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If anyone has replaced their factory wheels and put on 17 inch wheels or if you have any info regarding this swap I would like to know if the accuracy of the speedometer is affected by the addition of a larger diameter wheel.

Also, whether it has any affect on the ride or the gase maileage.

Thanks in advance.
Arizona Charlie
 

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arizona charlie said:
If anyone has replaced their factory wheels and put on 17 inch wheels or if you have any info regarding this swap I would like to know if the accuracy of the speedometer is affected by the addition of a larger diameter wheel.

Also, whether it has any affect on the ride or the gase maileage.

Thanks in advance.
Arizona Charlie
Hey Chuck!
I got chrome 17's a couple months back and everything seems fine. There was an option in the screen somewhere, forgive my not remembering exactly where, but mentioned something about reconfiguring for new tires. I think it may be referring more to a rotation, but yeah. After I had first gotten them on, the gps showed me driving through buildings, etc. I had it run the reconfig and it caught on and was up and fine again, I would say within 5-10 miles at most.

If anyone can remember right off where that reconfig setup is, let me know, or I guess, let Chuck here know. I believe it was top button to the right of the screen, second menu down? Something like that, I THINK.

I hope you're getting chrome 17s if you have a black one! It looks AWESOME!!
http://profiles.yahoo.com/bradca21
 

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I don't own a 2004, so I don't know how many systems are affected by the wheel calibration procedure that bradca21 referred to, but you should always be careful about changing the wheel size on any car, because it could make the odometer and speedometer read incorrectly (7.5% low, in this case). A speedometer that reads low will make you more likely to get speeding tickets, and selling a car with an inaccurate odometer reading is also illegal.

The 2004 Prius stock wheels are P185/65R15, which means the sidewall height is 185 mm * 0.65 = 120.25 mm = 4.73", and the overall tire height (unloaded) is 4.73" + 15" + 4.73" = 24.47". This means the circumference is pi*diameter = 76.9". This means the 15" wheel will make 824 revolutions per mile, because 1 mi = 5280' = 63360".

A 17" radius wheel will have C = 83.15" (assuming the tire's sidewall height is the same) and will make only 762 revolutions in the same mile. If the odometer is calibrated for a 15" wheel, it will display 0.925 mi when the 17" wheel has actually pushed the car for 1.00 mi (7.5% low). (In other words, when you sell the car after driving it 100,000 mi, the odometer will only say 92,500 mi.)

Similarly, if you travel this mile in 1 minute, the speedometer will read 60 mph on the car with 15" wheels, but the speedometer on the car with 17" wheels will think the car is only traveling 0.925 mi / 1 min = 55.5 mph (7.5% low).

I recommend you perform the following test:
1. With the stock 15" wheels installed, drive alongside another stock car for 1 mile at 60 mph to make sure your odometers and speedometers read the same.
2. After installing the 17" wheels and performing the calibration procedure, repeat the comparison against the same stock car and make sure the odometers and speedometers still match.

Of course, if you can find tires with smaller sidewalls, such that the overall diameter remains the same, you won't have to recalibrate anything. For example, P185/50R17 (if they can be found) will have nearly the same overall unloaded diameter as P185/65R15 tires.

Douglas (2002 Silver, Wisconsin)
 

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I compared a 15" tire vs a 17" tire at 1010tires.com and feel this would be fine as long as you got the lower sidewall height on the 17", this would keep the overall diameter nearly equal. The speedometer will still be slower but it looks like it is less than 1%.

TIRE #1 - 185/65R15
TIRE #2 - 215/45R17

Section Width: 7.28 in 185 mm
Section Width: 8.46 in 215 mm

Rim Diameter: 15 in 381 mm
RimDiameter: 17 in 431.8 mm

Rim Width Range: 5 - 6.5 in
Rim Width Range: 7 - 8 in

Overall Diameter: 24.46 in 621.28 mm
Overall Diameter: 24.61 in 625.09 mm

Sidewall Height: 4.73 in 120.14 mm
Sidewall Height: 3.80 in 96.52 mm

Radius: 12.23 in 310.64 mm
Radius: 12.30 in 312.42 mm

Circumference: 76.84 in 1951.7 mm
Circumference: 77.31 in 1963.6 mm

Revs per Mile: 850.3
Revs per Mile: 845.1

Speedometer1: 60 mph 100 km/h
Speedometer2: 59.6 mph 99.3 km/h

Speedometer Difference: 0.615% too slow
Diameter Difference: 0.61%
 

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arizona charlie said:
Also, whether it has any affect on the ride or the gase maileage.
I would imagine that changing to 17's would make mileage worse. First, you cannot find 185's or even 195's to put on 17's. This means that you will be adding rubber (the heaviest material on the tire/wheel set) at the furthest distance from the wheel center. You will now need to create more energy to get the tires rolling, thus decreasing your MPG.

Next, the tires on 17's are probably made of softer rubber. This grips better on the pavement, but also increases friction (which also decreases MPG).

Last, wider tires block more wind (increasing wind resistance). I think this is negligable, but nonetheless noteworthy.

What I think would be ideal is to find 195/50-17. Good luck finding these.
 
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