rwthomasjr

Recently changed from stock 16"x7" wheels and 225 75 16 tires to 15"x8" wheels and 30" tires. I noted on Quadratec's website that I needed a 33 tooth spedometer gear (assuming I have stock 3.55:1 rear). Bought a 33 tooth gear and went to change it out and found out it already had one. Question is, is the 33 tooth gear stock? I assume it is but based on Quadratec's website, it seems that a 32 tooth gear would be stock for the stock wheel/tire diameter. Probably wouldn't be a big difference between 32 and 33 anyway in terms of spedometer accuracy, mpg calculation, etc.

derf

I don't trust Quadratec's chart.

I'd actually run a 32 tooth for a typical 30x9.50/15 tire. The smaller the tire, the more teeth on the speedo gear you want. The 33 is pretty good for the 225/75R15. I might even run a 34 tooth for a 225 depending on the tire. Not all 225/75R15 tires are the same size. They vary from tire to tire.

If you want to do the math yourself, look up the tire on someplace like tirerack.com. Look in the specs and find the "revs per mile" and do the following math:

revs per mile * axle ratio * 13 / 1000 = x

Drop the fraction and that's the number of teeth you want on your speedometer gear.


So for a BFG all terrain 30x9.50, the revs per mile are 705. 7.05 x 3.55 x 13 / 1000 = 32.53575 => 32

That puts your speedometer as close to accurate as you're going to get.

rwthomasjr

Thanks. All said and done, it's likely not worth the $50 to go from a 33 to a 32 given the minimal difference its going to make. I even debated doing a swap at all. I appreciate you sharing the gear calculator.

derf

That's fair. I'd do a test run at 60-70 MPH with a GPS to make sure you know how far it's off.

4.3L XJ

The real definitive test is to use a free speedometer app for you phone and travel at a steady speed and see if the speedo matches the phone. Then do a little math and go from there

lawsoncl

Exactly!

EuroJeep

Hello,

I know I should count as explained in this thread.

but according to the book how many teeth on a

- 87 XJ Wagoneer
- 4.0 renix
- transfer case 242
- 225/75R15
- 1.5 lift

At the back my differential looks like that, But I wonder Which one it is.

thanks for you help.

and btw, why is this little pignon so exensive ??
any source ?

EuroJeep

I ask the question because my local « jeep specialist » says I need 31 teeth.
but Derf says 33 or 34 (and I tend to trust him)

my car currently has a 36 teeth. What were the tires,of the PO ??

4.3L XJ

If you want to know which is right, use the method I posted above

EuroJeep

Can you re-explain the math ? Please ?

I can drive the car and look at the difference between the speedo And the GPS.

- at what speed ?

- and then ?

Sorry for asking again
the pignon is expensive, I want to be sure.

4.3L XJ

Any speed, around 60mph is good

Desired tooth count = current tooth count times observed speed divided by actual speed

lawsoncl

I agree they are expensive for what they are. Looks like an 87 requires the "long" pinion gear and they're around $50 on Amazon. Might want to double-check the length of your's too.

derf

Everything is built aroud the sensor turning 1,000 revolutions per mile.

The shaft in the transfer case tail is a worm gear that is effectively 13 teeth. Work back from there.

The biggest problem is figuring out exact on-the-ground diameter of the tire. A tire advertised as a 31" won't be anywhere close to that. Even the "outside diameter" (usually 30.5" for a 31" tire) will be off.

In the specs for most tires is a "revs per mile" spec. Usually in the high 600s or so for a 31" tire. That tends to work out pretty close to how big the tire really is when its mounted and rolling down the road.

From that number, it's arithmetic. The axle shaft makes the revs per mile that is listed for the tire. The axle ratio affects how fast the drive shaft turns. The drive shaft spins the transfer case tail shaft which has the 13 tooth gear on it. Then its just a matter of figuring out how many teeth you need to make the sensor spin at least 1,000 revolutions per mile.

You want it to be over so your speedometer doesn't read low. Also it affects the odometer and having it read low means the odometer is low and legally that's fraud. But not many people care a lot about that.

But everything starts with having an accurate tire size. If that number is off, you won't get the right gear.

In practice, keeping the stock gear in and adjusting after you get the gears and tires installed and rolling down the road is a good way to be sure you get the right gear. Just use your GPS to tell you how fast you are really going and compare that to the speedometer. Then its current teeth x speedo speed / GPS speed.

IJM

To get the effective diameter of the tire, wouldn't you just measure the distance from the ground to the center of the hub on one of the rear wheels? To be extra precise, measure both and take the average. Then do your computations using that radius figure. The other way you could do it is use the roll-out method to get the effective tire circumference. Mark a spot on the edge of the sidewall/tread at bottom dead center, roll the Jeep forward until the tire makes one complete revolution, and measure the distance on the ground between the two points where the tire mark is at the bottom dead center. That's how I do it to calibrate my bicycle speedometers, and it might be a little more accurate than just measuring the radius as I describe above.

derf

That works too.