speedo check? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

highboy_04 · 08-24-2004, 06:15 PM

i've been trying to find a place to check my actual speed. i've got a different gear ratio now and need to figure out how off the speedo is. the tires are the same size so i need a calculator to do the ratios. thanks, Steve

toddtheodd · 08-24-2004, 06:18 PM

Try this.
Todd

cdowns · 08-24-2004, 06:23 PM

the easy way to do a rough check is go out on an interstate that has mile markers with a stopwatch. with your speedo at a steady 60mph your time between mile markers should be 1minute

*Gee_Emm* · 08-24-2004, 06:40 PM

Get a buddy that has a GPS and go for a ride

MACKL · 08-24-2004, 06:50 PM

GPS is the way to go. Takes all of the guess work out, no math. It will even give you a reason to buy one. Then it is a tool, not a toy!

da-burb · 08-24-2004, 07:11 PM

If you are not all that worried about if it was right in the first place, all you will need to do is divide your new ratio by your old ratio.

4.10/3.73=1.10 A 4.10 ratio is 110% of the 3.73 ratio. 3,000 rpms with a 3.73 will be 3,300 rpms with a 4.10 ratio. Your speedo would be off by 10%. 60mph would now be at 66mph on the speedo.

3.08/4.10=.75 if your 4.10 gear was 3300 rpms at 60mph a 3.08 would be 75% of the old 4.10 gear or 2475 rpms. your speedo would be off by 25% You would now be doing 60mph when the speedo says it is doing 45mph.

hopefully I didn't turn any of the numbers around, but it should be correct.

Here are a couple of other sites I have used to help figure this out before.
http://www.novaresource.org/speedo.htm
http://www.bgsoflex.com/speedo1.html

**smitty62** · 08-24-2004, 09:18 PM

You guys are way overcomplicating this--Cdowns has it right.
If you are really picky, take the deviation in seconds from 1 minute to travel measured mile (+ 0r - seconds) divide that by 60 for the percentage it deviates--that's the percentage your speedo is off.
IE: 58 sec. = 2 seconds deviation, div. by 60 = 3% fast or 61.8 actual mph. Close enough for government work and you never have to get out of cab or buy a $200+ piece of equipment!

highboy_04 · 08-24-2004, 10:14 PM

alright thanks for the help guys according to that site i am suppose to be going 80 at 3000 rpms with the 3.08 gears but with 3.73 i'm only goin 69.9. thanks, Steve

zooza · 08-24-2004, 10:20 PM

I checked mine today , parked my crown vic. beside the road and showed the wife how to lock the radar. Then I drove by at 55 mph. she locked it when I flashed my headlights. Found that the 95 mustang gt is off by 5 mph and the 72 c-10 is off 4 mph
55 on the speedo = 50 mph.
My suggestion is to just ask an officer to check you. I do it for people... sometimes when they ask sometimes not! LOL

08-24-2004, 06:15 PM	#1
highboy_04 Registered User Join Date: Mar 2003 Location: kansas Posts: 1,071	speedo check? i've been trying to find a place to check my actual speed. i've got a different gear ratio now and need to figure out how off the speedo is. the tires are the same size so i need a calculator to do the ratios. thanks, Steve __________________ '67 c10 350/th350

08-24-2004, 06:23 PM	#3
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	the easy way to do a rough check is go out on an interstate that has mile markers with a stopwatch. with your speedo at a steady 60mph your time between mile markers should be 1minute __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

08-24-2004, 06:50 PM	#5
MACKL Almost done! Join Date: Apr 2004 Location: Woodstock GA Posts: 193	GPS is the way to go. Takes all of the guess work out, no math. It will even give you a reason to buy one. Then it is a tool, not a toy! __________________ The 1970 SWB Project Removed 6cyl manual trans, manual everything V8-350 Auto/PS/PDB Blazer Fuel Tank Behind the Marker Light Fuel Fill

08-24-2004, 07:11 PM	#6
da-burb "Ochre Ogre" Join Date: Apr 2000 Location: Springfield, MN Posts: 3,558	If you are not all that worried about if it was right in the first place, all you will need to do is divide your new ratio by your old ratio. 4.10/3.73=1.10 A 4.10 ratio is 110% of the 3.73 ratio. 3,000 rpms with a 3.73 will be 3,300 rpms with a 4.10 ratio. Your speedo would be off by 10%. 60mph would now be at 66mph on the speedo. 3.08/4.10=.75 if your 4.10 gear was 3300 rpms at 60mph a 3.08 would be 75% of the old 4.10 gear or 2475 rpms. your speedo would be off by 25% You would now be doing 60mph when the speedo says it is doing 45mph. hopefully I didn't turn any of the numbers around, but it should be correct. Here are a couple of other sites I have used to help figure this out before. http://www.novaresource.org/speedo.htm http://www.bgsoflex.com/speedo1.html __________________ Bowtie Truck Stop Inc. Mid-West GM Truck Restoration Parts Supplier Your Key Parts, Auto Metal Direct, Dynacorn, and Goodmark dealer. like us @ www.facebook.com/BowtieTruckStop 1971 C-10 Suburban (Ochre) 1971 K-10 Suburban (Ochre) 1972 C-10 Suburban (Ochre) 1972 K-20 Suburban (Yellow- that just aint right!) Springfield, Minnesota 56087 Last edited by da-burb; 08-24-2004 at 07:13 PM. Reason: typo

08-24-2004, 09:18 PM	#7
smitty62 Registered User Join Date: May 2002 Location: North Carolina Posts: 1,234	You guys are way overcomplicating this--Cdowns has it right. If you are really picky, take the deviation in seconds from 1 minute to travel measured mile (+ 0r - seconds) divide that by 60 for the percentage it deviates--that's the percentage your speedo is off. IE: 58 sec. = 2 seconds deviation, div. by 60 = 3% fast or 61.8 actual mph. Close enough for government work and you never have to get out of cab or buy a $200+ piece of equipment! __________________ Stepsides look like real trucks! Smitty

08-24-2004, 06:18 PM	#2
toddtheodd The oddest Todd around Join Date: May 2004 Location: Miami, Florida Posts: 1,418	Try this. Todd

08-24-2004, 06:40 PM	#4
Gee_Emm Account Suspended Join Date: Sep 2000 Posts: 4,002	Get a buddy that has a GPS and go for a ride

08-24-2004, 10:14 PM	#8
highboy_04 Registered User Join Date: Mar 2003 Location: kansas Posts: 1,071	alright thanks for the help guys according to that site i am suppose to be going 80 at 3000 rpms with the 3.08 gears but with 3.73 i'm only goin 69.9. thanks, Steve __________________ '67 c10 350/th350

08-24-2004, 10:20 PM	#9
zooza Registered User Join Date: Jul 2004 Location: LINDEN, TN. Posts: 260	I checked mine today , parked my crown vic. beside the road and showed the wife how to lock the radar. Then I drove by at 55 mph. she locked it when I flashed my headlights. Found that the 95 mustang gt is off by 5 mph and the 72 c-10 is off 4 mph 55 on the speedo = 50 mph. My suggestion is to just ask an officer to check you. I do it for people... sometimes when they ask sometimes not! LOL



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