Trouble getting my speedo to read correctly. Put a new gear in, still too fast

[Corellian Corvette]

I'm trying to get my speedometer to read correctly in my 68 Charger.

I did a search, and found out there is a calculation to determine the correct gear.

My car has 3.55 rear end, with 295/50 15 tires on the back.

According to my calculations, my gear teeth (mathematically) should be 35.25790461, so I ordered (and had installed) the 35 tooth gear.

The car is still reading fast. So either I did my math wrong, so something else is going on. I checked against a local radar sign, and it said I was doing 35mph when my speedo was reading 40. That's about 15% off.

Also, when the car is at 60mph (according to my tach) I'm at about 2700 RPM.

Is there any way to use those numbers and figure out what's going on? If my final drive ratio is 1:1, how can I figure out my correct speed vs. my RPM?

Any help appreciated!

[flyinlow]

When new the speedo only had to be within 10 percent. Add 4 decades to wear on the speedo parts and here you are.

Try a 30-31 tooth gear.

The radar cart may be off. Do you know someone with a GPS that you could use to check which is off?

If you leave it the way it is, passengers will think your car is faster and you might not have to pay the go fast tax as often.

[oldkimmer]

.........Less teeth on the speedo gear will throw it out more, u need 37 teeth.............kim.............

[flyinlow]

Yea, got it backwards . Try a gear with more teeth.

[maxwellwedge]

And make sure the pinion adaptor is rotated to the right mark....for mesh issues.

[Corellian Corvette]

So here's what's strange. I drove the car this AM and took some RPM readings at various speeds, because I found a calculator that will give you road speeds at various RPM ranges.

http://www.wheelspin.net/calc/calc92.html

35 mph = 1700 RPM
44 mph = 2000 RPM
55 mph = 2500 RPM
60 mph = 2700 RPM

Assuming that my tach is reading properly - I KNOW my tires are 295/50r15 (26.61 diameter) and I KNOW my final drive ratio is 1:1. I recall that my rear gears are 3:55 (at least that's what I asked for)

Given that, according to the calculator my speedo is right on the money. Yet, I'm surprised how far off the local Radar unit is, and I have to admit it FEELS like it's running fast.

What do you guys think?

[John_Kunkel]

35 is correct for your combo.

In many cases speedo error has nothing to do with the pinion selected, the speedo is just incorrect. The best way to check for the correct pinion is to use the odometer instead of the speedometer. If the odometer correctly logs a known distance, the pinion is correct and the speedo needs work.