Speedometer Question

[varmint2506]

Hey guys, I have a calibration question.

 

Whenever I pass one of those portable radar speed limit units, my speedometer reads correct. (within 1 or 2 mph) I have gone from a 265 to a 315 tire. I never calibrated anything. I had just figured it was off about 5 mph or so. I have never checked it at speeds over 55 though. I know it only reads 9 tenths for every mile according to mile markers on the interstate. If I'm only reading 9 tenths for every mile, wouldn't the speedometer have to be off?

 

I went from a 245 to a 265 on my Tahoe also. It reads perfect on the radar and the mile markers.

[Shaners]

It's hard to tell exactly with the mile markers along the interstate.  Check it at the 5 mile mark (you should read 4.5 miles) and then check it at the 10 mile mark (you should read 9 miles) according to your "short" calculations.  If at the 10 mile mark you have gone only 9.5 miles, then your speedo is off 5%.

 

So if you are off 5% and you drive by a radar sign that states you are going 40 MPH, then your speedo would register around 38 MPH.

[City_Slicker]

Just speed by a highway patrol car right at 100 and see what he clocks you at.

 

Oh wait dont do that, he will probably say 120!!

 

 

 

[z71ff]

I still have the factory tires, and my speedo reads about 3 mph high at 70-80 according to GPS.

[varmint2506]

I still have the factory tires, and my speedo reads about 3 mph high at 70-80 according to GPS.

I was wondering about that myself. Before I changed tires I noticed that people passed me a lot even though I was running over the limit. Seemed like mine was reading high also.

 

Good idea on the GPS.  :thumb:

[Wingnut]

You should also take into account that those units sitting on the side of the road probably don't get calibrated nearly as frequently or as well as the ones uses by policemen and troopers.

 

I wouldn't put much faith in their accuracy down to an exact mph.