I know when you change your wheel and tire size and it is not equivalent to stock that it throughs your speedo off.

What about milage ? Will the milage info be accuarte ?

Thanks

 

I'm not a techie, but from what I remember, yes your mileage will be off.

 

Yes, it will change both the speedometer and odometer readings.

However, what a lot of folks don't realize is that the speedometer and odometer aren't always accurate in their stock condition. I've checked my xB and find that the speedometer is slightly optomistic (67MPH actual is 70MPH indicated) and the odometer is slightly pessimistic (100 miles driven shows as 95 miles on the odometer). Your car may vary from mine!

Easiest way to check it is to borrow a GPS and take a trip with it. It will show you right away how much your speedometer is off and over the course of the trip it will give you can figure out a correction for your odometer. Much easier than the Olde Dayes when we did it with a stopwatch and mile markers.

I am a bit disappointed at the inaccuracy in the xB, as it is greater than I've measured in just about any car except my 30+ year old Volkswagens.

If I were to go to larger tires, the effect would be that it would make the speedometer more accurate and the odometer less accurate.

One effect of the pessimistic odometer is to make my fuel economy look 5% worse than it actually is. This is about 1.5MPG, a significant difference.

George

 

Well that pessimistic odometer may be a good thing at resale time. Not that I'm plannin' on selling it...

 

thats why i went with the larger diameter...fat tires setup :twisted: :twisted:

 

Will it make much difference to my buyer if the odo reads 270,000 miles or 280,000 miles?

I think that the optimistic speedo/pessimistic odo combination is relatively normal. Manufacturers don' t want to be blamed for speeding tickets and they don't want to be accused of using "fast" odometers to get out of warranty obligations early. The xB just seems to be a bit further from reality than the average.

George

 

Hey I'm not questioning you exactly, since I'm not a GPS expert, but I thought GPS isn't exactly pinpoint precise either. . . I think there is a margin from a few inches to a couple of feet, and that's military grade GPS with military tracking equipment. I know it's pretty damn good, but not exactly laser precise. If that's true, the accumulated differences over time may distort the your speed by 2-3mph, it is feesible. I know that most speedometers in any car aren't precise either. . . there is a 2-3 mph margin if I recall. If this is true, then the best way, and probably most difficult, would be to use some sort of calibrated laser speed detection system, like what some cops have.

But either way the speedo is probably off on every car by a little bit, it's pretty normal. Also don't forget that the gains in resale of plus size wheels and tires more than likely will be offset by the increase in fuel consumption. You increase unsprung weight and the effects are more than double, if not exponential. ie, adding 20 lbs to your total wheel weight would be MUCH more than 40 lbs total, and I don't remember the exact numbers but it's more along the lines of 100lbs if I remember correctly. You're gonna have to use more gas to accelerate and are going to use more gas cruising. . . but most of you guys already know this. . . most race cars use 15" wheels and some but not many use 17. But yeah, the increase in value of your car isn't going to be huge. BUT IT LOOKS GREAT, hahaha and I can agree with that!

 

You are correct in that consumer GPS does have an error of a few inches to a few feet. However, this error isn't cumulative and it is always kept to within a few feet of the actual position, so the position indication is remarkably accurate. Assume that you took a 200 mile trip from north to south. If the error was 20 feet north when you started and 20 feet south when you finished, the total error would be 40 feet in 200 miles.

For velocity measurements, the speed of the change of the position error is always very slow, so it does not affect the speed calculated by the GPS.

There used to be an intentional inaccuracy built into GPS called "Selective Availability" or "SA". The idea was to prevent the Bad Guys from putting GPS on their missiles for precision guidance. Several years ago, the govenment finally figured out that the Bad Guys were more likely to ship via FedEx than build a missile so they turned SA off.

George

 

gotcha. . . makes sense