When I installed larger 31" tires on my Blazer, I wondered how much it affected the actual speed of the vehicle. I used the GPS I normally carry in my work truck to determine the difference vs the speedometer on the Blazer.
At 100 Kmh or 60 Mph on the speedometer I registered roughly 105 Kmh or 63Mph on the GPS. This is not enough of a difference to land anybody with a speeding ticket. But it did bring two questions to mind:
1 - How accurate is a common GPS unit (Garmin Nuvi) when used as a speedometer?
2 - How accurate is the speedometer on a Blazer anyways?

 

A GPS should be very accurate. It's measuring actual distances and time elapsed rather than counting rotations and calculating speed.

I'm not sure how accurate the stock speedometer is though. I upgraded from 235/70r15 to 31x10.5x15 and used my Garmin and iphone to see how far off my speedometer was and it identical to both GPS readings

 

I use a gps everyday. My speedo died AGAIN so i said screw it and i use my gps.

 

lol wow chris your cluster broke again ?

 

Eh i dont know whats wrong. i think the actual cluster died. I did the conversion and rechecked my wiring and it was good.

 

wow are you going to get it repaired ? or find one at a junkyard

 

Im going to steal the one out of my dads,see if its the issue then give it back to him lol. Ive just been to lazy to do it.

 

haha your dads 92 has a analog one ? or digital ?

 

My TomTom usually says I'm going 1 or 2 miles per hour faster than my speedo does. and I have stock tires.

 

John he has analog,so at least i can see if its the cluster its self.