Is MPH accurate if all filled in information is correct. Trying to figure out how far my I'm off and who's lying,  my speedo, GPS or quad. 

My truck info 6spd, 3.55, 285/70/17 Tire circumference comes out to 102.7 in. I have to have it at 97 in. to match my speedo at 70mph, GPS says I'm going 70.7 mph. If i set it to 102.7 quad will jump to 75 mph. 

My vote is with the GPS.  Doesn't make any difference what tire size you are wearing or gearing you are running.  Adjust the tire diameter input as needed to make the speedo match the gps reading  and you will be good.

It's fuzzy math in the quad.  Compare to the GPS.

Why would it be fuzzy math in the Quad? 

unless you go out there and measure actual circumference then it is kind of a guess as to tire size vs circumference

I bumped mine to 98 it's about perfect now. I'm happy. Funny thing is I tried GPS on my Honda and if I'm going 65 GPS is telling me I'm going 61 so somehow it's off even with correct size tires. Truck was opposite lol.

So putting it to 98 has to matching gps with 285/70/17’s?

Mines been pretty damn close since I set it to 98

41 minutes ago, notlimah said:

So putting it to 98 has to matching gps with 285/70/17’s?

Seems to be really close +-1/2 mile play with it a bit to get it where you think it needs to be. I still don't know if I want to trust GPS fully.

40 minutes ago, Dieselfuture said:

Seems to be really close +-1/2 mile play with it a bit to get it where you think it needs to be. I still don't know if I want to trust GPS fully.

 

You could always test with another GPS if you're worried, but to my knowledge, there aren't many methods of land speed measurement that are more accurate than GPS. Even radar can come up short if the angle is not 100% perfect.

Edited April 5, 20187 yr by kzimmer

Ok, I was thrown off because for that tire size it says circumference is 102.7. I adjusted to 98 and I’ll go by the nearest speed trap area and check it lol

Ok did my speed trap run and the posted sign said I was 2-3 faster then speedo and quad said. Bumped it 99.5 and still 1-2 slow so I think I’m going to be good at 100.5. Weird that we’re that far off with same size tires. 

Some speed signs are off by few ml too. 

True, but when I matched my stock speedo using the smarty it was right on, so I think it'll be close enough. At least I can try and argue that my speeds were close enough to what the sign said they were! lmao

If remember right @Me78569 mentions that the Quadzilla does some fuzzy math to get the speed number. Like my truck I'm running 99.7" tires and it 2-3 MPH off. The fuzzy math is a bit off for sure. 

it is a trial and error thing to get it perfect.  Your tire might be 31.2512" high vs the calc of 31.25"  that will result in a noticeable difference in what you need to input into the quad.  .