Tire Size

[Longshot270]

I know, aren't these numbers fun?

I used pegasus and inputted the same amount of squish as my truck and I got about 2380. Your tires are slightly larger than mine (235/70-17) and mine come pretty close to 2332. I think that number is about 1/10 of a mile off at 25 miles.

FYI, I used about 5% tire squish on pegasus. That program is set up a bit different compared to the hand calculations.

I have also had issues with mapquest, google has been right on target when I zoom in and manually pick my two points and the route to them. If you type in two addresses there is sometimes error from the points not matching up to the gps they used to map it.

[Paw Paw]

Thanks JackandJanet . I have Michelin LTX M/S 2 tires . Tire Rack says the diameter is 32.1 " and Revs per mile is 648 .
Thanks for all the help.
PAW PAW

[Jackpine]

Great! You found the page I was referring to. The 32.1" diameter is with no weight on the tire (and you need this value to check for clearance at the top of the wheel well or at the sides when the wheels are cornering). The revs/mile figure IS with weight on the tire and works out to a TS of 2482mm. I think that is going to be VERY close to the number that would be dead accurate.

- Jack

[Paw Paw]

Thanks JackandJanet . I am at work right now and do not have my figures in front of me . I will check my numbers when I get home and get back with you .
THANKS AGAIN FOR ALL THE HELP !!!!
Paw Paw

[Paw Paw]

OK , JackandJanet. I made it home and found my work sheet that had the figures on it.
WHOO HOO ! JackandJanet, you are exactly right . I found the conversion factor for miles to mm and did the math . I divided the mm per mile by the revolutions per mile and came up with 2483.
I did not know that the revolutions per mile in the chart already had the tire squish calculated in. Thanks for the information .
THANKS AGAIN - JACKANDJANET

PAWPAW

[Jackpine]

You're most welcome! Again though, this is really a starting place (that is pretty close). Your truck may weigh more or less than the weight the tire manufacturer assumes for their table. You may also inflate your tires higher or lower than the cold inflation pressure they assume.

And, even speed has an effect! At 75 mph, the speed limit on the interstates here in AZ, the centrifugal force (and heat) on the tire will cause it to "expand" and increase its "rolling radius" compared to the same tire driven at a more sedate speed of say 55 mph. You can see this "radius expansion" quite easily when a drag racer does a "burnout" to condition his tires. Of course that is an extreme rotational speed, but there will be an effect like this even at lower speeds. This is one reason I don't care for the "measure one revolution of the tire on the driveway" method. I think the result will be low.

So, you are probably within about 2 mph of an "exact" value for speed and distance at 2483mm. If you can do a LONG run like I just did (for 70 miles), you can make it a bit more accurate for YOUR particular weight/speed/pressure setup.

- Jack

[agood1]

There must be an easier way than all this math.
I just did a 400 mile round trip and with my programmer in the canned towing mode. I was 2 MPH off from my GPS. So I decided to try all this math and change the tire size. I have 275/70/18 tires....I came up with 2646. Well that didn't work....Now my Speedo and programmer are 4-5 MPH off from the GPS.

[Jackpine]

2646 is much too much of a change for a 2 mph error (which is about 2-3%).

You did not say if the error was fast or slow. What I'm asking here is is the speedometer saying you are faster than the GPS, or is it saying you are slower? And, are we talking about the dashboard speedometer or the Gryphon speedometer?

If the Gryphon speed was 2 mph slow, try 2533mm. If the Gryphon speed was 2 mph fast, try 2433mm.

But if the dash speed was 2 mph fast, you were probably dead on.

And, according to your FIRST post in this thread, your tires were 275/65-18s. Now they've "morphed" into 275/70-18s? The 70-series tires are a bit over 1 inch larger than the 65s.

- Jack

[agood1]

My Speedo and my Gryphon seemed to be sync'd pretty well, but they were both 2 MPH slower than the GPS. Once I changed it to 4626, none of them were sync'd.

No "morphing" here, my tires are 256/70/18 and always have been.

[Jackpine]

Originally Posted by agood1

My Speedo and my Gryphon seemed to be sync'd pretty well, but they were both 2 MPH slower than the GPS. Once I changed it to 4626, none of them were sync'd.

No "morphing" here, my tires are 256/70/18 and always have been.

Ah, my mistake. I thought I was still talking to Paw Paw. OK so, agood1, YOUR tires are either 275/70-18s or they're 256/70-18s?

If it's the former, they are 33.1" tires. If the latter, they are 32.1". That's an unloaded diameter of course. I would use a value of 2560mm if you have the bigger tires. If they're the ones you said you have in your last post though, try 2483mm.

In either case though, 2646mm sounds way too big (that's the larger tire with NO "squish" effect). You'd have to be driving a VERY light truck!

- Jack