things aren't adding up... - The 1947 - Present Chevrolet & GMC Truck Message Board Network

GMC Guy · 02-02-2011, 11:25 PM

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 · 02-02-2011, 11:37 PM

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 · 02-03-2011, 12:24 AM

Quote:

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** · 02-03-2011, 01:30 AM

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 · 02-03-2011, 01:51 AM

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 · 02-03-2011, 06:31 AM

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 · 02-03-2011, 08:30 AM

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** · 02-03-2011, 10:41 AM

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

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

GMC Guy · 02-03-2011, 08:50 PM

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 · 02-03-2011, 09:12 PM

Quote:

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 · 02-03-2011, 09:20 PM

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 · 02-03-2011, 09:39 PM

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 · 02-03-2011, 10:49 PM

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 · 02-03-2011, 10:52 PM

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 · 02-03-2011, 11:10 PM

Quote:

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

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

68longhorn · 02-03-2011, 11:12 PM

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

GMC Guy · 02-04-2011, 02:03 AM

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 · 02-04-2011, 01:06 PM

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 · 02-04-2011, 03:33 PM

yes that measurement is with the truck sitting still.

and ya i wish

02-02-2011, 11:25 PM	#1
GMC Guy Registered User Join Date: May 2009 Location: Sparks, NV Posts: 913	things aren't adding up... 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 __________________ Greg 1974 GMC 1/2 Ton SWB Fleetside L33 5.3/4l65e .................__ ............._____l __\__ ..............l_(O)_ l___(O)]

02-02-2011, 11:37 PM	#2
y5mgisi Second Chance Program Join Date: Jun 2006 Location: Portland, OR Posts: 2,642	Re: things aren't adding up... 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".

02-03-2011, 01:30 AM	#4
Corts60 Just here to tinker Join Date: Jun 2008 Location: Holland, MI Posts: 3,692	Re: things aren't adding up... 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. __________________ 78 C10 SWB Diesel #1 (wrecked)http://67-72chevytrucks.com/vboard/s...1978+silverado 78 C10 SWB Diesel #2 (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=650751 60 GMC Suburban (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=445526 60 GMC 1000 (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=298235 67 GMC 1500 (sold) http://67-72chevytrucks.com/vboard/s...d.php?t=803695 86 Suburban(summer unit) http://67-72chevytrucks.com/vboard/s...d.php?t=810697 88 Suburban 4x4(winter unit)https://67-72chevytrucks.com/vboard/...25#post8900725

02-03-2011, 01:51 AM	#5
GMC Guy Registered User Join Date: May 2009 Location: Sparks, NV Posts: 913	Re: things aren't adding up... 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 __________________ Greg 1974 GMC 1/2 Ton SWB Fleetside L33 5.3/4l65e .................__ ............._____l __\__ ..............l_(O)_ l___(O)]

02-03-2011, 06:31 AM	#6
S10Fan Old Heap Driver Join Date: Mar 2008 Location: Newport News, Va Posts: 2,640	Re: things aren't adding up... 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 __________________ _____________________________ Bryan '99 Silverado 1500, 4.3, 5-speed, reg cab, short bed '50 Chevy 2DR Hard Top, 350/350, M2 Front End, 3:08 gear, cruiser. '40 F**d Sedan, all Chevy power, Heidt's front end, TCI rear, nice driver.

02-03-2011, 08:30 AM	#7
xrcr Registered User Join Date: Dec 2008 Location: Foristell, MO Posts: 418	Re: things aren't adding up... 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.

02-03-2011, 10:41 AM	#8
Corts60 Just here to tinker Join Date: Jun 2008 Location: Holland, MI Posts: 3,692	Re: things aren't adding up... http://www.sccoa.com/faq/speedgr.html here is another calculator link that involves speedo teeth... __________________ 78 C10 SWB Diesel #1 (wrecked)http://67-72chevytrucks.com/vboard/s...1978+silverado 78 C10 SWB Diesel #2 (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=650751 60 GMC Suburban (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=445526 60 GMC 1000 (sold)http://67-72chevytrucks.com/vboard/s...d.php?t=298235 67 GMC 1500 (sold) http://67-72chevytrucks.com/vboard/s...d.php?t=803695 86 Suburban(summer unit) http://67-72chevytrucks.com/vboard/s...d.php?t=810697 88 Suburban 4x4(winter unit)https://67-72chevytrucks.com/vboard/...25#post8900725

02-03-2011, 08:50 PM	#9
GMC Guy Registered User Join Date: May 2009 Location: Sparks, NV Posts: 913	Re: things aren't adding up... 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? __________________ Greg 1974 GMC 1/2 Ton SWB Fleetside L33 5.3/4l65e .................__ ............._____l __\__ ..............l_(O)_ l___(O)]

02-03-2011, 09:20 PM	#11
S10Fan Old Heap Driver Join Date: Mar 2008 Location: Newport News, Va Posts: 2,640	Re: things aren't adding up... 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?" __________________ _____________________________ Bryan '99 Silverado 1500, 4.3, 5-speed, reg cab, short bed '50 Chevy 2DR Hard Top, 350/350, M2 Front End, 3:08 gear, cruiser. '40 F**d Sedan, all Chevy power, Heidt's front end, TCI rear, nice driver.

02-03-2011, 09:39 PM	#12
GMC Guy Registered User Join Date: May 2009 Location: Sparks, NV Posts: 913	Re: things aren't adding up... 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... __________________ Greg 1974 GMC 1/2 Ton SWB Fleetside L33 5.3/4l65e .................__ ............._____l __\__ ..............l_(O)_ l___(O)]

02-03-2011, 10:49 PM	#13
f.monroe Senior Member Join Date: May 2004 Location: Caldwell,Tx. Posts: 3,648	Re: things aren't adding up... 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 __________________ Born and proudly residing in a Red State !

02-03-2011, 10:52 PM	#14
GMC Guy Registered User Join Date: May 2009 Location: Sparks, NV Posts: 913	Re: things aren't adding up... 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 __________________ Greg 1974 GMC 1/2 Ton SWB Fleetside L33 5.3/4l65e .................__ ............._____l __\__ ..............l_(O)_ l___(O)]

02-03-2011, 11:12 PM	#16
68longhorn Registered User Join Date: Jun 2007 Location: Cape Coral, Florida Posts: 1,123	Re: things aren't adding up... f.monroe beat me to it! I was going to say the same thing, especially if you have a higher stall conv. __________________ 1986 Custom Deluxe 10 1978 Caprice 2 door - sold 1937 Chevrolet 2 door sedan - project

02-04-2011, 02:03 AM	#17
GMC Guy Registered User Join Date: May 2009 Location: Sparks, NV Posts: 913	Re: things aren't adding up... 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? __________________ Greg 1974 GMC 1/2 Ton SWB Fleetside L33 5.3/4l65e .................__ ............._____l __\__ ..............l_(O)_ l___(O)]

02-04-2011, 01:06 PM	#18
71swb4x4 Senior Member Join Date: Nov 2003 Location: Brookings, SD Posts: 10,497	Re: things aren't adding up... 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. __________________ Some people are like slinkies, they aren't good for anything, but you can't help but smile when you see one tumble down the stairs.



Register or Log In To remove these advertisements.

02-04-2011, 03:33 PM	#19
GMC Guy Registered User Join Date: May 2009 Location: Sparks, NV Posts: 913	Re: things aren't adding up... yes that measurement is with the truck sitting still. and ya i wish __________________ Greg 1974 GMC 1/2 Ton SWB Fleetside L33 5.3/4l65e .................__ ............._____l __\__ ..............l_(O)_ l___(O)]

Register or Log In To remove these advertisements.