So, for the first time in 10 years, I have a working speedometer. First time since I owned the car.
Also just installed a Sun Supertach II.

When I installed the new speedo cable, I also installed a new 33 tooth speedometer pinion. Based on what people here told me, and after looking at some online charts.

So, now that everythings working, at 60mph, I'm turning 3350 rpm.

That's a 318/904, with 3:23 gears and a 25.6" (225/60/15) rear tire.
At 70, I'm turning 3800 rpm.

These numbers sound about right? Seems to me the rpm is a little high for the speed, but I guess the gauges don't lie.

with a known tire diameter/circumference, and a known gear ratio, and being in any selected gear, one could mathematically determine rpm's, per given speed.

sorry, my first post went live before I was done typing. More info there now.

my 518 with 3:91 gives much lower numbers. have to calculate my final drive at tailshaft though.

here's a very quick, "down & dirty" calculator, to help you out a bit. CALCULATOR

http://vexer.com/automotive-tools/speed-rpm-calculator

That calculator is a little faster and easier to mess with.

Seems like something is off. Speedo, speedo gear, or tach.

60 mph should be 2600-2700 rpm.

Just bring a GPS deal in your car and see if it matches your speedo. There are phone apps out there too for GPS speedo, for your phone. These are great tools to see if the speedo in the car is accurate, or if you need to change pinion gears. I found that my car's speedo was WAY off, but using the GPS speedo in my phone, I was able to eventually get my car's speed pretty close to accurate, and what should have been in there for pinion wasn't even close to what I ended up with.............and no, I didn't get the second pinion right the first time either -- I believe I went through 4 different pinions to get the closest one. But it worked............

Good luck!!!!!!!!!!

Ok, so just to check the speedometer I downloaded one of the free speedo apps for my phone, and went for another ride.

Seems at highway speed, I'm off my 5 mph.

When car speedo reads 60, the phone app reads 55
So I'm not even going as fast as I thought, so that makes the high rpm reading seem even more off.
Wonder if the tach is bad. It's an old Sun Supertach II that I've had laying around for years.

I checked out both of those online rpm calculators, and they both say at 60mph, I should be turning like 2500-2600 rpm.

I need to find a way to check out this old tach,

Sounds like something is amiss. I would check the in dash tach against a timing light tach that you know is good. I think you're getting to the bottom of this deal..........

yep, definitely way off. I had the same rear gears and tire size in a (4-speed) A-body some years back, and 60 mph = 2600 rpm.

As others have pointed out, you have too many "variables". First you need to know if the tach is accurate. You already found the speedo to be nearly 10% off. Once those are both calibrated, then if it's still high the only thing left is converter slippage.

A better speedo pinion accuracy tester is the odometer. It's possible for the odometer to be dead nuts and the speedo to be way off.

Boy am I dumb !

I got to thinking, this tach was on the car when I got it. And the car was a /6 back then.

Turns out there's a little hole on the back of the tach, with a small switch in it, that you're suppose to use to switch it from 4cyl, 6cyl, or 8cyl.
It was set on 6-cyl

Didn't know it was there. When the mounting bracket is on the tach, it covers up this hole with the switch in it.

Took it for another drive and sure enough, at 60mph on the phone app, the tach is sitting pretty at 2600 rpm.

Can't beat a cheap fix and I be this never happens to you again, btdt.

Glad you found it don't feel bad, we've all done stuff like that!

You have a PM on this

Yes, we are working on old cars but we don't need to use old tools (charts). Take advantage of modern technology, it's cheap and easy to use.

Similar to what Kunkel said. Odometer is better, simply because "speedometer is off by 5" is not an accurate observation. (unless the needle is clocked wrong). If yo were off by 5, it would be off by 10 at double the speed and it's would be correct at 0.
Speedometers are off by a certain percent. Use a GPS to confirm then adjust up or down by the speedometer gear tooth count.