| Author | Message | ||
     Curt Perlman (Cnote) New Member Username: Cnote Post Number: 16 Registered: 01-2004 |
My DII came stock with 16" rims. When I took them off to put on some BFG's, I put on stock 18" Prolines in the meantime. I have a Garmin GPS that showed my speedo about 5 mph slow at almost all speeds with the stock 16's and XPC's. Now with the 18's my speedo and GPS show the same speed. My question is how and why is this possible? The two wheels both have stock XPC's and are the same overall diameter (I think...I have never take out the tape measure). Any ideas? | ||
| Musky Rover (Gumarcel) Senior Member Username: Gumarcel Post Number: 1201 Registered: 11-2002 |
The GPS is wrong. Mine is about 3-5mph slow. And I have put it in a few other cars and it was still 3-5mph slow. | ||
| Curt Perlman (Cnote) New Member Username: Cnote Post Number: 17 Registered: 01-2004 |
That's fine that the GPS is wrong, but why has my Disco's speedo changed with the new wheels and tires if they are the same overall diameter as the old ones. | ||
| Dean Brown (Deanbrown3d) Senior Member Username: Deanbrown3d Post Number: 1409 Registered: 02-2002 |
Take out the tape measure.... Measure with the wheels ON the vehicle. | ||
| Leslie N. Bright (Leslie) Dweb Lounge Member Username: Leslie Post Number: 3005 Registered: 02-2002 |
The GPS is more accurate than your speedometer. Maybe it'll be jumpy occasionally, if you're switching satellites, or as you're driving around mountains/trees/buildings are temporarily getting in the way.... but usually, the GPS will be more accurate than the speedometer. If you look at the ratings on tires, it tells how many revs per mile at a given speed that the tire does. For example, http://www.bfgoodrichtires.com/assets/pdf/all_terrain_ta_kd.pdf , you'll note that a 235/85R16 has a count of 654, as does the 265/75R16. So, if you switch between the two, the speedometer reading should be the same. A 235/70R16 has a rev/mile count at 45mph of 715, because the shorter tire is having to spin more to go the same speed. A 225/75R16 has a count of 707, so it'll have to spin a bit less than the 235/70. A vehicle that is calibrated to a 225/75R16 (so your vehicle is showing that over a mile your tire is making 707 revolutions over that distance) but is using a 235/70R16, the speedometer will show that you're going faster than you really are because after 707 revolutions, you won't be as far along. Realize too, that each tire size for each manufacturer for each particular style of tire is going to vary. Just because in this example, the AT/ko happens to have the 235/85R16 and the 265/75R16 as a dead-on match, if you change to a different tire it probably won't: http://www.bfgoodrichtires.com/assets/pdf/mud_terrain_ta_km.pdf Here, the 265/75R16 has a 652, but the 235/85R16 is a 650, so it's a hair taller. Same brand, just a different tread. So, a 235/85R16 in a MT is a bit taller than the same size in the AT. So, when you switched to an 18", you happened to find a diameter that matched your vehicle's speedometer's calibration. If you figure out the rev-count for that tire, then get a tire with the same rev-count, you *should* have the speedo and the GPS match (assuming that the 2nd tire has it's rev-count calibrated the same way the other tire's count was, to the same degree of accuracy....). All it takes is just a little difference to really play with the speedometer accuracy. And, what isn't a noticable descrepancy at in-town speeds will be more pronounced at highway speeds. FWIW..... -L |
