My speedometer is reading 10mph below what it should be. Why would this happen? I've heard of getting them adjusted but it's not something that is necessary to get tuned and pay for. Any ideas?

Dan

 

Did u change tire sizes ?? what year is it ???

 

it's a 92 f150. It's not miles per gallon it's miles per hour. The speedometer reads, for example, 50mph when I'm really doing 60!!! It's very nice if I want to get a ticket for doing 10 over. I didn't know what I could do to remedy it.

 

We understand it's miles per hour and not gallions. He asked you about the tires because they will throw off you're speedo if you get a tire with a different diameter than stock. There is a tech article link on the right as to how to re-program the speedo to the tire size.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

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Justin - One Happy FTE Member

 

If you changed tire size you have to recallibrate the speedo. I belive the 92 is electronic not mechanical. If so there is an excellent article on this site on how to do it. I did mine and made a big difference !! If you drive an f-150 you know gas mileage is a moot point. If it's not any of those then you might have a bad speed sensor.

 

For more complete info, read this:
http://bbs.off-road.com/ubbthreads/showflat.php?Cat=&Board=offroadfordbroncolate&Numb er=898100&page=0&view=expanded&sb=5&o=&fpart=

 

my speedo was off by 10 or 15 when i put my 36's on, now it doesnt even work at all.

 

Orange peel, yours is mechanical and requires a new plastic speedo gear in the tranny. Easy to change but a lot of times they get worn easily and wear out and stop working.

Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

Check out my Gallery for a look-see.


Justin - One Happy FTE Member

 

Well, that answers both of my questions I had. The other was the odometer being off. So from what I gather, the computer determines the speed from the number of revolutions of the tire. Seems quite a bit to be off since I matched the tire sizes exactly when I had them replaced. Even if they are off a slight bit would that equate to 10 mph over what it reads?

 

I agree that this is an excellent article:

https://www.ford-trucks.com/articles/december2001/speedorecal.php

It helped me solve my problem as well. I had a bouncy speedo on a truck I was about to buy (odometer was off as well) and it turned out to be a faulty PSOM (fortunately it was replaced under an extended warranty by the seller). When I got it back from the dealer, everything was still off. It turns out the dealer never checked the tires (stock P235/75/R15) to tell the service center what to preset the constant at (9.68-9.72).

I hope it is a programming issue (easy) and not a faulty PSOM (expensive).

 

Sweet! Thanks. I'll try it out today.

 

The odometer reads from the speedo, so that will be fixed when you fix the speedo.


Primary rig is Green Thunder:
95' F-150 XLT 4x4, 302, 5 spd, MSD 6A, Flowmaster Exhaust, Sunroof, Clear corners w/ Diamond headlights, CD player with 2 10" subs and some 32" BFG Muds .

Check out my Gallery for a look-see.


Justin - One Happy FTE Member

 

Well, I just ran out and tried the test. I have P23575R15 tires. The code is 972 and is exactly what is in the computer. The processor has the 'o' in it for overseas. How do I change this? This might be my problem?

 

>Well, I just ran out and tried the test. I have P23575R15
>tires. The code is 972 and is exactly what is in the
>computer. The processor has the 'o' in it for overseas.
>How do I change this? This might be my problem?

Well, it seems as if (I could be wrong) your truck was set up to run in Canada for the metric (kph). Therefore, the conversion constant of 9.72 is too high and must be lower to make up for the kph to mph difference (0.621 miles per km).

P23575R15 tires are around 720 revs/mile which is the same as 447 revs/km. I am not sure if you could "trick" the speedo by putting in a dummy conversion constant to make up for the metric conversion. If so, I would see if it could accept a constant of 6.04, assuming Ford's table is linear. In other words, call the 447 revs/km actually 447 revs/mile, multiply by 0.0135 and get 6.04 as the constant. (Those would be some GIGANTIC tires).

I do not know if Ford has a different conversion constant table for metric tires, if so, that would be the proper number to put in.

The real way to know is to check what your odometer measures a kilometer as when compared to a highway mile marker.

Good luck. I have not done this before and it is pure speculation.