Calibrating speedometer??? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Derrickbwg · 10-18-2005, 03:08 PM

I'm using 275/60/15 tires and my speedo shows about 9 miles an hour less than actual speed (as referenced by other vehicles). Is there a way to calibrate the unit to show actual speed?

hgs_notes · 10-18-2005, 03:19 PM

You can try this...http://www.chevytalk.com/tech/transm...calibrate.html

HG

**kwmech** · 10-18-2005, 03:38 PM

9 miles an hour at what speed? Speedo calibration works on a percentage. May be as simple as a gear swap in the back of the trans

Derrickbwg · 10-18-2005, 05:49 PM

at 55 and 65. I paced a car going 55 then told them to go to 65, I didn't see much difference (still about 9mph) between the two error wise. Maybe I wouldn't notice that small a diff between 10mph.

**pjmoreland** · 10-18-2005, 06:27 PM

Do you know what gear ratio you have in your rear end? It might be listed on your glovebox door options sticker. What kind of transmission do you have?

You will need to replace the speedometer drive gears located on the output shaft of your transmission, as already suggested by "hgs notes".

Derrickbwg · 10-18-2005, 06:40 PM

I'm not sure of the gearing, the tranny is a 350.

modifiedGMC · 10-18-2005, 09:33 PM

One way I've found to find the percentage spedo is off is get on a road you can drive 10 miles between mile marker posts (that way you KNOW you went 10 miles). note your odometer starting and the mile marker starting. go until you have gone exactly 10 marker miles. note the distance traveled on your odometer. multiply 10 X odometer distance = percentage of accuracy example: 10 marker miles X 7.5 odometer miles = 75% accurate. You can use this to "gauge" your speed by your speedometer. if you don't want to exceed say 65 MPH and your speedo is 75% accurate then multiply 65 X 75% = 48.75 MPH on your speedometer. Before anyone says "why do all that just calibrate it

" This is just some info I figured out after driving my truck the first time after changing out the rearend. I was driving about 50 or 55 MPH but was keeping up with people doing 65+. and I would get bored so I started checking mileage and figuring percentage of accuracy and came up with the above info. It will help you figure out how much to change the speedometer cable drive gear and/or driven gear inside the tranny.

10-18-2005, 03:08 PM	#1
Derrickbwg Registered User Join Date: Sep 2005 Location: Lexington KY Posts: 64	Calibrating speedometer??? I'm using 275/60/15 tires and my speedo shows about 9 miles an hour less than actual speed (as referenced by other vehicles). Is there a way to calibrate the unit to show actual speed? __________________ 1971 Chevy swb Chevy trucks and import cars Last edited by Derrickbwg; 10-18-2005 at 03:09 PM.

10-18-2005, 03:19 PM	#2
hgs_notes GEARHEAD Join Date: Aug 2003 Location: MN Posts: 6,122	Re: Calibrating speedometer??? You can try this...http://www.chevytalk.com/tech/transm...calibrate.html HG __________________ If no one knows what you're doing, they can't tell you you're doing it wrong HG's Dream Car Build - Shelby Cobra Kit Build your own adjustable track bar 71 Long Fleet C/10 72 Jimmy 4x4 (Junkyard Jimmy) HG's Plow Truck '78 K35 Dually Bigass Farm Truck HG's thread of miscellaneous stuff

10-18-2005, 03:38 PM	#3
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,499	Re: Calibrating speedometer??? 9 miles an hour at what speed? Speedo calibration works on a percentage. May be as simple as a gear swap in the back of the trans

10-18-2005, 05:49 PM	#4
Derrickbwg Registered User Join Date: Sep 2005 Location: Lexington KY Posts: 64	Re: Calibrating speedometer??? at 55 and 65. I paced a car going 55 then told them to go to 65, I didn't see much difference (still about 9mph) between the two error wise. Maybe I wouldn't notice that small a diff between 10mph. __________________ 1971 Chevy swb Chevy trucks and import cars

10-18-2005, 06:27 PM	#5
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,426	Re: Calibrating speedometer??? Do you know what gear ratio you have in your rear end? It might be listed on your glovebox door options sticker. What kind of transmission do you have? You will need to replace the speedometer drive gears located on the output shaft of your transmission, as already suggested by "hgs notes".

10-18-2005, 06:40 PM	#6
Derrickbwg Registered User Join Date: Sep 2005 Location: Lexington KY Posts: 64	Re: Calibrating speedometer??? I'm not sure of the gearing, the tranny is a 350. __________________ 1971 Chevy swb Chevy trucks and import cars

10-18-2005, 09:33 PM	#7
modifiedGMC Registered User Join Date: Oct 2004 Location: Walker Co. Alabama Posts: 474	Re: Calibrating speedometer??? One way I've found to find the percentage spedo is off is get on a road you can drive 10 miles between mile marker posts (that way you KNOW you went 10 miles). note your odometer starting and the mile marker starting. go until you have gone exactly 10 marker miles. note the distance traveled on your odometer. multiply 10 X odometer distance = percentage of accuracy example: 10 marker miles X 7.5 odometer miles = 75% accurate. You can use this to "gauge" your speed by your speedometer. if you don't want to exceed say 65 MPH and your speedo is 75% accurate then multiply 65 X 75% = 48.75 MPH on your speedometer. Before anyone says "why do all that just calibrate it " This is just some info I figured out after driving my truck the first time after changing out the rearend. I was driving about 50 or 55 MPH but was keeping up with people doing 65+. and I would get bored so I started checking mileage and figuring percentage of accuracy and came up with the above info. It will help you figure out how much to change the speedometer cable drive gear and/or driven gear inside the tranny. __________________ If it don't fit, force it,...If it still don't fit , modify it 'till it does fit !!!



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