Upgrade to 7"/9" wheels results in inaccurate odometer. ?
 

My car was upgraded to 7" fronts and 9" rears.
The odometer seems to be clocking too many miles.
In 50 miles, it might add a couple of miles too many.
So the error is upwards of 10%.

I want to calculate how much phantom mileage was incorrectly added since the wheels upgrade.
Can anyone calculate exactly how much error is being introduced by replacing 6s and 7s with 7s and 9s ?

Speedometer also seems too fast by about 10%

I can get the speedo/odometer re-calibrated to factor in the 7/9 wheels, right ?

Wheel size is not what matters, it's your rear tire diameter. To see the effect on your speedo/odo with different tire sizes, go here - Tire size calculator

Can't get the ancient Java applet to work.

OEM: 225/50/16 on the 7" rears.
Current setup: 245/45/16 (9” on the rears)

Rear tire diameter went from 24.86" to 24.68"

There is no way this tire/wheel swap is the cause of this problem, the diameters are not that different.


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1% diff in tire diameter. Speedo would change by .5 mph at 50 mph. Adding two miles in 50 would be 4% and not 10%. Typical speedos read about 3% higher than actual with oem size tires.

Should you be using diameter or circumference to calculate error in distance traveled?

It's really more about the circumference than the diameter. i.e. the distance the car moves in inches for one revolution of the tire
Just ran the circumference on 225/50 16 vs 245/45 16.
The 245/45 circumference is 77.538"
The 225/50 circumference is 78.09"
I know it sounds nuts, you put on a "bigger tire" but it's really smaller in circumference!

So your speedo is calibrated to a 78.09" circum, but now you are using 77.538"
(it's the 45 profile aspect that gives the smaller circumference)
Therefore the 45 aspect tire has to turn more to go the same distance as the 50 aspect tire. Ergo more revolutions indicating a higher speed than actual on the speedo.
In one mile the 225/50 tire turns 811.37 revolutions
In one mile the 245/45 tire turns 817.14 revolutions


Gbox has a real neat excell spreadsheet for doing gear and tire calculations. You just plug in the numbers. GBOX Transmission-Gearchart
Download and try it, it's fun!

"In one mile the 225/50 tire turns 811.37 revolutions
In one mile the 245/45 tire turns 817.14 revolutions"
Everything you said is right but it is still a 1% difference.
811.37 / 817.17 = .992938786 or .7061214%
Both tires are very close to stock size of about 24.7" diameter

If his speedo is off by 10% then it does need to be recalibrated. My autocross tires are 225/45-15 and about 23" diameter which is about 7.4% smaller than stock. Add that to a speedo that already indicated 2-3% high and it is about 10% higher than actual.

After 50 miles, it was showing 52 or 53 miles.
So, it’s off by about 5%.

Always liked the willtheyfit.com calculator - this shows a speedo error of +0.72% so unlikey to make that much of a difference (ignore some of the data as the offsets inputed aren't known and are irrelevant in this case anyway).

Online Wheel and Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

To calculate your phantom mileage, we'd need to know how many miles your racked up since the change. It's pointless though. What would you do with that number? Roll the odometer back?

You assume your odometer was correct before the change. Most aren't.

Also, you assume your tire diameter was 24.86 before the change. Two problems with that. First, you need a measured diameter, not a "theoretical diameter". Different brands of tires in the same size will not have the same actual diameter. The sizes are more approximate, than exact. Secondly, your tires were probably worn, which meant they were no longer the original diameter. They could have had enough wear to make them smaller than the tires that replaced them.

Take the car to a speedometer shop and see what they can do for you. Fixing the speedometer will likely be easier than fixing the odometer.

Or, just forget it, like most of us do.

JR

Come on man, if you put fat rubber on the rear and are focusing on the mileage difference you're not getting your money's worth from that upgrade. Enjoy the load of extra grip!

Circumference is a function of diameter, so it makes no difference which one you use to calculate it. Circumference = diameter x Pi (approx. 3.14)

245/45 will make the speedo read higher than 225/50, but not much to care about.

I always check my speedometer error, regardless of what size tire I'm using. I need to know my actual speed in order to avoid getting any more tickets. http://forums.pelicanparts.com/support/smileys/nuts.gif

Here's the thing, It may not even be just 5%, I mean that odometer CRANKS the mileage at an shocking rate. It says I have put 4000 miles on the car in a year, and I know that's no where close to real, considering I put 20k miles on a commuter car I've been driving the hell out of for 2-3 years.

Sad part is, I just had the odometer gears fixed when I got the car last year. I told them about the odometer racking up way more miles than reality, perhaps b/e of the tires, or maybe just b/c something just needs to be recalibrated. They said to send it back for no charge, so I might as well. So, even if the tires are not the cause, the odo/speedo might just need to be recalibrated? It's a shame they didn't do that when it was in for the odo gears fix.

So, you've measured this? Sending the speedometer in isn't something I would do, unless you have actually gone out and measured the odometer against a known distance. Not hard to do, takes 10 minutes.

JR

http://forums.pelicanparts.com/uploa...1441146368.jpg

California Speedo can re-calibrate if you provide wheel size.

I say send it back to the original vendor with rear outside diameter info. Give them a chance but I would call them first.

Mine is out a good 8-9% and I have no unusual rubber on my car. The error is also linear.

Yes, the example above was something I specifically measured against a known route.
It costs me nothing to recalibrate, just remove and pack and mail. I can do that in 30 mins.

Well, if you have actually measured it, it should be a known percentage off. You seem to dither a bit in your assessment of it.

JR

That's full of awesome. Thanks for posting that! :D

Agreed that these older Porsche speedos are wacky. They can be way off for no apparent reason. A friend with '87 Carrera has mentioned for years his speedo is considerably off in terms of reading too fast.

Agreed with JR regarding the tire diameter thing. The actual dimensions of the tire vary by manufacturer, wear and load/speed. So while the calculator is a nice tool to give you an idea of what changes you can see? You really want to use specific actual tire dimensions to give you a better indication, and even then it's not going to be 100% accurate.

The confusing thing is, his car had 245/45 rear tires on it when he bought it, as I recall. So, nothing much has changed since he owned it. The tires he put on the rear are essentially the same size as other choices in that size, so that's not the problem. A 245/45 will usually have a smaller diameter then a 225/50, but a new 245/45 will be larger than a worn 225/50, so we are not talking a huge difference. Way less than 5%.

JR