I'm going to swap my peg-leg 2.73 for Richmond 3.23 and take-off posi this weekend. I researched the speedometer gear computation, and here's what I decided:

stock 2.73 = 17(drive) ; 37(driven) w/ 215/65/15's

2.73/3.23 = 84.5%

37/44 = 84.1%


So, I just need to change the driven gear from 37 teeth to 44 teeth, right? Thanks for checking my math, guys.
:rockon:

 

Okay, so now I did the calculation with RB83L69's "SMUC" formula, and I get a different answer.

12 * 5280 / 26 / Pi = 775.7 tire rotations per mile

775.7 * 3.23 = 2505 driveshaft rotations per mile

2505 / 1000 = 2.505 speedo gear ratio

44 / 17 = 2.588 = bad

43 / 17 = 2.529 = better

40/16 = 2.5 = disco


I made my first calculation based on the FAQ's on this site. Which calculation is right?

 

Make sure you get your tire size accurately. Measure from the center of the wheel to the ground with the car's weight on it.

Both calculations are "right". What happened, is when you did the first calculation, you multiplied the round-off error that was already built-in, in addition to making the correction.

In any case, even with your "bad" calculation, your speedo is still onl;y about 3 and a fraction percent off..... less than 2 mph at 60. Maybe not quite perfect, but not too terribly awful either.

 

I just used the nominal tire size based on 215/65/15 (26 inches). The tape measure actually says about 25 3/8 +/-.

Running this through the SMUC, I get a speedo gear ratio of 2.567. I would rather just change the driven gear, for simplicity. I'm leaning toward 44 teeth. Would you recommend 43 or 44 teeth?

Thanks for your help.

 

It works out to 43.6 with those tires, so 44 would be closer. That would put you to well within ½ mph at 60, reading slightly higher than your actual speed.

After you make the change, check the accuracy of your odometer against mile markers on the freeway; use a good long stretch, like 10 miles or so. That's the best way to check your speedo gears, better than the speedo itself. The odometer is a more accurate instrument than the speedo. After all, it's just a bunch of gears itself; arranged such that for every 100 revs of the cable you get one-tenth mile, for every ten of those (1000 revs) you get a mile, etc. Your speedo itself can be out of calibration, but the odometer basically can't be inaccurate as long as it works.

 

Right on. One more thing... what about the VSS? Will it 'see' the new speedometer gears? Or will it need some attention, too?

(Woohoo! I'm officially a TGO member.)

 

Yes. It "sees" them, quite literally in fact.

It uses an optical system that reflects off of the piece of the speedo that rotates. It's a little bar with magnets on it. It's shiny, and the thing behind it (the steel drum that's attached to the pointer and magnetically coupled to the spinning bar) is black. So each time the cable rotates, the optical pickup sees 2 ends of the magnetic bar go by.

The optical thing is the little yellow box mounted on the back of the cluster.