You can input the original tire size data and the data for the tire you are changing to; input an mph figure, say 10, and have the speedometer difference displayed. Now, whether or not you recalibrate your speedometer (in the cases where it is possible) or not, that's up to you.
Tire sizes are usually displayed T/A_W (235/45_R17), where 235 is the rim diameter, 45 is the aspect, and 17 is the tire width. Yes, I presented car tire sizes here, but that's what I'm working with at the moment; rv tires are no different as to how they present the figures.
For instance, these 3 tires will fit on a 2006 Audi A4, and as far as the owner's manual is concerned, they are all stock tires:
235/45_R17, 215/55_R16, 235/40_R18
To be going 10 mph actual speed, the 17 will read 10 mph, the 16 will read 10.9 (8.45% difference), and the 18 will read 10 mph (well, 10.002, but at 10 mph you won't be able to discern the difference). (.02% difference).
While we already think about whether or not a tire will fit into the space provided, whether or not the same modification may require a speedo recalibration or some other way to keep us from getting speeding tickets, knowledge is power.
By the way, using your GPS to discern your speed has its own issues. Besides a sampling rate per second rather than an instant one (and that rate is different depending on the age and maker of the unit), GPS also presumes you drove in a straight line from point A to B.
2000 Pace Arrow Vision, 36B, 2 slides, Ford V10
2006 Audi A4 3.0L Quattro Cabriolet
2003 Indian Chief
Patriot Guard Rider
Iron Indian Rider
Indian Riders Group
American Legion
USAF, Retired
Over sized Tires = $60 speeding ticket in Spokane Washington... trust me on this...
John Harrelson
Carson City, Nevada
fulltime since 1977
93 Ford 350 4wd Diesel
95 Prowler 30.5 ft 5th wheel w/slide
TWO CENTS WORTH
The story goes that a man died and was approached by the Devil who told him that he could buy his soul back for a dollar. The man searched his pockets and could only come up with 98 cent. While begging the Devil to forget the two cent he was short, an Angel happened by and hearing the Devil laughing, asked the man, "Would you mind if I put in my two cents ?" The Devil got so mad that he exploded in a puff of smoke and the man's soul was saved. The moral: Sometimes putting in your two cents worth makes a difference.
JOHN "the cook" 1997
Old vehicles are easily recalibrated with either a $10 gear change that takes 5 minutes or an adapter attached to the speedo cable. Every car I have built I calibrated the tire size, gear ratio and installed the correct gear, sometimes by trial and error. All my vehicles drove withing 2 mph of the speedo reading.
The current vehicle I am building will be computer driven and it is just a matter of having the computer "calibrated"
"The great challenge of adulthood is holding on to your idealism after you lose your innocence."
– Bruce Springsteen
Tire sizes are usually displayed T/A_W (235/45_R17), where 235 is the rim diameter, 45 is the aspect, and 17 is the tire width. Yes, I presented car tire sizes here, but that's what I'm working with at the moment; rv tires are no different as to how they present the figures.
WHAT? 235 is tire width in MM not diameter and 17 is rim diameter not width.
hpk
2000 Ford F-350 SRW 4X4 PSD Jayco Super Lite 29.5RKS (31') 5th wheel 50 gal X-ferflow in the bed tank. Banks big exhaust and Stinger kit.
And you're right, I did identify the width and rim size elements backwards.
As far as a 10 dollar plastic gear fix to modify the speed shown, you're right. Except that won't work on Audi's, VW's, Porsche's, and a few other makes.
I still like the way I can recall on the fly with my bike. After going with a different size tire, I just stop, press a button for 7 seconds, press once more and drive exactly 2 miles under 30 mph, and then press the button again. It's now calibrated for that tire size (speedo pickup is magnetic from the tranny, not from the front wheel).
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