[morfn321]

Can anyone tell me what size in mm I should input. I run a 275/70/18 in the winter and a 35/12.50/20 in the summer. I tried the formula from edge but spedo is still 4mph high. Thanks greg

[JackandJanet]

Originally Posted by morfn321

Can anyone tell me what size in mm I should input. I run a 275/70/18 in the winter and a 35/12.50/20 in the summer. I tried the formula from edge but spedo is still 4mph high. Thanks greg

Greg, when you have the Tire Size entered correctly, the dash speedo will read about 2 mph higher than a GPS at 70 mph, due to mechanical errors. However, the odometer will read correctly. My guess is, you are not adjusting for "squish" - the flattening effect of weight on the tires that reduces the effective size.

The Edge formula does not account for squish, the Gryphon formula does. Try multiplying the TS you used by 0.97 and adjust from there if needed. If the speedo still reads too fast, reduce the TS more. If too slow, increase the TS.

- Jack

[Jim Allen]

Why not measure and enter the exact loaded circumference and set it to the MM?

I could swear this was in the Edge instros:

1) Set the truck on a flat, fairly smooth concrete surface with a little room to roll. Make sure tire pressure is set to your correct or chosen pressure.

2) Use a large carpenter's square to find wheel centerline of one rear wheel and then mark both the tire and the ground in line with the wheel center and perpendicular with the ground with a magic marker.

3) Roll the truck forward exactly one tire revolution and then use the square again to line up the mark on the tire with the centerline of the wheel and then mark the ground opposite the tire mark.

4) Measure the distance between the marks on the ground and you have the circumference.

This method accounts for tire pressure, tire wear, vehicle load, even temperature if you want to be that pedantic. It will change a few MM with tire pressure and treadwear, I know because I've checked. If you did it with a very hot vs a very cold tire, you could account for that as well.We are talking differences in a few MM. I found a difference of about 15mm between two identical size (but different brand) of tires

FWIW, I find GPS speed readings inconsistent if you are looking for perfection. I use them for spot checks mainly

[Longshot270]

Originally Posted by Jim Allen

Why not measure and enter the exact loaded circumference and set it to the MM?

I could swear this was in the Edge instros:

1) Set the truck on a flat, fairly smooth concrete surface with a little room to roll. Make sure tire pressure is set to your correct or chosen pressure.

2) Use a large carpenter's square to find wheel centerline of one rear wheel and then mark both the tire and the ground in line with the wheel center and perpendicular with the ground with a magic marker.

3) Roll the truck forward exactly one tire revolution and then use the square again to line up the mark on the tire with the centerline of the wheel and then mark the ground opposite the tire mark.

4) Measure the distance between the marks on the ground and you have the circumference.

This method accounts for tire pressure, tire wear, vehicle load, even temperature if you want to be that pedantic. It will change a few MM with tire pressure and treadwear, I know because I've checked. If you did it with a very hot vs a very cold tire, you could account for that as well.We are talking differences in a few MM. I found a difference of about 15mm between two identical size (but different brand) of tires

FWIW, I find GPS speed readings inconsistent if you are looking for perfection. I use them for spot checks mainly

If you use this method I would do three revolutions and divide the final length by 3. Helps to increase your accuracy. I would never completely trust just one of any sample.

If you use a GPS, do not use the speed function, like Jim Allen noted, there are inconsistencies while moving. Use the odometer of the GPS and the odometer of your truck to get them aligned.

[Dark Rider]

can I adjust the tire size while im driveing to get the correct speed or do i have to stop and turn off my truck every time?

[Longshot270]

Originally Posted by Dark Rider

can I adjust the tire size while im driveing to get the correct speed or do i have to stop and turn off my truck every time?

You have to stop to reprogram the truck so yes.

[Dark Rider]

thanks

[Dark Rider]

I dont have lvl ground to check the distance for tire size and i am run 35 12.50 17 tires but the 12.50 is not a ratio so how do i fugure the tire size?

[ticopowell]

being that the Gryphon wants the circumference that is 2*pi*radius, so it should come out to about 109.96 inches, which turns into 279.29 cm, which is 2792.88 mm, then times that by .97 gives you 2709.1, but since your gryphon cant do the .1, enter as close to 2709 as possible and you should get a fairly accurate speedometer.
The other way to figure it out is to find out how many revolutions per mile that your tires make and then divide 1609344 (mm per mile) by that number and you should get pretty close to 2709 but with less calculations, kinda like bill says on this page.
Hope that helps!

[JackandJanet]

Nicely stated, ticopowell!

- Jack