speedo calibration?

[turboranger91]

i did a quick search, but it's a bit of a pain browsing on my phone. figured i'd just make a thread. recently lifted with 33's. no idea how fast i'm going. how do i fix this?

 

[Tbars4]

...Just click the "How To" found in my signature and you will find a link to the info you seek...

 

[Maniak]

The short answer is you change the driven gear on the end of the speedometer cable.. but IIRC, the link in Tbars sig has info on how to figure out which size gear you need.

 

[Tbars4]

...You are correct...:biggthump

...It has pictures and includes all the info for this fix..

 

[turboranger91]

right on, guys. thanks. i just wasn't sure if it was a speedo gear or some wacky computer crap. lol.

 

[Flip4ford]

Takes less then 5 minutes to change the gear--- super duper easy!!

 

[turboranger91]

Takes less then 5 minutes to change the gear--- super duper easy!!

changing the gear is no biggie... diguring out which one i need.... that's another story. lol.

 

[Maniak]

I always figure out which one I need by doing math...

for example.. If only the tire size changed... say from 235x75-r15 to 33x12.50-15 you would do this...

235*75-r15 is actually 29" tall (give or take a little).. A 33" tire is usually 32.5" tall (each manufacturer is a different)...

29 / 32.5 = .89 which means there is an 11% difference in size..

Now look at your speedo driven gear (the one on the end of the cable).. lets say its a 17tooth gear.. You need to slow the speedo down so you need a speedo gear that is 11% bigger.. which is about 18.87 teeth.. so a 19 tooth gear..

I don't know if that helps, but thats the way i've figured out which gear I've needed over the years.

~Mark

 

[scrapper]

Hi, I am having a problem with what do you do if the list don't go up high enough. I figured out that I need a 24 or 25 going from 3.08 to 4.10 gears. I have a 19 tooth gear and the list went up to 21. Will I need to take the tail shaft off and change the other gear or do they go higher than the list? I want to put 15 in. tires on but it will be awhile. I don't plan on much more than 245's I still have a couple years on the 14" on there now. Got any ideas? I don't know anything about it. If I have to change the inside gear is it easy? Thanks

 

[Tbars4]

...Maniak can probably answer the mathematics to that...

 

[Maniak]

I'm confused...

Your going from 3.08 to 4.10... with that big of a jump w/out changing tires too means you would need to go from your current 19 tooth gear to a 24 tooth gear which I don't think they make.

If you "know" your going to get bigger tires very soon, then I would wait and just remember your speedo is 25% off until you put on the different tires. You mentioned 15" tires but I think you meant 15" rims.. I need to know current tire size and the new tire size to tell you what size gear to put in there.

If you want the 4.10's but will keep the current tires for a while you can have a speedo shop make an adapter for you. the adapter goes between your drive gear and the driven gear. basically, its a short speedo cable with a set of gears in an adapter to change the ratio for you. Its expsensive and they don't last that long (we got 50k miles out of ours before needing it rebuilt).

 

[scrapper]

I ask awhile back and got some different answers. It all seemed to matter on the gear that was installed but someone thought that the inside gear may have to be changed. I finally checked 19 teeth and I guess it was pink , I thought it was white. Color blind but sure it said 19.
so Mr. Maniak any thoughts?

 

[scrapper]

When I am going 85 I am really going 65. I can wait I have 2 15" rims from an explorer the steel spare tires. I was waiting to pick up a couple more before I switch. I just got these tires about 6 months ago and will have to get my moneys worth out of them. I did not know if they made more gears sizes that is way I ask. We posted at the same time earlier. I haven't desided what size tires I will end up with. I guess it depends on the economy. Thanks for the answer.

 

[Maniak]

AFIAK, there is only one size "drive gear".. the driven gear is the one you can change..

to figure out what you need and or how far you will be off we need to know two things..

How many teeth on the driven gear
What was your gear ratio
what size tires did you have..

With the above information AND knowing your speedo "WAS" correct we can figure out what size driven gear you need with the new gear ratio and tire size..

~Mark

 

[scrapper]

I think I did it right but I can be wrong alot. 3.08 changed to 4.10 tires are 225 14" wranglers. speedo worked fine. I did mean at 80 mph my actual speed is 65 mph. I got a 23 from tbars formula and used a formula from the ranger station and got 25 tooth gear. It has a 19 tooth pink gear now.I guess I may put 15" rims on but it will be awhile, maybe next year. I want 31's but don't know, will see how the truck is doing next year. I don't remember the old tire size but I think they were Larado's or something like that. I can look on the door a little bit later.

 

[Tbars4]

...Just to clarify something...It will not matter if you have 14" or 20" wheels for the calibration...It only matters on the tire size and gears ratio...

 

[Maniak]

Without "knowing" exactly what size tires were on it before you can't do the math to figure out what size gear you need. You need all 3 numbers to figure how "tall" the tire really was. i.e. 225x75r14

If you don't have good starting numbers your only choice short of going to a speedo shop is to know what you currently have for the tire size and know what gear ratio you currently have. With those two pieces of information you can use some calculators to figure out what speed you will be running at what RPM (say 2000 rpms in OD, cruising down the freeway).. You can then know how far off your speed is and use math to figure what size driven gear you need.

I use the calculator here.. http://www.4lo.com/calc/gearratio.htm You fill in everything except the speed. I don't know if you have a manual or auto trans.. that will determine the transmission ratio.. The transfer case ratio will be 1:1, the RPMs will be some rpm you can run at (cruise) in 4th gear (od), the diff ratio will be 4.10 (your current ratio) and tire diamter will be the "height" of your current tire (actual height, not 31 for a 31" tall tire)..

~Mark

~Mark

 

[ShadowRaven]

So if I'm understanding this correctly, I totally F*%&ed up and got the wrong speedo gear if I got a black 20 tooth? I have a 2wd 02 Sport with 33's and 3.73, and I read on explorer4x4.com that I needed a black 20 tooth, but evidently I need a tan 20 tooth? Sigh... I guess I'll have to take the part back -_- One more thing though, I'm not too tailshaft savvy, so I honestly have no clue where I'm supposed to be looking to find where the speedo gear is. Thanks in advance guys.

 

[Tbars4]

...You are not as lucky...You have to have yours re-programmed I believe..:scratch:

...This would be a good place to read up and ask your question...
http://www.explorerforum.com/forums/showthread.php?t=154465

 

[ShadowRaven]

...You are not as lucky...You have to have yours re-programmed I believe..:scratch:

...This would be a good place to read up and ask your question...
http://www.explorerforum.com/forums/showthread.php?t=154465

Man, with technology comes more heart ache, I tell ya. Well, I suppose I'll have to run by Ford sometime next week and find out what I need to do. It's not necessarily that important, cus I have the actual vs read speeds figured out and memorized, but I'd like to get it done for piece of mind. Oh well, that's what I get for having a younger than 2000 model :/