I have a problem with my spedometer, it constantly runs 10-20 mph faster than what im actually going. so it messes up my mileage and it's just plain annoying. What can i do to fix the problem. (The truck is a 1972 f100 all stock except for the engine)

 

Kurt, Maybe it's not as stock as you think. I think your rear ratio has been changed as well. Follow your speedo cable to the tranny, and with a 3/8" wrench remove the holding nut. Pull the whole thing out and count the teeth on the gear. I suspect you might need one two teeth count different.

John

 

Sounds like someone changed the rear gear ratio. It might have had 3.25 or such and someone put 3.70 in it. That would slow the truck, but speed up the reading since you are turning MORE rpm to go the same distance.

You need to add a couple teeth to the cable gear. That will slow the speedo down. As mentioned, pull the cable from tranny(unbolt for stick, unscrew for auto) and count teeth(They are also colored coded:red/yellow/blue, etc).

You need to accurately figure out how far off. How someone pace you with new car, or something that is a known good reading. 10-20 mph is too wide of gap to figure ratio change needed.

truck is going 50 , but reads 60 = 17 percent off
truck is going 50 , but reads 70 = 29 percent off

if gear is 15 tooth...add 17% = 2.5 teeth, either 17-18 tooth will get you close
if gear is 15 tooth...add 29% = 4.3 teeth , a 19 will get you close.

There were also a couple different gears available inside the tranny. This will also affect ratio, but harder to change. If changing the small gear doesn't get you close enough, then tranny gear will need changed.

Hope this sheds some light.

 

make sure the tires are the right size, I had a mustang that had over sized "cheater" slicks that put my speedo off by about 7 mph

 

thanks guys,
i contacted the orriginal owner of the truck and he said that when he put in the new engine, he also replaced the tranny and rearend. Its from a mercury but he's not sure what kind. I figure that has caused the problem

 

It would be hard to have stock sized tires since the bias tires it came with would not match the height size of the current radial offerings, especially once the radial tries settled.

I used a GPS on a flat level highway to get my truck close to stock at 55 mph.

I think for an older truck, the best thing is to put in a F-600 dash and an electronic speedo/tach and sender.

 

I would bet it's the gear in the tranny if they changed them all. Depending on the rear ratio, they put a certain number teeth gear inside the tranny. Depending on that number is weather the small one can be changed enough.

Get back with some information and we'll try to get you set up closer.

 

My understanding is the speedometer cable has got to match the transmission, (i.e. there is a gear on the speedometer cable specific to the tranny that it fits into).
jz

 

I had this problem in my 70. I purchased the truck with 31" tires and later figured out the rear end was 4:10. The spedo was way out so when I put the right tires on the truck it was correct again. Then I changed the rear end to a 3:55 but thought ahead and grabed the spedo gear out of the transmission of the truck I got the rearend from. Someone also had told me you can get the spedo gears from a good Ford parts dealer you just need to know the rear end and transmission being used. Hope that helps.