Tire size,rear end, and speedometer - The 1947 - Present Chevrolet & GMC Truck Message Board Network

xb9rguy · 11-30-2011, 04:35 PM

So my speedometer is off by between 10-15 mph over 60mph. My tire size is 275/60/15 on all four corners and if I read right on another site that converts from original size to current I should only be about 1-2% off which unless i'm having a brain fart don't make sense as to why my speedo is so far off. So i'm wondering if someone in the past has changed out the gears in the rearend if that would make a difference and if so anybody have any idea what may be in it? Oh ya and it's in a 72 1/2 ton with 3 spd manual and sbc. Thanks for any help guys. I can't afford a gps speedo right now so I would like to figure out a cheap way to maybe get my speedo close to right.

domeier · 11-30-2011, 05:04 PM

Quote:

Originally Posted by xb9rguy

I can't afford a gps speedo right now so I would like to figure out a cheap way to maybe get my speedo close to right.

Have your ladyfriend drive alongside you on the freeway and give you a thumbs-up when she hits 60. Mark your speedo with a grease pencil.

Chris4x4Gill2 · 11-30-2011, 05:53 PM

you can figure your rear end ratio:

Jack up rear end so that tires are off the ground.
Mark a line on the driveshaft, another on the tire.
Put in nuetral and rotate the tire 1 complete revolution, count the number of times the driveshaft turns.

xb9rguy · 12-01-2011, 05:11 AM

Quote:

Originally Posted by domeier

Have your ladyfriend drive alongside you on the freeway and give you a thumbs-up when she hits 60. Mark your speedo with a grease pencil.

I would but the last lady friend I had is the reason I'm broke now

Andy4639 · 12-01-2011, 09:44 AM

The old speedo's in these trucks are a good guestment at best. Mine in my 71 never did read right. I used it up until back in August when I bought a GPS one. Have you got a GPS Garmin. If so use it on the map setting and it gives you speed. It's a easy way to check the truck.

The spring inside the speedo after 30 some years is wore out. I suggest a rebuilt one are a new one may or may not help you.

11-30-2011, 04:35 PM	#1
xb9rguy Registered User Join Date: Feb 2010 Location: Tacoma, Washington Posts: 86	Tire size,rear end, and speedometer So my speedometer is off by between 10-15 mph over 60mph. My tire size is 275/60/15 on all four corners and if I read right on another site that converts from original size to current I should only be about 1-2% off which unless i'm having a brain fart don't make sense as to why my speedo is so far off. So i'm wondering if someone in the past has changed out the gears in the rearend if that would make a difference and if so anybody have any idea what may be in it? Oh ya and it's in a 72 1/2 ton with 3 spd manual and sbc. Thanks for any help guys. I can't afford a gps speedo right now so I would like to figure out a cheap way to maybe get my speedo close to right. __________________ Jeramy CURRENTLY OWN, 72 c10 project/driver 2010 Kia Forte Koup SX daily driver 88 cherokee wheeler and dog hauler 2001 drz 400 Miss my 2004 firebolt (stock) and 2001 superhawk with a ton of modifications and many other previous vehicles.

11-30-2011, 05:53 PM	#3
Chris4x4Gill2 Registered User Join Date: Apr 2006 Location: Alabama Posts: 794	Re: Tire size,rear end, and speedometer you can figure your rear end ratio: Jack up rear end so that tires are off the ground. Mark a line on the driveshaft, another on the tire. Put in nuetral and rotate the tire 1 complete revolution, count the number of times the driveshaft turns. __________________ "Lifetime" 72 C10 Project

12-01-2011, 09:44 AM	#5
Andy4639 Old member Join Date: Jan 2002 Location: Liberty, & Garden City S.C. , U.S. Posts: 19,936	Re: Tire size,rear end, and speedometer The old speedo's in these trucks are a good guestment at best. Mine in my 71 never did read right. I used it up until back in August when I bought a GPS one. Have you got a GPS Garmin. If so use it on the map setting and it gives you speed. It's a easy way to check the truck. The spring inside the speedo after 30 some years is wore out. I suggest a rebuilt one are a new one may or may not help you. __________________ 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.



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