Tire size /odometer error - Off-Road Forums & Discussion Groups

broncobilly69 · #1 09-24-2002, 05:46 PM

If the stock tire size is 30 and you run 33's.The odometer reading will be lower than the actual mileage correct?

Billy

#2 09-24-2002, 05:59 PM

Yeah that seems right. Seems like it'll be off by the difference of tire circumferences per tire revolution.

F150 · #3 09-24-2002, 06:12 PM

All of the equations used to find the circumference and distance traveled are linear. So you can just get the percentage difference in tire sizes and your odometer will be off the same percent.

So a 33" tire is 10 percent larger than a 30" so your odometer will be off by 10 percent.

So for every 1000 miles the odometer reads you have actually gone 1100 miles.

TheJuice · #4 09-24-2002, 06:24 PM

The equation may be linear. Unfortunately a 30" tire is not necessarily 30" exactly. Revolutions per mile are a better way...there was a thread on this a while back.

Broncobilly, yes, you are correct

#5 09-24-2002, 06:33 PM

That's correct. I am running 33's and my odometer is off about 7MPH give or take a few at highway speeds (50-70).
I think <a target="_blank" href=http://tellico.off-road.com/ubbthreads/showflat.php?Cat=&Board=offroadfordtrucks& Number=329714&page=&view=&sb=&o=&a mp;vc=1> this thread</a> is the one <font color=blue>TheJuice</font color=blue> is thinking of.

F150 · #6 09-24-2002, 06:36 PM

Sure there will be some differences due to different manufacturers and wear, but 10 percent should put you real close.

And if you have an accurate enough measurment of your tire to get the revolutions per mile, the percentage difference of you tire hieghts figured into your odometer will give you the exact same result as figuring the revolutions per mile. This will save some time converting circumference with miles and revolutions.

#7 09-24-2002, 07:32 PM

If, however, the previous owner (assuming it was bought used)had it recalibrated for a certain tire size, then the circumference equation would possibly be inaccurate. I had mine recalibrated by Ford and was told they could only do it 6 times and even then could only calibrate it to the largest tire Ford put on the Bronco for that year. In my case it would have been 31-10.50s. Since I was running 33s at the time it didn't help much and I was still 10% off. So, when I went to 35s, I used the mileage check points on the freeway. Did it with a set of 35" BFGs and 35" Les Schwab tires. Both showed me to be off around 14%. So Broncobilly, when I am cruising at 65 on the freeway per my speedometer, I am actually going 74 and for every 100 miles on the odometer, it is actually 114 miles. I did just the reverse on my daughter's Honda. Put smaller diameter tires on it and she thinks she is going faster than she actually is (lead footed girl).

F150 · #8 09-24-2002, 07:51 PM

Yes that is assuming the odometer was accurate for the 30 inchers.

red61on, I don't know about the 96s, but on 92-95s you can calibrate your speedometer yourself for any size tire. But only 6 times.

Sixlitre · #9 09-24-2002, 08:03 PM

BroncoBilly

Quick way to check the error rate is; take a drive onto the interstate and find a milemarker. Pull off and draw up level with it. Set the trip meter to zero and drive off, go 10 miles and pull over at that milemarker.

If you had shorter tires like my 275/60/15s you'd be over reading by 6.6%. In other words my odometer would indicate 10.66 miles, when the real milemarker proves it's only been 10 miles. Knowing this I just multiply whatever the speedo says by .934 and I get a dead accurate reading of what I'm actually doing.

BTW that must be why I can't figure out what all the fuss is about in the 5.Slow thread, because my real axle ratio is now 3.78 not 3.55, giving me much better take off. The other day I toasted a yellow super charged Nissan truck no trouble at the light. He was rather upset (high payments and all I guess).

Sixlitre

#10 09-24-2002, 10:47 PM

I'm really not sure at this point F150 but I do know that my '96 is OBDII and creates somewhat of a pain for me when trying to find performance items. I don't know if my PCM is an issue with the calibration for my speedo or not with this system. Actually though, it cost me $69 when I did have it done so won't be doing that again and I am so used to knowing about how fast I am going anyway (not to mention less mileage showing on the truck) I won't bother. Curious though as to there being any difference between the 92-95 calibration vs the '96. How are the 92-95 calibrations done?