Speedometer gear help

DanEC · #1 (**permalink**) 10-02-2017, 05:56 PM

I used a GPS app to check out my speedometer and it appears to be off even more than I suspected. It appears that 40 speedo indicated is actually about 46 mph and 50 speedo indicated is about 58 mph - roughly 15% off. Some of this may be due to the 255-60/15 tires I run in the rear but the difference compared to a 295-50/15 is only about 1.5% based on circumference. With a TL and 3.31 rear the speedometer calculators I checked all say I need a 17 tooth gear with the 7 tooth drive gear.

I didn't count the teeth on the gear ERA provided but based on the 3.31 rear I figure it was around a 16 tooth for a 295-50 tire. Nothing on my order indicates the drive gear I was provided.

I can't see the difference between a 16 tooth gear and a 17 tooth gear to account for a 15% correction. If I have a 15 tooth gear by some chance the math seems to work out fairly close.

I doubt many of you have 60 series tires but does a 17 tooth gear sound about right for a TL with 3.31 rear and a 27 inch tire?

strictlypersonl · #2 (**permalink**) 10-03-2017, 05:26 AM

According to page 14 in the ERA manual, you need an 18 tooth gear.

my427cobra · #3 (**permalink**) 10-03-2017, 01:08 PM

Hi guys. I have always searched for a simpler explanation regarding the selection of speedo gears.

Yes, all those calculations and charts work, if you know all the data points (axle ratio, trans output no. of teeth, speedo gear teeth, tire rolling dia etc).

But if your speedo is reading, say as in this thread, 15% high, wouldn't a simple approach be:

1) Take out speedo gear.

2) Count speedo gear teeth (or consult speedo gear color charts)

3) Calculate, as was suggested here, 15% more teeth. ( more speedo teeth turns the speedo slower. Right?)

4) Buy and instal the speedo gear with 15% more teeth.

I'd be interested in responses from the forum members as to how many have accurate speedo readings even with the "right" speedo gear?

How many of you have used a speedometer "correction gear box" as often suggested by the speedo experts (Nisonger etc)?

Cheers
Greg

my427cobra · #4 (**permalink**) 10-03-2017, 01:17 PM

OOPPS. I misread Dan's post. I got it reversed.

Dan's speedo is reading LOW ( Speedo: 40 mph. GPS: 45 mph).

Therefore he needs to spin his speedo faster.

Therefore 15% fewer teeth than he currently has. (fewer speedo gear teeth spins the speedo faster).

Right?

Cheers
Greg

DanEC · #5 (**permalink**) 10-03-2017, 03:57 PM

Quote:

Originally Posted by strictlypersonl

According to page 14 in the ERA manual, you need an 18 tooth gear.

Bob - I assume that's what should be in it now? I ordered the car with a 3.31 rear by ERA, fitted for a toploader and the speedometer cable and gear was included.

I know the best thing to do is pull it out and verify what's in there first. I was just hoping to save a trip under the car.