Speedo doesn't need correction

[Romer]

My truck came with 275'son18 inch tires which equates to a 33.2". Interesting that according to my GPS, my speedo is correct. I asked the PO if he corrected the speedo and he said he didn't

Curious. I wouldn't have thought it calibrated to the nav GPS.

Thoughts?

 

[Trunk Monkey]

I've wondered the same thing. My 99 the speedo should be off similarly to what my 80 was, which was 3-4 mph, but it's only off about 1.

 

[CappyKD]

Hmmm, I'm non-nav and I have 33's my speedo is off 5-6 MPH. Hmm.

 

[hoser]

I looked into this awhile back and my recollection is that car manufactures are allowed to be something like 10% over on their speedometer readout but 0% under. Since speedometers seem to be +/- 6% by nature, all vehicles will read over actual speed.

 

[Romer]

I looked into this awhile back and my recollection is that car manufactures are allowed to be something like 10% over on their speedometer readout but 0% under. Since speedometers seem to be +/- 6% by nature, all vehicles will read over actual speed.

If thats true, then the bigger tires where the speedo correction

It's hard to tell if it's Dead on, but its close enough when I compare it to a GPS or the Police mobile radar units (Your speed is ...) that I can't tell a difference

 

[neoworm]

I'm running the 275's on 18's as well. My speedo reads 6mph low.

 

[hoser]

I'm running 4.88/4.3= 13.5%
My tires are 35.1/31.3(stock)= 12.1%

My speedo is currently dead on with my GPS with no correction.

Here's a read:
Speedometer Scandal! - Feature - Auto Reviews - Car and Driver

 

[Romer]

I'm running 4.88/4.3= 13.5%
My tires are 35.1/31.3(stock)= 12.1%

My speedo is currently dead on with my GPS with no correction.

Here's a read:
Speedometer Scandal! - Feature - Auto Reviews - Car and Driver

Interesting read. Thank you

Guess Im just luck it all works out

 

[arbetrader]

I have 275/70/18, and my speedo is off around 5-6mph at 60mph. Granted, it was out by a couple already before the tire change.

 

[RobRed]

Lets remember that tires in the same stated size are not dimensionally the same across models/brands. Air pressure plays a role as does wear.

In my case on the 285/75-16 Duratrac, new, 44 psi; 69MPH indicated equals 73.4 GPS. Test sustained for 3 miles.

According to Tire Rack specs;

Duratrac 285/75-16: 634 revs per mile / 32.8 diameter

Michelin LTX 275/70-16: 666 revs per mile / 31.3" diameter

Duratracs are 4.8% less revs per mile than stock.

GPS test: Speedo is 6% low versus GPS

 

[1loudLX]

I'm running 4.88/4.3= 13.5%
My tires are 35.1/31.3(stock)= 12.1%

My speedo is currently dead on with my GPS with no correction.

same.

 

[bkgee]

...Air pressure plays a role ***as does wear***.

This is a big factor. The most common tire on this forum, BFG AT 285/75R16 comes new with 18/32 inches of tread, and the wear bars are at 2/32. That's 3% difference in circumference... or 1.5 mph difference at 50 mph gage.

I'm not sure how much air pressure plays, as the 10 ply of steel should be pretty inelastic... even though the tire will probably always measure less than its effective diameter when parked, it might float up to near its effective diameter when its rolling at speed?..

 

[Romer]

Looks like I was all wrong on this. Just did a trip to the Maze and back and with the GPS builtinto my iPAD, it showed I was 6% consistantly faster than what the speedo showed

 

[Layonnn]

I am running 295BFG ats and compared to a Garmin my speedo is ~5 off at 70.

 

[Anthony.L]

Looks like I was all wrong on this. Just did a trip to the Maze and back and with the GPS builtinto my iPAD, it showed I was 6% consistantly faster than what the speedo showed

That sounds more like it. Mine is 7% off and I've seen 6-7% as very common here on the forums the past years.

Instead of going through the complicated and expensive task of correcting the OEM speedo I just drive by the speedo on my Scan Gauge with correction built-in.

 

[Layonnn]

Instead of going through the complicated and expensive task of correcting the OEM speedo I just drive by the speedo on my Scan Gauge with correction built-in.

Same.