things aren't adding up...

[GMC Guy]

i'm really not sure what is reading inaccurate. i'd just like to know how fast i'm going lol

1. just replaced the speedo cable, re-greased, no more boucing
2. 28 inch tall tires, bfg 275/60/15
3. doubled checked rear gear ratio. keep coming up with 3.08

here's where i'm confused, when i use the RPM calculator, it doesn't match up with what my tach and speedo are showing me on the road. (new Auto Gage tach btw)
http://www.csgnetwork.com/multirpmcalc.html

according to my gauges, here's what i'm doing on the freeway:

65mph: about 2700 rpm
70mph: about 2850 rpm
75mph: about 3000 rpm
80mph: about 3300 rpm

these numbers don't add up with what i should be doing with 3.08's, but they are almost the same as the numbers for 3.42 gears...

sorry if i'm not making sense guys, just confused
just looking for anyone's advice on what isn't reading accurate, thanks ahead of time

[y5mgisi]

Most likely cause your tires arent actually 28" tall. Thats what they spec out to, but i bet, if you measure them, they arent actually that tall and are closer to 27".

[86k20mudddigger]

Originally Posted by y5mgisi

Most likely cause your tires arent actually 28" tall. Thats what they spec out to, but i bet, if you measure them, they arent actually that tall and are closer to 27".

Agreed^^

Plus them one leggers you always do dosent help the tire size either haha

[Corts60]

Keep in mind that the teeth on the speedo (in the tranny) need to match your gears and tires. There are different combos available and there are also reduction units that could go between your speedo cable and where it hooks to the tranny to help offset the tire/gear combo. I have a '78 and I also run 275/60/15 BFGs. My speedo is between 5 and 10 mph off.

[GMC Guy]

how do i know what speedo gear to use?
th350, 3.08 gears, 28" tall tires.

and even if the tires were a little under 28" it wouldn't throw the speedo reading off by over 5mph

[S10Fan]

Use a GPS to check your speedometer, have someone with you so you're not writing and driving. Make note of the GPS Vs Speedometer readings. Pull the speedometer gear out of the transmission, count the teeth, make note of the color, and check the color of the gear on the transmission shaft. Take all of that info along with your tire size and check with these guys................

http://www.transmissioncenter.net/sp...n_______va.htm

BTW.....I love my 700R.....70mph=2000rpm

[xrcr]

I was going to suggest timing between mile markers and checking tach and speedo numbers (distance/time=mph) 60mile pr hr= mile pr minute so 60seconds in minute x 60minute in hr = 3600 therefore 3600/60 = 60 mph
This won't help you dial in your guages but it will show you where you're at.

[Corts60]

http://www.sccoa.com/faq/speedgr.html

here is another calculator link that involves speedo teeth...

[GMC Guy]

ok, i borrowed my friend's garmin gps and took note of the speed readings of the gps vs. my speedo on the way to school. come to find that the readings are almost right on...my speedo is within 0-3 mph slow of the gps reading. this still doesn't explain my tach readings at the relative speed. they seem to be 200-300 rpm off (on the high side) of the given speed.
ex.
75 mph should be approx. 2850 rpm
but instead, at 75mph the tach is reading around 3050 to 3100 rpm

could my brand new $70 tach be off?

[71swb4x4]

Originally Posted by GMC Guy

could my brand new $70 tach be off?

That's a silly question. Have you ever bought a new product that was defective? I think it's happening more and more. New doesn't mean it works right. So many people in this world are bent on the price they pay, and not the quality of the product. It's messing stuff up for the rest of us.

[S10Fan]

OK, I miss interpreted the original issue. I thought you were trying to tackle a speedometer issue.

Now that you know the speedometer is accurate I wouldn't be concerned with the Tach. Seriously, 200-300 RPM is probably as good as it's going to get for an inexpensive tach. Unless you're running a Pro-Stock 200mph race car 300 rpm doesn't make a difference.

There's an old saying in racing, "How fast can you afford ($$$) to go?".

"How accurate can you afford ($$$) your tach to be?"

[GMC Guy]

this is unfortunately true...
my problem is that my speed doesn't match what my rpm's should be with 3.08 gears and 28 inch tires.

but out of curiosity, i just went outside and measured my back tires from the ground to the top. they measured at approx. 26 3/4 inch...

[f.monroe]

What trans are you running? Keep in mind that most calculators use a 1:1 ratio for drive. A manual trans or a trans with a lock up converter will give a true 1:1. A TH350 or TH400 without a lockup will probably still have slippage at highway speeds and is probably the 200-300-rpm you are looking for .
Frank

[GMC Guy]

ohhhh that makes sense. lol i just have a standard th350 without lock-up. just can't wait to get my 700r4 installed with lock-up

[y5mgisi]

they measured at approx. 26 3/4 inch...

That adds up to alot! Have you re calculated it at that measurement?

[68longhorn]

f.monroe beat me to it! I was going to say the same thing, especially if you have a higher stall conv.

[GMC Guy]

does 26 3/4" seem normal for 28" diameter tires?
and as far as the stall rating on the converter, i have no idea. i doubt it's very high. is there any way of finding out?

[71swb4x4]

Is the 26 3/4" with the truck sitting still and the tire squatting?

What you really should do is get an ex-girlfriend to hang out of the bed and measure it while you are driving down the road so you get a true measurement of the size while you are driving.

[GMC Guy]

yes that measurement is with the truck sitting still.

and ya i wish