Whodat

I was once told that you need to adjust or recalibrate your odometer to reflect a larger (in most cases) rim. Say if you go from a 17 to a 19 inch rim, the odometer isn't exactly 'correct' (presumably a smaller 17" rim will turn less than a 19" to go the same distance?). Has anybody else ever heard of this and/or do you guys bother to recalibrate the odometer after mounting larger rims?

chixdiggit

I think you have it reversed. Larger rims will turn less to travel the same distance of a smaller rim. This is because the circumferance of the larger rim is greater than the smaller rim.

For example, The 19" inch rim will move a distance of 119.32 inches (circumference of a circle is diameter x Pi) with one revolution of the wheel. The smaller 17" wheel will travel a distance of 106.76 inches with one revolution. Since speed is the distance travelled over a fixed period of time (hence kilometer per hour and miles per hour) the larger wheel would then travel a greater distance over a fixed time if we were to hold the number of revolutions made by the wheel constant. Thus the larger wheel will travel at a faster speed.

The car calculates the speed by the number of revolutions made at the wheel. If you were to use the larger wheel your car will be calculating the speed based on the smaller wheel. This in turn will cause the speedometer to understate the speed at which you are travelling. This understatement becomes compounded as speed increases, therefore the faster you are travelling, the more inaccurate the speedometer will become. That is why with large changes to wheel size you may need to recalibrate your wheels.

Hope this helps

deyainrdy4ds

um...so if u put bigger wheels on...will it make ur odomoter read more or less than its supposed to?

like if i traveled 100 miles...will it reflect something more like 105 or something more like 95 with the bigger wheels...?

chixdiggit

That is correct.

When it's regular city driving speed, I think the difference is negligeable. Keep in mind as your speed increases, the difference becomes increasingly larger as well, so unless you're constantly driving at 100mph or faster, I don't think the difference would be that great.

If you did use the larger wheels; theoretically if you the speedometer says 40 you may actually be going like 42. If the speedometer says 100mph you could be going 110mph; and as it increases to say 150mph you would then really be travelling at 175mph. The numbers are just pulled out of the air for example purposes. If you really wanted to you can calculate the number using basic physics by figuring out the number of revolutions of the wheels required to travel a mile and work it from there.

You would have to talk to a technician about the accuracy. I don't think many people have their speedometers recalibrated for a 2 inch difference though, but it's a simple change that can be done by the dealership or shops with the correct computer programs.

Over here we need to take cars that are over 5+ years old for emissions inspections. Usually they put the car on somthing that looks like a dyno and they drive it at a given speed and measure the emissions from your vehicle. When they realize that you have larger or smaller aftermarket rims, they need to recalibrate the speeds to get a true reading.

chem1441

Doesn't the smaller tire thickness on larger rims compensate for any difference, so no calibration is necessary?

Whodat

chem1441: See, that's what I was told. That the middle size number or ratio of your tire (Using the "40" in 245 40R-17 for example.) would have to be smaller if you go with bigger rims. So in other words, if you go, let's say, 18s for example, you get a 255 35-18 tire to compensate. Is that the way you understand it?

Sorry folks but I'm just trying to understand all this tire/wheel stuff thoroughly before doing anything to my ride.

Whodat

Oh, ok. I guess I did have em backwards huh?...haha. Shame on me! You really broke it down. I appreciate it!
So does this mean that the guys (and gals ) that have put 19s on and not recalibrated their odometers actually have MORE MILES than what's showing on the odometer (since the 19s are covering more ground)?

chixdiggit

That is a very valid point and yes it does help compensate.

Tire sizing makes a difference in this equation by compensating for the increase diameter of the rims. However, with tires, like wheels, different folks prefer different strokes. The height of the tire walls do come in various sizes. It is possible to put low profile tires around a smaller rim as well as larger comfy tires around larger rims. You can always keep the original wheel size by selecting the appropriate tires.

Keep in mind that the second number isn't an independant size. It is a calculated ratio based on the width of the tire. You can not directly 100% compensate for the change in rim size by selecting a tire with lower or higher aspect ratios. This is because tires only come in certain sizes and dimensions. In the end if you select the appropriate tires, the changes would be negligable. If you want to read more on tire sizes and how it works, you can probably find a lot more helpful information in the stickied postings by Luke from tirerack in the wheels forum. I found them really helpful for reference materials when I was looking into buying wheels.

I was exagerating the numbers a bit to make a point. However as rims increase in size the difference in speedometer accuracy diminishes unless you recalabrate. This would be like going from 17" rims to the larger sizes like 22" or 24" etc... (I do believe the max size you can get out there is 32" that would only fit hummers made by either Asanti or Giovanna). That is where most of the difference will be because the stock wheels are a certain size and you can't add a negative size tire for compensation.

chixdiggit

Okay let's really put this into perspective. Time for a fun physics exercise… (I need a break from work).

Let's say the stock wheels and tires all together are 17" in diameter. We would like to buy some really big shiny oversized rims and the rims and tire amount to 19". Therefore there is an increase of 2" overall in wheel size. For the sake of argument we live in the city and put 20,000 miles on the vehicle per year on average. So how much additional mileage are we adding on the car that is not registered due to an uncalibrated vehicle?


Given:
1 mile = 5280 feet = 63360 inches

The distance around the wheel is known as the Circumference.
Circumference of a wheel (C) can be determined by the equation: “C=2πr” where π is a constant (3.141592) and “r” is the radius of the entire wheel (including tire).


Calculations:
The circumference of the 17” wheel is then: C17 = 2x(3.141592)x17
C17 = 106.81428 inches. Therefore one full revolution of the 17” wheel will travel 106.81428 inches or 8.9012 feet.

The circumference of the 19” wheel is then: C19 = 2x(3.141592)x19
C19 = 119.3805 inches. Therefore one full revolution of the 19” wheel will travel 119.3805 inches or 9.9483 feet.

The difference in distance of one revolution between the two wheels is:
d = 119.3805-106.81428
d = 12.56622 inches

We can now find the number of revolutions of the 17” wheels is required to travel 1 mile.

1 mile = 63360 inches
1 revolution = 106.81428 inches
63360/106.81428 = 593.17911 revolutions per mile.

1 mile = 593.17911 revolutions, 20,000 miles = 11,863,582.2851 revs.

The car calculates distance and speed by the number of revolutions made by the wheel. We can assume by not calibrating the vehicle, the car will measure distance and speed with the same variables between the 17 and 19 inch wheels. The number of revolutions required to travel 20,000 miles will be the same between the two different size wheels based on this assumption.

11,863,582.2851 will be valid for the 20,000 miles traveled on the 19 inch wheels as well. But remember for the 19 inch wheel 1 revolution of the wheel = 119.3805 inches or 9.9483 feet.

Therefore the distance traveled when the wheel rotates 11,863,582.2851 times will be:
=11,863,582.2851 x 119.3805
= 1,416,280,384.9822 inches

1,416,280,384.9822 inches / 63360 inches per mile = 22,352.91011 miles.

22,352.91011 miles – 20,000 miles = 2,352.91011 miles.


Therefore:
By increasing the overall wheel size by 2 inches and not calibrating the car’s sensors, the odometer will be off by approximately 2,352.91011 miles for every 20,000 miles traveled.

deyainrdy4ds

2352 more or less than stock

whoover

When fitting larger rims you indeed aim for as close to the same total wheel/tire diameter as stock. The idea is to replace rubber with metal. Significantly changing the overall diameter will totally screw up the handling (steering geometry), tire wear, ESP, BAS, speedometer and even nav.

In general a "+1" fitment will go up one inch in wheel diameter, up 10-20 mm in tire width and down 10% in the aspect ratio (middle number). For +2 you double everything. Again, the correct way to do this is to aim for greater width (within the constraints of the car) and as close to the original outer diameter as possible.

There are calculators that will compute the difference, which can be plus or minus, so long as it's small. Here's one:

http://www.canadiandriver.com/tools/...c/tirecalc.htm

Usually it's best to let a pro suggest wheel and tire fitments.

Whodat

2,352 MORE than stock.

Whodat

chixdigit: Now that's some math for ya *** *LOL*! Physics must have been your favorite subject! Thanks. Thats about what I was thinking but I had no idea anybody would be able to break it down in such a detailed fashion. Again, THANKS!

Whodat

Cool! That's a great tool! Thanks man. You know that billy is bookmarked already!

chixdiggit

Glad I could help. That's why we're one these forums.

To be honest, I couldn't stand physics anymore and opted out of sciences and ended up in business...lol.

Whodat

Ah. And what a well-rounded young man you are! Again, many thanks.

deyainrdy4ds

damn so odometer will be reading out more than you actually traveled...that kinda suxx

da_guitarist

Very nice physics lesson...keep in mind though, that with the appropriate size tire decrease with wheel size increase, the numbers are MUCH less exaggerated. According to some back of the envelope calculations i get tha w/ 125000 miles on 17's w/ stock tires would translate to 127700 miles on 19s w/ appropriate tires. IE, 2700 miles difference in 125,000. So in 20000 normal miles, it would only be off about 500 miles.