How close is your speedometer? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

pritch · 05-19-2013, 07:15 PM

When mine is on 60 I am doing more like 64-65. I base this on several of those radar speed signs they have before sharp turns on the Interstate. I know these are probably not the most accurate radars, but there are 3 of them that I drive past routinely and they always agree, always the same.

Point is, I need to get new tires soon. Would I need a little taller tire or a little shorter to dial in my speedo?

rogergodding · 05-19-2013, 07:21 PM

I'd say a little shorter.... mine is right at 10 mph off, but that didn't change with tire size...

hyatt · 05-19-2013, 07:28 PM

If you have a I Phone there is a free speedometer app It will show you exactly how fast you are going

69GWC · 05-19-2013, 07:39 PM

Yeap speedo app, mine is off 15mph with my 33s.
Posted via Mobile Device

Tbruz · 05-19-2013, 08:02 PM

Once you fgure out how fast or slow your Speedo actually is via phone app, pace car or GPS, you can add a speedometer ratio adapter for about $40 between the end of your speedo cable and where the speedo cable screws into your tranny housing.
Tell them your actual speed and what the speedo reads(or distance for that matter) and they can build proper ratio adapter.
Did it with my Blazer, speedo and GPS reading are now within 1%
Thanks
Tbruz

Indyuke · 05-19-2013, 08:10 PM

Mines about 1-2 mph off at 60... very close. But I have the truck set up pretty close to stock.

**MidLifer** · 05-19-2013, 08:11 PM

You can also change the speedometer gear - here is a calculator to help figure out which gear you need:

http://www.tciauto.com/tc/speedometer-gear-calculator/

turp mcspray · 05-19-2013, 08:19 PM

FYI...On a th350, or any other trans with the small driven gear, 1 tooth difference = 1/2 tenth on the odometer. (or 5% change) On a th400, or anything else that uses the large driven gear, 2 teeth = 1/2 tenth.

**Already Gone** · 05-19-2013, 10:38 PM

I buried my speedo one tme and figured I couldn't be going be going that fast cause the truck rode and handled perfectly. Got the GPS out and with my speedo showing 95mph the GPS showed just under 94.
Posted via Mobile Device

Andy4639 · 05-20-2013, 08:04 AM

Well if you buy a speed hut GPS speedo from them you will be spot on no matter what size tire. gear, transmission you have. I have a full setup in my 71 and love them.
U can also just buy the GPS speed o and hook the hot wire and ground up and your done. Instant speed directions 1/4 mile speeds

pritch · 05-20-2013, 09:07 PM

I may be out of luck. No smart phone or GPS. Oh well.

**LockDoc** · 05-20-2013, 09:18 PM

-

with what Andy4639 said.... Here is the install of mine. Doesn't matter what tire, rear ratio, trans, or anything else you are running it is always right on the money...

http://67-72chevytrucks.com/vboard/s...d.php?t=421745

LockDoc

pritch · 05-21-2013, 08:49 PM

That is a good idea. Now I remember your original thread on this. I might do something like this on my Panel, but for my beater truck, I don't know if I want to go to the trouble. Thanks, for the reminder, though

**special-K** · 05-21-2013, 09:40 PM

How close is my speedometer? From where,my eyes? I'd have to measure,but prolly right about 24" Is my guess.

Ok seriously,mine have always been accurate till I go to bigger tires,such as on my 4wds.

bugmasta · 05-22-2013, 12:18 AM

Don't know, not connected. Bounces all over the place when I connect the cable. Just try and go with the flow of traffic and make sure some one is always passing me.
Posted via Mobile Device

AustinScott · 05-22-2013, 12:47 AM

My speed-o was close -5mph when I had the TH400 and 3.08 setup.
Then I changed to a TH2004r and 3.73.. my speedo was a bit off.. +10-15 mph.
When I did my valve body & governor swap on the 2004r I got the correct speedo gear for my current setup.. Man now the thing is perfect at all speeds... well within 1 mph when tested against I-phone app.

I had to swap from RED TO GREEN on the trans gear... pretty easy fix if you ask me

Scott

**70cst** · 05-22-2013, 06:40 AM

Good thread. I never thought about this

Andy4639 · 05-22-2013, 06:51 AM

www.speedhut.com has a small speedo also. I have been thinking about buying it for my golf carts so I can swap it out to whichever cart I'm using at the time. It's like a tach mounted on the steering column.

**special-K** · 05-22-2013, 08:01 AM

Speedos don't get off by a set amount of MPH,they get off by a percent. So the mph difference increases with speed. When I put 37" tires on my 4.10 truck it reads 10% off,or says 50 when I'm doing 55. I pass 9 mile markers and my odo says 10 miles.

I got pulled over for doing 40 in a 25 going downhill the first night I put the tires on. I told the officers that and that I hadn't had a chance to figure the difference it made or have my speedo recalibrated. We discussed how it worked and the one was saying it would be off by a set amount of mph and I said the number would increase with speed. They offered to follow me to this less traveled road where they would have me drive by at 25 and they'd read my actual speed. So,I drove by doing about 36 on my speedo and they got about a 40 mph reading and let me off the hook

That was back when no one had tires that big yet.

Mr Handy · 05-22-2013, 09:09 AM

Quote:

Originally Posted by special-K

Speedos don't get off by a set amount of MPH,they get off by a percent. So the mph difference increases with speed. When I put 37" tires on my 4.10 truck it reads 10% off,or says 50 when I'm doing 55. I pass 9 mile markers and my odo says 10 miles.

I got pulled over for doing 40 in a 25 going downhill the first night I put the tires on. I told the officers that and that I hadn't had a chance to figure the difference it made or have my speedo recalibrated. We discussed how it worked and the one was saying it would be off by a set amount of mph and I said the number would increase with speed. They offered to follow me to this less traveled road where they would have me drive by at 25 and they'd read my actual speed. So,I drove by doing about 36 on my speedo and they got about a 40 mph reading and let me off the hook

That was back when no one had tires that big yet.

That's funny!
Mine is off about 7mph at 60/70
I downloaded one of the iphone apps and I paced my wife home one night...
I think when I put new tires on I'm going to go to 235/75 r15. If my math is right that should shorten the gap by about 2mph and put me 5 under at 70ish. I have worn out 225/75s on right now.

bugmasta · 05-22-2013, 02:48 PM

Quote:

Originally Posted by special-K

Speedos don't get off by a set amount of MPH,they get off by a percent. So the mph difference increases with speed. When I put 37" tires on my 4.10 truck it reads 10% off,or says 50 when I'm doing 55. I pass 9 mile markers and my odo says 10 miles.

I got pulled over for doing 40 in a 25 going downhill the first night I put the tires on. I told the officers that and that I hadn't had a chance to figure the difference it made or have my speedo recalibrated. We discussed how it worked and the one was saying it would be off by a set amount of mph and I said the number would increase with speed. They offered to follow me to this less traveled road where they would have me drive by at 25 and they'd read my actual speed. So,I drove by doing about 36 on my speedo and they got about a 40 mph reading and let me off the hook

That was back when no one had tires that big yet.

did the exact same thing once with the local ticket writer after putting 35's on my bronco. let me go with a fixit ticket to have the speedo recalibrated.

srvivor71 · 05-22-2013, 04:23 PM

Quote:

Originally Posted by 69GWC

Yeap speedo app, mine is off 15mph with my 33s.
Posted via Mobile Device

SWEET! Android has it too. This will come in handy because mine is way off - it shows at least 10 MPH under actual speed.

74baggedbug · 05-22-2013, 07:47 PM

Quote:

Originally Posted by Tbruz

Once you fgure out how fast or slow your Speedo actually is via phone app, pace car or GPS, you can add a speedometer ratio adapter for about $40 between the end of your speedo cable and where the speedo cable screws into your tranny housing.
Tell them your actual speed and what the speedo reads(or distance for that matter) and they can build proper ratio adapter.
Did it with my Blazer, speedo and GPS reading are now within 1%
Thanks
Tbruz

Where did you get the adapter at? Thanks
Posted via Mobile Device

**LockDoc** · 05-22-2013, 08:13 PM

Quote:

Originally Posted by 74baggedbug

Where did you get the adapter at? Thanks
Posted via Mobile Device

Not sure where he got his but they are available here...

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

Plus a lot of other stuff & info on the site.

LockDoc

knomadd · 05-22-2013, 10:18 PM

My GTO had about 5 mph difference. Wound up changing the speedometer gear in the transmission. can't remember if it's the drive or driven gear. It has a TH400, but I found something online that helped me determine which gear I needed. TCI has quite a few different gears to swap yours if that's what you want to do.

If you still want to change tire height, taller is faster, shorter is slower. Check out this website. http://www.rimsntires.com/specs.jsp You can compare a size you have with a size you will consider switching to. It even tells you how much difference the new tire will make in your speedo.

05-19-2013, 07:15 PM	#1
pritch Registered User Join Date: Mar 2004 Location: Hoytsville, Utah Posts: 3,365	How close is your speedometer? When mine is on 60 I am doing more like 64-65. I base this on several of those radar speed signs they have before sharp turns on the Interstate. I know these are probably not the most accurate radars, but there are 3 of them that I drive past routinely and they always agree, always the same. Point is, I need to get new tires soon. Would I need a little taller tire or a little shorter to dial in my speedo? __________________ '68 Panel Project Boogie Chillin' '68 C-10 Ol' Green

05-19-2013, 07:21 PM	#2
rogergodding Registered User Join Date: Aug 2007 Location: Sachse, Texas Posts: 614	Re: How close is your speedometer? I'd say a little shorter.... mine is right at 10 mph off, but that didn't change with tire size...

05-19-2013, 07:28 PM	#3
hyatt Registered User Join Date: Dec 2012 Location: sparta tennessee Posts: 345	Re: How close is your speedometer? If you have a I Phone there is a free speedometer app It will show you exactly how fast you are going __________________ 1946 car hauler 1948 surburban 1937 chevy sedan 1940 olds sedan (project) 1967 ss396 chevelle 1999 dually crewcab(24000 miles)

05-19-2013, 07:39 PM	#4
69GWC Registered User Join Date: Jun 2012 Posts: 1,089	Re: How close is your speedometer? Yeap speedo app, mine is off 15mph with my 33s. Posted via Mobile Device

05-19-2013, 08:02 PM	#5
Tbruz Registered User Join Date: Feb 2010 Location: Windham, NH Posts: 226	Re: How close is your speedometer? Once you fgure out how fast or slow your Speedo actually is via phone app, pace car or GPS, you can add a speedometer ratio adapter for about $40 between the end of your speedo cable and where the speedo cable screws into your tranny housing. Tell them your actual speed and what the speedo reads(or distance for that matter) and they can build proper ratio adapter. Did it with my Blazer, speedo and GPS reading are now within 1% Thanks Tbruz

05-19-2013, 08:10 PM	#6
Indyuke Registered User Join Date: Feb 2008 Location: Lake Stevens, WA Posts: 623	Re: How close is your speedometer? Mines about 1-2 mph off at 60... very close. But I have the truck set up pretty close to stock.

05-19-2013, 08:11 PM	#7
MidLifer Senior Member Join Date: Apr 2011 Location: South NJ Posts: 1,264	Re: How close is your speedometer? You can also change the speedometer gear - here is a calculator to help figure out which gear you need: http://www.tciauto.com/tc/speedometer-gear-calculator/

05-19-2013, 08:19 PM	#8
turp mcspray Registered User Join Date: May 2007 Location: Rapid City South Dakota Posts: 2,359	Re: How close is your speedometer? FYI...On a th350, or any other trans with the small driven gear, 1 tooth difference = 1/2 tenth on the odometer. (or 5% change) On a th400, or anything else that uses the large driven gear, 2 teeth = 1/2 tenth. __________________ Turp Mcspray New life for an old 2wd, farm blazer http://67-72chevytrucks.com/vboard/s...d.php?t=505987 My Blazer build http://67-72chevytrucks.com/vboard/s...d.php?t=342299

05-19-2013, 10:38 PM	#9
Already Gone 70+ ( Old Skool Club ) Join Date: Nov 2003 Location: Saskatoon, Saskatchewan ,Canada Posts: 9,105	Re: How close is your speedometer? I buried my speedo one tme and figured I couldn't be going be going that fast cause the truck rode and handled perfectly. Got the GPS out and with my speedo showing 95mph the GPS showed just under 94. Posted via Mobile Device __________________ 1972 C10 Custom/Deluxe 613 Highlander 406/700R4 1999 White Tahoe LS 4x2 with Z56 Police Package 1992 K1500 GMC Suburban Members I have personally met: MusicMan70 - HeavyD - ChewyChevy67 - StingRay -71SWB4x4 - 67 Burb - DeadheadNM - too much stuff - bc65 - das601 " Circumsatances Do Not Change Responsibility " " The Sky is not the Limit, Your Mind is." Marilyn Monroe .. RIP Charlie Watts 1941 - 2021

05-20-2013, 08:04 AM	#10
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,936	Re: How close is your speedometer? Well if you buy a speed hut GPS speedo from them you will be spot on no matter what size tire. gear, transmission you have. I have a full setup in my 71 and love them. U can also just buy the GPS speed o and hook the hot wire and ground up and your done. Instant speed directions 1/4 mile speeds __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

05-20-2013, 09:07 PM	#11
pritch Registered User Join Date: Mar 2004 Location: Hoytsville, Utah Posts: 3,365	Re: How close is your speedometer? I may be out of luck. No smart phone or GPS. Oh well. __________________ '68 Panel Project Boogie Chillin' '68 C-10 Ol' Green

05-20-2013, 09:18 PM	#12
LockDoc The Older Generation Join Date: Aug 2006 Location: Montezuma, Iowa Posts: 25,394	Re: How close is your speedometer? - with what Andy4639 said.... Here is the install of mine. Doesn't matter what tire, rear ratio, trans, or anything else you are running it is always right on the money... http://67-72chevytrucks.com/vboard/s...d.php?t=421745 LockDoc __________________ Leon Locksmith, Specializing In Antique Trucks, Automobiles, & Motorcycles (My Dually Pickup Project Thread) http://67-72chevytrucks.com/vboard/s...d.php?t=829820 -

05-21-2013, 08:49 PM	#13
pritch Registered User Join Date: Mar 2004 Location: Hoytsville, Utah Posts: 3,365	Re: How close is your speedometer? That is a good idea. Now I remember your original thread on this. I might do something like this on my Panel, but for my beater truck, I don't know if I want to go to the trouble. Thanks, for the reminder, though __________________ '68 Panel Project Boogie Chillin' '68 C-10 Ol' Green

05-21-2013, 09:40 PM	#14
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,862	Re: How close is your speedometer? How close is my speedometer? From where,my eyes? I'd have to measure,but prolly right about 24" Is my guess. Ok seriously,mine have always been accurate till I go to bigger tires,such as on my 4wds. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

05-22-2013, 12:18 AM	#15
bugmasta Registered User Join Date: Aug 2009 Location: North Bend, WA Posts: 272	Re: How close is your speedometer? Don't know, not connected. Bounces all over the place when I connect the cable. Just try and go with the flow of traffic and make sure some one is always passing me. Posted via Mobile Device __________________ 68 c20 build: http://67-72chevytrucks.com/vboard/s...d.php?t=358130 Club longbed ...cause girls like'em long!!!!!



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05-22-2013, 12:47 AM	#16
AustinScott Registered User Join Date: Dec 2007 Location: Southington CT Posts: 548	Re: How close is your speedometer? My speed-o was close -5mph when I had the TH400 and 3.08 setup. Then I changed to a TH2004r and 3.73.. my speedo was a bit off.. +10-15 mph. When I did my valve body & governor swap on the 2004r I got the correct speedo gear for my current setup.. Man now the thing is perfect at all speeds... well within 1 mph when tested against I-phone app. I had to swap from RED TO GREEN on the trans gear... pretty easy fix if you ask me Scott __________________ 68-SWB, 327, hooker headers, spintec mufflers, disk brake 3.5/5 drop, buddy buckets, 200-4R, 2200 stall, 3.73 12bolt Eaton Posi, My 68-SWB Photo Build Thread All my Google Albums

05-22-2013, 06:40 AM	#17
70cst Senior Member Join Date: Feb 2001 Location: New Madison, Ohio Posts: 21,365	Re: How close is your speedometer? Good thread. I never thought about this __________________ A husband can be right...or...A husband can be happy. 67-72 Chevy and GMC Trucks...The Classic Truck for the Classic Folk. 1970 CST Two tone green, 402BB, 400 Automatic, Tach, Buckets, AC, AM-FM, Tilt, GM CB, GM 8 Tract, LWB, etc JOHN 17:3...The better side of "LIFE" Remember: Everyday is a good day...Some are just gooder!

05-22-2013, 06:51 AM	#18
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,936	Re: How close is your speedometer? www.speedhut.com has a small speedo also. I have been thinking about buying it for my golf carts so I can swap it out to whichever cart I'm using at the time. It's like a tach mounted on the steering column. __________________ 1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs.

05-22-2013, 08:01 AM	#19
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,862	Re: How close is your speedometer? Speedos don't get off by a set amount of MPH,they get off by a percent. So the mph difference increases with speed. When I put 37" tires on my 4.10 truck it reads 10% off,or says 50 when I'm doing 55. I pass 9 mile markers and my odo says 10 miles. I got pulled over for doing 40 in a 25 going downhill the first night I put the tires on. I told the officers that and that I hadn't had a chance to figure the difference it made or have my speedo recalibrated. We discussed how it worked and the one was saying it would be off by a set amount of mph and I said the number would increase with speed. They offered to follow me to this less traveled road where they would have me drive by at 25 and they'd read my actual speed. So,I drove by doing about 36 on my speedo and they got about a 40 mph reading and let me off the hook That was back when no one had tires that big yet. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

05-22-2013, 10:18 PM	#25
knomadd Registered User Join Date: Jul 2012 Location: Rosenberg, Texas Posts: 3,506	Re: How close is your speedometer? My GTO had about 5 mph difference. Wound up changing the speedometer gear in the transmission. can't remember if it's the drive or driven gear. It has a TH400, but I found something online that helped me determine which gear I needed. TCI has quite a few different gears to swap yours if that's what you want to do. If you still want to change tire height, taller is faster, shorter is slower. Check out this website. http://www.rimsntires.com/specs.jsp You can compare a size you have with a size you will consider switching to. It even tells you how much difference the new tire will make in your speedo. __________________ -Duane C/10 Club Texas member 1971 Chevy C10 "Brutus" SWB Fleetside 2016 6.0L (L96) 6L90E (driver/project) 1965 Pontiac GTO "Royal Goat" 400/TH400 w/AC (my baby)

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