Our tach reads 3000rpm at 55mph. We have 355's in the rear, th400 (3 speed), 15" wheels on the back.
Does this seem high or about right for our configuration?

Not sure but I also have 3.55 rear w/15" rims and 225/70 tires. I turn 3k RPMs at 60mph. I used my laptop with Streets & Trips and the GPS module to make sure our calabration on the speedo was correct. I am also running an M21(M22 conv.) so I am not sure how that plays into the whole calculation...

rpm = (trans drive ratio x rear gear x mph x 336)/ tire diameter inches.

Add about 150rpm for a stock non-lockup torque convertor, or more (up to 300rpm) for a high stall. If for some reason it's quite a bit off, borrow a GPS and check your speedo.

If I plug in 26" for the tire, it looks like your running about 10% higher rpm (even considering torque convertor slippage).

I'm thinking your speedo is reading about 7 - 10 mph too slow.

As mentioned above, use a gps to double check.

Ok,
We changed the driven gear and checked our speedo with the GPS. At 55 mph were are dead on at 3000 rpm.
The only other thing is to check the Tach to make sure it's reading right.

Wait...

You stated your initial rpm/mph info was 3000 rpm at 55MPH....then you changed your driven gear and now show 3000/55 using the GPS. So, your first gear was correct? How many teeth? How many teeth on the new one?

3000 RPM sound high for 55MPH...but, I could be mistaken.

That sounds about right, the transmission type doesn't matter, all are 1:1 final drive, rearend and tire size determine engine rpm at any given speed. I've got 3.08's and at 55 I'm just under 3k but with 235/60 R14's on the back.

Only way to improve is taller gears in the rearend or an overdrive unit to change final drive ratio.

Yes, our speedo was off but I knew the actual speed because I had a GPS. So while I changed out the driven gear to get the speedo accurate, we still run 3000 rpm at 55 mph.
We were wondering if something was wrong with our tranny or if we were running in 2nd gear to have the rpm's that high. After reading the comments above, I dont think that's the case. However, we might have a tach that's off a little and need to recalibrate.

I could not get my Rally Speedo to read correct with my 2.56 rear end and 4 speed, I was readin 40 when I was actually doing about 60, according to my GPS.

I ordered the 44% increase adaptor. Actually just came in today so will be putting it on tonight.

Ratio Adaptor

My Pontiac buddy is the one who figured mine out, he calculated all the math and went to his little plastic container of gears, picked one out, popped it in and told me to take it for a spin and use my "gadget (GPS)" to check if it was correct. Lesson learned, I no longer doubt him about anything when it comes to cars! Or much of anything else for that matter... LOL

Trans does make a difference if you consider a non-lockup convertor. Non-lockup will have some "slippage" and that adds rpm. See my previous post.

BTW, the above numbers don't make sense for your combination. You should be around 2250rpm, or if a non-lockup TC about 2400rpm (or you have a very liberal idea of "just under.)"

Excellent. I put in my 44% conversion unit last nite and the speedo now reads within a mile or two per hour...close enough...finally!

Checked mine this am. Same configuration as Colorado(except 2600 stall convertor). At 3000 RPM, I was going 60MPH.

Found out today our Hood Tach is off. According to the timing light tach we were at 2550 when the hood tach read 3000. So we feel better knowing that were not really 3000 rpm at 50 mph. Now we have to fugure out how to adjust the tach.

I wonder if you have a tac for a 6 banger?

They were different?
It does read correct at idle, just gets off as the rpm's increase.
S!*$, that would suck if it's for a 6 cyl.

Yes, they were different. Now you can generally switch them, like tune-up tachs, but the originals were either 6 or 8.

Actually, it would be pretty impressive seeing your tach wind up to 6700 rpm when you're only turning 5K...

See, you guys in the U.S. have an easy life I'm tellin' ya. Cheap gas, sunshine, straight roads, 3 grand @ 50!!....you wouldn't put up with that for long at $10 a gallon like I pay.
Not now needing to impress the girls from the stop light I try and enjoy the UK's wet and twisty roads with some somewhat taller gears in all my cars, today in the trusty '41 Hudson pickup with a 289ci Ford (sorry) I clicked off over 220 miles, with the tacho sitting at 3000 we were at 85mph, returning about 25 to the gallon. Hardly relative to your birds I know but maybe of interest. Do you guys care too much on fuel economy in your toys? I try not to, some are worse than others but they are to be driven and enjoyed right! just wondered if economy is an issue or not.

I pay $8 a gallon so no great difference there. For the 1500 or so miles I can rack up in a Michigan summer the price and the economy don't matter much. My 400 auto does have a 2.78 rear gear but still won't do better than 14 mpg in all-around driving.

If I drove my car as a daily driver I would likely have two cars, one for a daily driver and one for serious fun. Not that the two can't be the same car, but for the cost of meeting all goals in one car I can have two toys.