Thanks for the reply's folks. I was checking on how it would have to be done before changing tire size. Due to circumstances beyond my control I ended up with a regular cab, short bed 4x4 F-150 in a mild state of disrepair.

Stock tire size listed by the factory door seal is 235/70R16. I would like to run a 31" tall tire, how hard are the speedo gears to get a hold of and which one would I need to change it? Thanks.

 

They gear is in the rear tail housing of the transmission, driver side. Crawl under truck and you will see where the cable goes into tranny. Loosen the 10 mm bolt and pull out. The gear will be on the end. Count the teeth and let me know how many teeth. My local ford dealer carries them in Stock. Maybe call your local dealer and see what they carry in Stock.

 

Might be a few days before I can/will get to that, ground is wet and rain for the next few days. Thanks for the reply and help. When I can I will do it and let you know.

 

Probably something better to do is just put your new tires on see what percentage your speedo is off. That is what I did. I went from stock tires to 35's and changed rear gear from 3.55 to 4.56. I was off about 18% fast.

 

FYI the 97-98 speedo are electrical but use a analog signal. There is no cable.

 

Yeah. Wasn't really sure how it worked. I know my 97 doesn't have a cable that screws into the back of the speedometer but looks like a cable that goes into transmission tail shaft. Thanks carcrazygts2 for the clarification.

 

I didn't forget just took longer than I expected. The speedo gear is black colored with 20 teeth on it by my count.(trans is 4speed auto) The factory door specks list the rear as a H9 thats a limited slip 3.55 and the listed factory tire size is 235/70/16. I physicaly check and its a 3.55 rear on the truck.(opened the diff cover and read the actual numbers on the gears) The tires on it now are wore out BF Goodrich A/T 265/70/17. Acording to my friends GPS phone app. at 60Mph by the speedomiter on the truck the GPS said 63mph. I plan on going back with the 265/70/17 tires in the future, my question is what gear do I need to make it right with those tires?

 

A low cost way of dealing with this problem; especially with an old truck like my 1998 that is not work throwing a lot of money at if it does not change the appearance and driveability is the following.

Go out on the freeway with a lot of mile markers at a time when there is not a lot of traffic.


Have either a stop watch or a watch with a second hand.

Set your truck at 60 miles an hour (1 mile a minute) and clock the the time it takes you to go one mile between the mile markers. Do this for a number of miles so you know you are getting a good time.

After you think you have gotten a good time, divide 60 seconds by the seconds it took you to go one mile.

On my truck, this was 54 seconds or put another way, I was traveling about 11 percent faster than my speedometer is showing. So at 60 miles per hours I am really going about 67. At 35 I am going about 40.

This is not the most accurate method of determining speed but close enough that you won't be tieing up traffic or blowing the doors off the cars you pass. My truck is only worth about $1500 so for me it is not worth getting my speedometer fixed to give 100% accurate readings.

 

Thanks for the reply Frank Ford. I have priced the speedo gears and for the between 10 to 20 dollars including shipping I don't mind spending it to get it pretty close to right on my truck. It just takes like 5 minutes to take the wire plug off and undo the 10mm nut and you can pull it out and change the gear in like a minute. Just making sure wich gear to get, with all the smart folks here that are willing to share information like yourself. Between it being wet for like a 6 month span here and time to get to the truck hasn't been easy.