Speedometer reading 18% fast

[Roger Slater]

My 1986 Comanche 2.5 has 195/65/15 tyres. The smaller section makes the speedo read 18% fast.  At GPS 60 mph the speedo reads 70mph. Could someone kindly advice the speedo cable gears required to lower the speedo by 18%

Thank in advance

 

Roger

[Eagle_SX4]

You will need to pull the gear out of you truck and just multiply the number of teeth by .18 then add that number to the tooth count. Then round to the nearest whole number.

 

Example using a 32 tooth gear

32x.18=5.76

32+5.76=37.76

Round up to 38 for the gear count.

[derf]

Basic equation is straightforward:

 

Tire revs per mile × axle ratio × 13 ÷ 1000 = teeth.

 

Drop the fractional part.

 

I looked up the tire size and got 832 revs per mile.

 

I'm assuming a 4 cylinder runs 4.10 axle gears.

 

That gives me 832 × 4.1 × 13 ÷ 1000 = 44.3456

 

Drop the fraction and you get 44 teeth.  By dropping the fraction, the speedo will stay just a hair above actual speed.  If you round, you can be running faster than the speedometer reads.  But only by a hair.

 

For 215/75R15 it's 751 revs per mile.  That's 40.0283, or 40 teeth

 

235/75R15 is 721 revs per mile and 38.4293, or 38 teeth.

 

A 31x10.5/15 tire is around 685 revs per mile giving you 36.5105, or 36 teeth.

 

I believe that the Km/H speedometers are calibrated the same.  They just have a different number scale painted on them.  But I would double check.

[Roger Slater]

Thank you gentlemen.  Do I need to change the gears as a pair or will just changing the driven gear do the trick.

 

Thanks

[derf]

I only replace the driven gear attached to the sender.  They're not matched.

 

Getting to the other one is always a bit involved.

[Roger Slater]

Thanks for the valuable info.

Roger

[derf]

Keep in mind that there are number ranges cast into the mount for the gear.  Those tell you how to rotate the mount by how many teeth are on the gear you're installing.

 

Different gears are bigger (more teeth) or smaller (fewer teeth).  The gear mount piece is offset.  You need to rotate it to the right number setting after swapping out the gear.  That gets the new gear the right distance away from the other gear.

 

There are 4 slots.  The retainer has two tabs that lock it in place in 2 of the slots.  You have 4 choices for rotation, at 90 degrees.

 

Pay attention to how it comes out.  It'll probably be covered in gunk but when you clean it, you should be able to find the numbers.