How far is my speedometer/odometer off? Here's your answer!

[LINY4Runner]

When you run oversize tires, the vehicle actually moves faster & farther than it "thinks". This is because when the diameter of the tire increases, so does the circumfrence. Here's the easy way to figure out the difference:

New tire size / old tire size = difference
speedometer x difference = actual speed

here's an example: most Runners came equipped with about a 30" diameter tire, and the most popular up-size is a 33"
33/30 = 1.1
55mph x 1.1 = 60.5mph ACTUAL SPEED

an example from my truck (because it came with 15's and 27" tires eewwww)
33/27 = 1.22
55mph x 1.22 = 67.1mph ACTUAL


You can also use this formula for gas mileage...an example on my truck:
12.4mpg x 1.22 = 15.12 ACTUAL MPG

NOW! Some people are regeared and wondering what the difference is now...
Well wonder no more, because it's the exact same formula. The final drive ratio has nothing to do with the speedometer because the speed sensor is not in the transmission, rather there's a wheel speed sensor at each wheel (at least on ABS models, not sure about non-ABS). So the only variable that will throw off a speedometer is tire size.

 

[downpour11]

I've heard of this little formula before but in my experiences I've never been able to convince myself that by putting on 33's, my speedometer would be off by 5mph??? One of the times I've been pulled over I was going 88-90 steady and the cop comes up to my window and after I asked him "why did you pull me over" lol he said, "i clocked you at 89."
This was on I-5 in cali by mt. shasta and i was driving a 2000 highlander edition on 33's w/ 3" suspension and 2" BL.

Has anyone else noticed their speedometer seems to be off? Liny have you?

 

[KeatonMP]

...or everyone could just use this little tool:

Tire size calculator

Enter the stock tire size...then enter the size your running now...it will calculate everything and give you a % or error and tell you how far off you are at 60MPH.

 

[004Gunner]

You can also use this formula for gas mileage...an example on my truck:
12.4mpg x 1.22 = 15.12 ACTUAL MPG

NOW! Some people are regeared and wondering what the difference is now...
Well wonder no more, because it's the exact same formula. The final drive ratio has nothing to do with the speedometer because the speed sensor is not in the transmission, rather there's a wheel speed sensor at each wheel (at least on ABS models, not sure about non-ABS). So the only variable that will throw off a speedometer is tire size.

That's pretty good fuel economy considering your setup

 

[piperoo]

I've never been able to convince myself that by putting on 33's, my speedometer would be off by 5mph???

Has anyone else noticed their speedometer seems to be off? Liny have you?

I went from 15" wheels with 255/55/15 to 16" wheels with 265/70/16 and anything above 50 mph I am off around 3-5 mph. I tested it with my GPS....my Tom Tom has the speed I am traveling and the faster I go the more off my speedometer was.

 

[brian2sun]

Well, now I'm wondering if my speedo is calibrated to the 265/70/16s that were on it when I bought it or if it's calibrated to the 225/75/15s that it says on the door sticker. I do know that when I drive my Honda to work it is 4.8 mi., but in the 4runner it's only 4.0 mi. so that means my odometer is ~17% off. So... I have calculated ~15.9 mpg average in the runner over the last 6 months, Im really getting ~18.6!! That's great gas mileage for a lifted, heavy-ass 3rd gen on 33.5s!!

 

[Martinjmpr]

I tested it with my GPS....my Tom Tom has the speed I am traveling and the faster I go the more off my speedometer was.

Since most of us have GPSs these days (don't we?) that's the easiest way to figure it out. Zero your GPS, then zero the trip meter. Drive 100 miles as indicated on the GPS. Then take a look at the trip odo. That will tell you your error percentage right there.

 

[FoneRunner]

Using my GPS, I calibrated my scanguage so that it reads the right MPH...but before I did that I was driving through a section of Pennsylvania where they have a marker for every tenth of a mile and when my Odo said I had gone a mile, the markers said I had gone 1.1 miles...thats 10% off with 285/75's...

 

[sportscarfan]

Another good tire/wheel calculator:
Wheel / tire size calculator / comparer - RIMSnTIRES.com

 

[JohnsD90]

I use the Speedometer app on my iphone, mine is 3mph off stock, the app, and many radar things on the side of the road prove my factory calibrated speed to be off by 3mph at almost any speed.

 

[LINY4Runner]

Most factory speedometers are 3-5mph fast.

What I find interesting is that the speedometer % of error doesn't change after regearing...that's what I was mainly so excited about

 

[Spencer]

I use the Speedometer app on my iphone, mine is 3mph off stock, the app, and many radar things on the side of the road prove my factory calibrated speed to be off by 3mph at almost any speed.

3mph fast at 0?

 

[Spencer]

I went from 15" wheels with 255/55/15 to 16" wheels with 265/70/16 and anything above 50 mph I am off around 3-5 mph. I tested it with my GPS....my Tom Tom has the speed I am traveling and the faster I go the more off my speedometer was.

because its a fixed percent 10%@ 25mph is 2.5 mph and 10%@ 75mph is 7.5mph

 

[Spencer]

Well, now I'm wondering if my speedo is calibrated to the 265/70/16s that were on it when I bought it or if it's calibrated to the 225/75/15s that it says on the door sticker. I do know that when I drive my Honda to work it is 4.8 mi., but in the 4runner it's only 4.0 mi. so that means my odometer is ~17% off. So... I have calculated ~15.9 mpg average in the runner over the last 6 months, Im really getting ~18.6!! That's great gas mileage for a lifted, heavy-ass 3rd gen on 33.5s!!

so think automatically your speedometer is off 17%. The hondas can be fast by 7% and the yota slow by 10% = 17%

 

[Spencer]

Most factory speedometers are 3-5mph fast.

What I find interesting is that the speedometer % of error doesn't change after regearing...that's what I was mainly so excited about

I think thats strange. But cool, that the vehicle speed sensor is at the wheel

 

[Brutus]

When I first got my truck, it had 265-70-16 and the speedo read 4 mph off. After my new tires 265-75-16, it reads right on now according to my GPS.:shrug:

 

[brian2sun]

When I first got my truck, it had 265-70-16 and the speedo read 4 mph off. After my new tires 265-75-16, it reads right on now according to my GPS.:shrug:

Well, if your speedo was reading 4 mph too fast, then the bigger tires put it back 4 mph. That's the only way that would happen.

 

[thorridder]

i had 265/70/16 and traded up to 33/12.5/17 but doesnt seem to be any difference

 

[thorridder]

When I first got my truck, it had 265-70-16 and the speedo read 4 mph off. After my new tires 265-75-16, it reads right on now according to my GPS.:shrug:

dont know about yours or what is it but our gps is about 3-5 mph slow

 

[Spencer]

dont know about yours or what is it but our gps is about 3-5 mph slow

How do you figure..