Speedo Off After Rear End Replaced

[GMDriver4ever]

I have a 2002 Yukon XL 4WD that had the rear end inerds come apart. It had a GT5 4:10 ratio. I searched and found a used one at a local bone yard.

The old came out easily and the replacement went smooth. However, I recently discovered while on a trip and using my GPS that my speedometer is off. I timed the speedometer at various speeds to the GPS and here is what I found.

When the speedo reads ...........The GPS Speed is...............

31 35

40 45

49 55

53 60

57 65

66 70

If the rear end I got from the boneyard is the same 4:10 ratio wouldn't the speedometer read the same as with the old rear end? Is it possible they sold me the more common 3:73 rear end? What can I look for on the rear end itself to identify it as a 4:10 ratio?

 

BTW I have the front drive shaft out because I am going to replace the outboard axles and need to replace the Transfer case motor. Thanks for your help.

 

[MikeNH]

Just looking at the numbers I'd say it's plausible you got the 3.73 axle. Use this tool to help sort it out:

 

http://www.summitracing.com/expertadviceandnews/calcsandtools/gear-ratio-calculator

[Silverado4x4]

Looks to me also they sold you a 3.73 instead of the 4.10 just by looking at your numbers.

Without ripping it back apart jack the rear up with wheels off the ground mark the driveshaft have someone spin the passenger side wheel 1 turn (may have to have someone hold the other) and count how many time the driveshaft went around. If shaft went around 3 and 3/4 turn then its a 3.73 a little over 4 turns of the shaft then its a 4.10 which I dough.

 

Or you can go here if you rip it apart and count the teeth on the pinion and then ring gear. http://www.csgnetwork.com/rnggrpinioncalc.html

 

Did you save the old ring and pinion gears? You could go by the teeth count on that also.