Will going up one size correct speedometer?

Go here for a comparison with different size tires. It tells you the speedometer difference.

Tire Size Calculator - tire & wheel plus sizing

 

I did both speed and distance ground checks last summer with our 2010 Prius and found:

• indicated speed ~102% - 51 mph indicates is 50 mph true
• ground distance ~100% - I could find no significant ground distance error on the interstate I-65 from Ardmore TN to Nashville TN, ~90 miles

What you want to use is the revolutions per mile for your next set of tires. The first number is the tread width and not nearly as effective in changing revolutions per mile as the second number. So with my NHW11 (2003) Prius, I went from:

• 175/60R14 - 919 revolutions per mile
• 175/70R14 - 886 revolutions per mile

Now my speed indicated is 3% low so 60 mph indicated is ~62 mph. But I've always driven the speed limit or a bit low so this is not a big deal. But my next tires to test will be 195/70R14, 849 rev/mile, and these will more than double the error to 7%. At 60 mph indicated it will be ~65 mph and that is too much error for my taste.

I'm looking at three alternatives:

1. GPS speedometer - I found a nice, heads up unit for $165 and a couple of do-it-yourself units for less.
2. Garmin nuvi in speed mode - generates a large enough speed display and also provides navigation. However, I'm also looking at all GPS units.
3. iPhone GPS speedometer(?) - a wild-card, it could also be my MP3 player. But I won't contract with AT&T for phone service and it is a pricy solution. Still, it is easier to pull it in and out of the car than the oversized Garmin nuvi

I'm not sure which approach I'll go with since there are significant advantages and disadvantages of each approach. I'm still doing my 'trade study.'

GOOD LUCK!
Bob Wilson

 

Bob,

If I understand what you were saying correctly, with the initial tires, you saw a 1 mile per hour difference in rate, but no significant difference in odometer reading (which suggests that the two aren't necessarily "coupled").

If you switch to the 195/70/14 and see an approximate 5 mph difference in highway rate (65 mph vs 60 mph indicated) do you expect your odometer reading to still be accurate?

 

Maybe I'm just being simple here, but if your speedometer is fast, and it's affecting your odometer meter...which is tied to decisions about routine maintenance and overall mileage which will eventually affect the value of your automobile isn't the best answer just for Toyota to fix this? I think a reasonably accurate speedometer and accurate odometer isn't something beyond expectation in a vehicle.

 

The odometer reading will change too but I only adjust that when I enter my gallons and miles ... once per tank. I tend not to worry so much about distance. In contrast, speed errors can lead to a discussion with law enforcement as well as driving in avoidable, high drag speed regions.

Bob Wilson

 

Generally the odometer is accurate as it comes from the factory. Speedometers are usually a little optimistic. Both of these are regulated by law. In some jurisdictions it is required that the speedometer read high.

Tom

 

There was similar discussion regarding this on the Phaeton Forum. While the speedometer is geared to read 2-3 mph high, the odometer will be dead-nuts on.

This is so that, when using the largest size tire physically possible on the car, the speedometer will not read low (e.g.: you're doing 63 when the speedo reads 60). This is regulation in Europe and is common practice with most European vehicles. I've noticed the same discrepancy in my Prius, whereas the speedometer in my roommate's '10 Outback is dead on.

Having said that, the readings on the odometer are accurate, providing you're using the factory-specced O.D. tire. For example, using the 215/45R17 tire, my speedometer is about 3.3% fast at 60 mph, but the odometer is accurate. When I switch to the winter tires (195/65R15), the delta is 1.49% larger, which means the calculations will be 1.5% too slow, meaning the odometer is now logging miles 1.5% slower than normal and the speedometer is now only 2.2% fast at 60.

 

Just use the Scangauge. it shows the true speed.
Only the speedometer indication is 5% faster, everything else is correct except the mpg/tank average calculation is base on the instantaneous speedometer reading. It is about 5% higher too.

 

Tom,

This would make sense, but what about if someone changes their tire size, would the odo not be off slightly? I do not care, as I will drive my car till it cannot drive anymore, but I am curious to know.

 

Yes, that would throw off the odometer.

Tom

 

Generally, and for the first time in any car I've driven, the Prius speedo and my Garmin agree, and GPS velocity readouts, I'm told, are based on Doppler measurements of the satellites' carrier waves....

 

I've been heading back north from Florida for the past two days and have frequently checked the speedometer reading against my Garmin 760.
Shortly after getting my Gen III I discovered that the speedometer was off by 1 mph. I first noticed it when going by those speed read-out signs and then by checking it against the Garmin.
During this current trip I noticed that below 66 or so mph the speedometer is off 1 mph, yet after 68 mph up to the maximum speed I reached of 74 mph the speedometer and my Garmin both indicated the same speed.

 

I don't know of any common consumer grade GPS devices that measure Doppler speed. Consumer devices use track point time and distance to calculate speed. In other words, at time T1 you are at point P1; at time T2 you are at point P2. Find the distance between points P2 and P1, and divide by the time difference between T2 and T1, and you have the speed.

Obviously this technique suffers with positional errors, and curving paths will also introduce errors.

The Doppler units calculate speed by looking at the Doppler shift on the L1 signal from each satellite. Given that the satellites and the earth are all moving, this is no easy task. Some specialized commercial units can do this. It's only a matter of time until consumer units do too. Perhaps some high end units even now, but I'm not aware of any.

Tom

 

you are mainly worried about clearance?.... 205 is easy for a prius. my Gen II only rubs when i put 225/-/17's on... it's really the width that rubs, not height.

i currently run 215/50/17. the speedo is basically dead accurate and the look is much better than stock... the package V model of the gen II uses 215/45/17... i heard 18 inch rims will fit the gen III without rubbing, so i doubt you will have problems...

 

I currently run an 18x7 inch wheel with no clearance issues with 4 people and a trunk full of crap. i have 215/40/18 tires so they are stretched little rubber bands also.

 

There is a calibration tool in the GPS system on my 2007 Prius. So I would assume it is also built into the 2010 model. If you go into the setup in the GPS system you will see 3 screens. The tire calibration is at the end of the 3rd screen.

Marvinh

 

Keep in mind that this is only for the Nav system.

Tom