My car came with an original 2.76 gear, as of last weekend it now has a 3.54...anybody know what speedo gear I should now put in to correct the speedometer? The trans is a 904. Thanks

the tech archive at the top of the page has a speedometer gear chart you may find useful also.

I agree. I used it to figure out the collection of speedo gears I needed for my junk.

yours will depend on tire size but mine are

235/75/15

2.76 = 25 tooth
3.21 = 29 tooth
3.23 = 29 tooth
3.55 = 32 tooth
3.91 = 35 tooth

325/50/15

2.76 = 26
3.23 = 30
3.91 = 36

out in the parking lot put a rear valve stem at 6 o'clock then roll the car forward in a straight line till the stem returns to the same position (grab several bricks & your tape) & measure "rollout" then divide Pi into that number for your diameter then as said go to the chart & CNXT (SP) priorly posted a comprehensive chart that is in the tenths on diameter numbers & your gear ratio. Jack the vehicle way up on the DR side to minimize fluid loss.

Excellent! Not only did I change gears you guys thumped me into remembering that I went from 14" to 15" tires also...sweet! Preferred gear sources?

On Moparts or FABO or Ebay

eBay.

CRT has them in stock. and are a board sponsor.

Thanks, and looks like my calculations lead to a 33 tooth gear.

I'd be surprised if you could get a more accurate diameter by measuring roll out and calculating it than just looking up the tire specs. I'd think manufacturing tolerances are way tighter than trying measure roll out.
If you're going to go to that trouble you're better off taking the tire off and actually measuring it.

That would be the same one that is on my car. That's the yellow speedo gear part no. 2538933.

I would think that "rollout" would be the most accurate measurement cuz it would be exactly what the # of turns is that will be imparted to the drivetrain & something I remember from drag racers way back is that the tire might even grow some at speed. If you grabbed 2 bricks, your tape and a calculator & have ~8 ft of space in front of your car I think you could knock it out in 5 to 10 minutes. If you do it you might compare it to the tire listed dia & we can see for sure.

For one of our math whizzes out there.

Say you have a 275-60-15 inch tire mounted and aired up.

Laying flat on the ground, let's say measures 28 inch diameter.

Now mount the wheel on the car, lower to where is is sitting on the pavement.

As tire will flatten out a bit from the weight of the car, perhaps it is now 27 inches from the pavement to the top of the tire.

How does this effect the rolling circumference of the tire?

If no flattening, 1 full revolution would move the auto 28 inches times pi of 3.1416 equal 88.9 inches.

How far will the car move forward in one revolution allowing for 1 inch less diameter due to this flattening?

Often have wondered about this and how it effects odometer calculations.

Sxrxrnr- the diameter of the tire is not what is needed here - what you need is the "loaded tire radius".
If you measure from the exact center of the axle flange, hub cap, wheel cover - to the ground and do the correct calculations you will end up with the same information without the bricks or moving the vehicle. Either method does not take in to consideration the growth of the tire as speed increases but I doubt that growth is much at speeds less than 70 mph.

Thanks for all the replies guys, appreciate the input, I took the easy way out and used the tire size/gear ratio and just rolled with that. With the current speedo gear being calibrated for 2.76 and 14" tires you can imagine how far the speedometer is off!

Rapidus,

Say it ain't so!! You calibrated your speedo gear on 14" tires!!!

No wonder you are way off.

It is easy to calculate exactly the speedometer gear you need but you need to know the axle ratio and the tire size. The previous replies are correct about the importance of using the loaded tire size - also called the rolling diameter. The rolling tire diameter is about 3% smaller than the static (unloaded) diameter.

Tire Dimensions
Tire Upgrades

I disagree. The rollout (which computes to true diameter) will differ from the squatted radius.