How off will my speedo be if I change the stock 235-75-15's to 30X9.5's?? I have several tire cal programs, & websites, but I can't find any sizes in that range.
Thanks

Go to www.4lo.com. This place has the calculators that will tell you your difference. You shouldn't notice anything but about 2-3 mph difference.

I have a calculator that works pretty good..I figured it out for you...235-75-15 = 28.9" diameter
I compared that diameter with a 30" tire and came up wth the following figures for ya....
Speedo says= Actual speed:
20=20.8mph
35=36.3mph
45=46.7mph
55=57.1mph
65=67.5mph
75=77.9mph
85=88.2mph

What if you go from a 235-75-15 to a 31x10.50 r15? I know you just love sitting around figuring this stuff out for slackers like me:) Thanks

Nate

Great online calulator:

http://www.miata.net/garage/tirecalc.html?clkd=iwm

Nate,
If you got to 31's you will find a 10% difference (speedo says 50, really 55)

Checked it with a gps...

Jon

I sat by a guy in a class that got pulled over in a taurus and the speedo was off by 8-9mph at 70 mph. A mechanic told him that he could get out of the ticket because Ford speedos are only gaurenteed for 10% accuracy. When we had 10 Explorers and Matt Adam's F150 in a line( granted we had all sorts of tire sizes) at 65 mph we were all over the place. So Matt said "I have the correct speed with stock size tires" and it was not what I expected so I fired up the GPS and even his was off by a few MPH so just to let you know it may not be accurate from the start. All to make it a little more of a challenge:)

Funny thing about speedo calculation is that you may be off by 5mph at 70, but not necessarily at 55...thats why I wrote it at differant speeds for the first guy...for you...
speedo says : new tires
20=21.5mph
35=37.5mph
45=48.3mph
55=59mph
65=69.7mph
75=80.4mph
85=91.2mph
any higher than that and I would worry about other things if your riding on 31" meats
Hope that helps...