The speedometer on my truck is off. WHen i'm going 45 on the speedo I'm really going 40. When the speedo reads i'm going 70 I'm really going 62 MPH. I checked it with my GPS. What can I do to fix this. WOuld having 235's on the front and 245's on the rear mess it up? The PO had diffrent sizes for some reason.
Messed up speedo
- Thread starter Kris K
- Start date
Mr_Roboto
Full Access Member
Stock on your truck is 215/85R16. Shorter tires will make the speedo run faster, taller tires will make the speedo run slower than actual road speed.
Your stock tires are 27.7" tall. 245/75R16's are 30.5" tall. For maximum fuel economy, go with 235/85R16's the next time you get tires.
You can slow down the speedometer by changing the driven gear at the trans end of the speedometer cable. Pull your existing gear, and get a new gear with one more tooth than your existing gear. This probably won't get you dead on, but it will get you closer.
Your stock tires are 27.7" tall. 245/75R16's are 30.5" tall. For maximum fuel economy, go with 235/85R16's the next time you get tires.
You can slow down the speedometer by changing the driven gear at the trans end of the speedometer cable. Pull your existing gear, and get a new gear with one more tooth than your existing gear. This probably won't get you dead on, but it will get you closer.
Mont91
Full Access Member
Need the complete tire size to be sure but yes the 245s on the rear would throw the speedo off. Are they 245 85 R16 or 245 75 R16 or what HOLD THE PHONE what size tires are on it now? are they the ones in your sig, the 265s? If they are that is where the cause is. The bigger tires revolve slower to cover the same distance as the 235s so the speedo is showing this slower revolving speed. You need to change the gear on the end of the speedo cable, simple take out the retaining bolt slide off the clip slide on the new gear repace the clip slide back in replace retaining bolt.
What he said. if I was a faster typer I would have beat him to the answer
What he said. if I was a faster typer I would have beat him to the answer
Mr_Roboto
Full Access Member
I assumed 245/75R16 because it is a very common size, Ch*vy trucks used to use that size, as well as Ford vans. I also see that size on various SUV's. 245/85R16 is not commonly available if at all.
Remember kids, there are 3 numbers in a tire size. We need all 3 of them.
Remember kids, there are 3 numbers in a tire size. We need all 3 of them.
Mr_Roboto
Full Access Member
If you are going to use the taller tires on the back (235/85 and 265/75 are approximately the same height) I would try 2 extra teeth on the speedo gear. You'll probably overshoot a little, but speedo gears are a "closest fit" not an "exact fit".
tonkadoctor
Full Access Member
As a general rule of thumb on the speedo cable gear it's 1 tooth for 5 MPH at 55 MPH
Gear driven speedometers are not exact, close counts, and when they are off they are off by a pecentage...ie if it's 10% low and reading 50mph you're doing 55 but when reading 70 you're doing 77.
On the Ford manual trasmissions you have a choice from 16 to 21 teeth
C0DZ-17271-A16R Teeth (Burgundy)
C3DZ-17271-C17R Teeth (White)
C0DD-17271-B18R Teeth (Yellow)
C0DZ-17271-B19R Teeth (Pink)
C1DZ-17271-A20R Teeth (Black)
C4OZ-17271-A21R Teeth (Red)
Pull your cable as explained before and determine which gear you have and then get gear you need which should be one tooth more than what you already have, they are about $9 at Ford.
If you already have a red 21 tooth gear you're out of luck for finding a 22 tooth gear because it isn't made and need to contact somebody that can make an adaptor.
My situation is the oposite of yours. With my 3.08 geared rear I need a 15 tooth but it doesn't exist and an adaptor isn't available nor can it be custom built, (yes I tried to get one made for the 11% I'm off), because of the Ford engineered bastardized type of cable and sensor arrangement on the '90 model I have.
Gear driven speedometers are not exact, close counts, and when they are off they are off by a pecentage...ie if it's 10% low and reading 50mph you're doing 55 but when reading 70 you're doing 77.
On the Ford manual trasmissions you have a choice from 16 to 21 teeth
C0DZ-17271-A16R Teeth (Burgundy)
C3DZ-17271-C17R Teeth (White)
C0DD-17271-B18R Teeth (Yellow)
C0DZ-17271-B19R Teeth (Pink)
C1DZ-17271-A20R Teeth (Black)
C4OZ-17271-A21R Teeth (Red)
Pull your cable as explained before and determine which gear you have and then get gear you need which should be one tooth more than what you already have, they are about $9 at Ford.
If you already have a red 21 tooth gear you're out of luck for finding a 22 tooth gear because it isn't made and need to contact somebody that can make an adaptor.
My situation is the oposite of yours. With my 3.08 geared rear I need a 15 tooth but it doesn't exist and an adaptor isn't available nor can it be custom built, (yes I tried to get one made for the 11% I'm off), because of the Ford engineered bastardized type of cable and sensor arrangement on the '90 model I have.
tonkadoctor
Full Access Member
No, there 3 types different types of driven gears in the automatics that I know of as well as 4 different drive gears that are machined into the output shafts. We really need more info on your truck to help you. Year, Model, engine, transmission, rear gear ratio, tire size.....
There is also a special retrofit 23 tooth gear available that you can get but it has a slightly different gear pitch and only has a service life of about 25,000 miles
I don't know why I was thinking yours was a manual, must have been reading something else right befor I replied here.
Here's a link to a good source for these gears and adaptors. Nice people and good to talk to. http://www.transmissioncenter.net/speedometer_calibration_______va.htm
I think i figured it out my speedo cable broke yesterday. So it must have been messed up for a while before it broke which would mess up my speedo . I'm going to fix it today, and see if that mit be the reason for the speedo being off.
f-two-fiddy
Registered User
before you replace the cable. Make sure the speed sensor is free, no drag. The new cable will come with a plastic drive end for the speed sensor. If the sensor is sticking, it'll break the new cable end, in less than a mile.
f-two-fiddy said:
I got it put back together, and had no problem's. Now my speedo at 65 MPH is really 58MPH. So my speedo is reading 7 MPH faster. I'm putting my 265-75-16's on tonight instead of the 245-75-16's.
I wonder if it will correct the problem?
Ok got the 265-75-16's on last night. My speedo now reads 5 MPH faster then i'm really going. The gear in the t-case is red. So I don't know if I can change it to get the speedo right on. Another thing is my brakes suck now it seems i have to push alot harder to get it to stop. So hard my trailer brake controler light come's on. Is this normal? What can I do to fix it cheap?
