I have a 3.55 rear with 31" tires and replaced the trans a few months ago from a 1984 f150 that had a shorter driveshaft. The speedo says 35 mph, but i am really going 30mph. i don't want to change the speedo gear in the trans,(unless someone has one for me and stupid me for not changing it.) i would like to go with bigger tires about 33" to hopefully equal it out. WAny suggestions on what one would think what would be good to go the right speed. Thanks.

1987 F150 2wd
4.9L, 4 spd manual
31X10.5X15 BF's ATs
3" suspension lift
166,000 and counting

 

You'll probably need a 35 or 36 inch tire.

Bob
'66 7 Litre, top loader, 3.25 Traction-lock, Sidewinder Intake, SCJ Exhaust
'88 F150 LWB 4x2, XLT Lariat, 302, 5 speed, 3.08
'99 Ranger, 4x2, 3.0, 5 speed, 3.73 posi

 

I agree with bsprowl.
The error percentage is 5/35 (35-30)/35 = 14.3%.

The difference between 31 and 36 inch tires is 5/36 = 13.9%. Real close.

On the other hand if you can find a speedo gear for the cable end that goes into the transmission (driven gear) with one less tooth, you can use 33 inch tires. Then you would have about 6.1 % change, and changeing one tooth on the driven gear will give you about 6%. So you would be in acceptable range.

Winford

 

Thanks for the info. That equation is pretty good to know.

1987 F150 2wd
4.9L, 4 spd manual
31X10.5X15 BF's ATs
3" suspension lift
165,000 and counting

 

ok....i have a question for you since you seam to know what your talking about......i have 3.55 rear in my truck and 35 inch tires. what kind of gear change can i make in my tranny to make my speedo correct.

 

We need to know how many teeth are on the gear on at the end of the cable in the transmission.

Bob
'66 7 Litre, top loader, 3.25 Traction-lock, Sidewinder Intake, SCJ Exhaust
'88 F150 LWB 4x2, XLT Lariat, 302, 5 speed, 3.08
'99 Ranger, 4x2, 3.0, 5 speed, 3.73 posi

 

gui88ford: Like most other things in life, you have to know where you are before you can determine how to get where you want to be. Anyway, the previous post I made was information on changes, assuming that the original setup was accurate.
If you have only changed the tire size, then, assuming the original tires and rear-end ratio produced accurate speedo/odometer, the amount of gear change you would want would be the same percentage as your tire change. If your originals were 30 inch tires, your change would be (35-30)/30 or about 16.7 percent. Normally, the speedo drive gear in the transmission has 6,7,or 8 teeth, and the driven gear has from 16 to 21, depending on the vehicle. If you change the drive gear from say 6 to 7 teeth, you have a change of 1/6 or 16.7 %. If you already have 7 teeth and can find an 8 tooth, you would gain 1/7 or 14.3%. It is much easier to change the driven gear, but you would have to go DOWN by 2 or 3 teeth, which may not be possible.
Winford