 

Well I've heard that counting the number of teeth to determine if its been changed is not accurate, since they dont always put the same gear in similarly configured trucks. Thats what Ive heard anyway.

 

I just checked my speedo with my GPS, speedo says 72, gps says 70..Stock 255/70R17's, My motorcycle is 5mph's off.. Sorry no point to this, just making an observation..

 

Ok???

 

speedos are poorly calibrated from the manufacture in my opinion, I think Ford has the speedo reading faster than you are travliing in an attempt to slow you down, it's a mass consericy lol



p.s. Where the hell is the spell check on this thing?

 

Have you cosidered taking it in for a speedo calibration? I had it done to get out of a speeding ticket once. Cost me less than $35.00. Just a thought....

 

Johngs,
I too have been doing some reseach on this issue. I plan to get some larger wheels and tires for my 05 Screw and have a hard time beliving that some people are going out and buying $300 programmer to change the speedo reading. From previous experiance I have a supercharged V-8 t-bird that I've changed the rear end gear in and had to install a new speedo gear to reflect the correct MPH and it only cost me $20.

From the info I have read both our trucks have the 4r100 trans which should intern use the same speedo gear. Basically you will need to find out the # of drive teeth from the output shaft (unfortuneatly I don't know), your tire diameter, and your rear end gear and that should give you the # of teeth that you need for the speedo gear.

Found this site that may explain things a little better = http://www.tciauto.com/tech_info/speedo_gears.htm

Note: After I entered my data into the calculator that is supplied by the above website, with a 9 tooth output drive (just a guess) it comes up with a 21.386 speedo gear.

Now again that's with entering the stock sized tires I have (255/65/17 - 30.1") 3.55 rear end.

Hope that helps and if you find out anything else let me know.

 

I'm still trying to figure out what to use in my tuner to get the speedo right for my supposedly "stock" size Yokohamas.

Still haven't got it right...still reflecting "faster" than I'm actually going. Guess I'll need to drive around until I find one of those laser speed warning trailers and keep tweaking it until it's right.

 

Quick, your truck wont have the VSS gear like mine does, they switched to the digital in 99 I believe. You will have to get it re-programmed to correct the speedo.

My mine problem is I just cant find the dang gear

 

Well crap, there goes my theroy.