Just wondering what you guys are using for stock tire size on the smarty jr. I have 265/70r17's which converts to 31.6." I put that in the program and a speed radar sign clocked me at 63 when my speedo said 65. I want to double check with DTR to see who's wrong.

Thanks

 

Park the truck on good concrete with the tires at the hot tire pressure you want. Measure the center of the tire height, then plug it in the smarty.

 

Almost...but you forgot to multiply by 2.

For reference, my 315s are actually 33.2" instead of the 34.8" the manufacturer lists. My stockers were input in the system as 30.4", and my speedo was 1mph slow by LIDAR.

 

Huh? Maybe I'm not following...

I measure the height of my tires, they are 37's, but measure 35 3/4 on the truck, so I put in 35.75, and I'm pretty dang close...

 

He thinks you meant "measure from the center to the top of the tire" As in center of the rim to the top and multiply by 2 to get the true height.

 

Oh. My bust. I measure from the ground in the center of the bottom of the tire to the center of the top of the tire.

Maybe I should go take a picture. I am a Marine, after all... haha

 

The top half of the tire isn't necessary for figuring speedometer settings. If you measure from the ground to the center of the hub you get the static loaded radius (SLR). Multiply the SLR by two, and you'll get the overall diameter seen by your odometer/speedometer. The top half of the tire has no load and is going to be taller than the bottom half, which can screw up calculations.

Of course, things get really crazy when you start to get wide tires on marginally skinny rims...they have a tendency to balloon outward at higher speeds (think dragstrip funny cars) and change the overall rolling diameter resulting in a significantly different rotational load rating. This usually isn't a concern until you hit the 40" tall/14" wide range, and can really only be properly measured by driving the vehicle with chalk stripes at the desired speed, measuring the distance between marks and then using math to determine the diameter seen by the speedo.

 

Now that's good info Adam.

 

Throw this idea in the mix.....

No matter how squished the tire is, even if it is sagged down 6 inches, the circumference is the same. An oval can have the same circumference as a circle, but the distance from the center point will be much less at the closest point of an oval.

So...... a single revolution of a squished tire should be the same distance as a revolution of a round tire. Unless your tread is folding over, the circumference never changes. Over- Inflation will not stretch the steel bands.

The speedo is calibrated by the revolutions of the drive line. Inputting the tire size tells the speedo how far you have travelled when the driveline has made one revolution. Insert a tall tire / large circumference and the speedo indicates you travelled a longer distance per revolution and a faster mph.

While there are many good ideas on where to start, revs per mile is where the rubber hits the road.

To really make matters confusing, your speedo is probably off anyway, and as your tire wears (up to an inch at least! ) your indicated speed will change. This leads us right back to using a good starting point and then tweaking from there to get it spot on.

 

You make a very valid point. Taking these thoughts into condsideration, I now wonder if the best way to measure the diameter of the tire would be to measure it at its widest point.

I believe the most precise way to check the accuracy of your speedometer is to compare the miles driven shown on your trip odometer to mile markers on the road.

Example: at mile marker 31, you reset your trip to 0. You drive to mile marker 131. You have driven 100.0 miles. If your trip shows 102 or 98, obviously your tire size is incorrect.

With an error this small, you may not see a discrepancy between your speedometer and your speed on a GPS. Most of you probbaly don't care about such a small error, but I like mine to be as accurate as possible so that I get the most accurate MPG number when calculating fuel economy by hand. In this example, you are off by 2%. This means that your MPG calculation will be off by 2% also. So if you calculate 20 MPG, your actual MPG is 20.4 if you're off by -2%, or 19.6 if you're off by +2%.

Of course, all of this assumes 100% accuracy of the mile markers, which I'm not sure is a reasonable assumption. Just my $.02.

 

I take the over all height from the ground to the top of the tire and put that in. Then make adjustments until I am dead on with my GPS.

 

That sounds about right, I measured 30.5" on mine but the book said 31.6." It felt too fast when I had it set at 30.5" and I didn't want to ask the cop how fast I was going because at that point its too late .

I'll try it closer to 30.5" and see where I'm at.

Thanks for the help

 

My watch has a chronograph on it, so I can check my speed relative to the mile marker signs with that. The signs are not always perfect, but they are darn close most of the time.

I usually like to keep a steady speed around those radar signs we have all around Wisconsin. I lock in 30mph and see what the sign says. Remember, radar isn't exact all the time either.... But my gosh, how close do I need to be???

My wife gets at least one speeding ticket a year, so it's not a bad thing when my truck reads 3mph faster than actual, except I'll rack up a few more miles than I actually travelled....

 

I have mine set on 30.7 and speedo is right on with GPS.
Diameter drops as the tire wears too.

 

Mighty find reading, and lots to consider.

But if I may mention this ... when I was a kid, a cop told me to set my speedo at a constant 60 mph, (some cars did not have cruise control then). Using a stop watch, time one mike on your speedo.

You should travel 'one mile' at '60 miles per hour' in 'one minute'.

I have done this for years and twice had vehicles checked and were on the money.

When I increased my tires on this truck, using my Smarty, it took three adjustments for set it right.

Might be worth looking into ?