Speedo Calibration & Trim Level

Don't know the answer to this, but I have some auxiliary data - there are suspension differences. When you go through the parts databases, there are different part numbers for various suspension bits (sorry, I don't recall which) for the vehicles with 17" and 19" wheels.

What that relates to, I don't know. I guess stiffness adjustment for the different profile tyres would make sense. So they are fine-tuning, rather than just popping different wheels on the same car. So maybe they fine-tune the speedo too.

 

Historically, Prius speedos exhibit the opposite behavior: 60 MPH on the speedo equates to 58 MPH on the GPS/radar. You really don't want your speedo under reporting your speed. The traffic officer is not going to believe it when you say your indicated speed was only X when he clocks you at Y > X.

 

I experience the opposite. I'm about 2mph slower than the speedometer shows. My suspicion is they calibrated it to the larger wheels.

 

Highly doubt it.

I’m sceptical: all car manufacturers are legally obligated to ensure the speedometer displays at least slightly high, with any stock wheel/tire combo.

 

Chrysler speedometers are usually dead-on the GPS (in at least my experience with a few recently). But yes, the Toyota speedometers I've checked have all been on the order of 2mph above the GPS (that's good user experience... "I don't get as many tickets in my Toyota").

 

My Toyota speedometer reads high. BUT Toyota does have and use different speed calibrations for each factory tire size.

Now for the interesting part - the car’s ecus have very accurate speed - which you can access through the obd2 port.

Bottom line is Toyota wants you to stay under the speed limit and overstates the speed on the instrument panel. In my case by 3-4%.

As a result I monitor and alarm on the true speed and engine temp separately.



It is one reason why some people are annoyed by Prius drivers on the freeway. We may think we are doing the 65 mph speed limit but are actually driving closer to 62.

 

Much like everything else with Prius and monitoring aspects of our cars, it makes a difference what kind of GPS is being used for calibration. Is it a Garmin GPS device or an ODB11 app GPS reading or something else used.
Beyond that is speed linear across the range of mph typically driven or even the entire speedo range.
Seeing many calculations of almost all events posted by us here at priuschat are, static / one reading, observations, when many times these are things that are not always linear across the full range of data points.

I like the idea of comparing GPS speed readings to Speedometer observations.

Personally, I don't know if I'd trust my ODB11 app GPS readings as much as I'd assume one of the Garmin devices calculations.
I think at least a few readings within the typical mph range of each member participating would be better than only one.

I've noticed that extremely slow mph are not linear, but that is so far off normal everyday driving, that I never documented it, and it's not that far off to be noticeable easily, anyway.

 

On a single model of car?

When there’s more than one wheel/tire combo from the factory, AFAIK they keep the tire OD’s close.

 

That might explain why the car always displays gas mileage figures that are about 2% better than what I observe at the pump. For example if it says I got 50mpg for 300 miles, it might take 6.2 gallons to fill it up.All three of my cars did this (Gen2, Gen3, and Gen4).

 

I don’t think so. Speedometer error is by legislation, and the car knows the more accurate speed behind the scenes. MPG error I’d speculate is intentional, requested by Toyota executives, as “rosy” as they can get away with.

on my 3rd Gen, the very consistent “error” is around 7%, on the optimistic side of course.

 

Yeah, I'm skeptical on that statement too. Not least on what you define as "stock wheel/tire combo". There are no wheel choices offered on US Priuses, as far as I know, so no opportunity to switch between 17" and 19". LE/SE have 17", XLE/XSE/Ltd/Premium have 19".

So why would a car always sold with 19" wheels have any requirement for 17" wheels that are not an option for it? How can you apply a rule to something you don't offer?

Maybe if the two trim levels were bundled into a single homologation / type approval, but they're not - they're distinctly different cars, with different official efficiency figures.

 

Dealership told me that switching an XLE from 19” wheels to 17” wheels is not recommended. The LE and XLE do not have identical components. I guess I need to get more info. The extra Toyota tire warranty is only for initial 6 yrs. If I had free flowing money or lived in an area that had well maintained roads, I wouldn’t be so worried about this.

 

When dealerships say things like that, it really means "I don't know anything about the subject". It's not the 19 to 17 that is the concern. It's the diameter and width of proposed setup to the old. Having a 17 inch setup that is the same diameter and width of the 19 inch setup will do absolutely nothing to the drive components.

 

Earlier gens did have some differences. Our 3rd gen came with 17" wheels, has different steering and suspension. I think though with the advent of 4th gen they're all have same steering/suspension.

 

As I said above, in the parts database I did see some suspension part number differences between the 17" and 19" trims. Can't say what they were or why though.

I'm not worried about getting smaller wheels, but maybe going from 17" to 19" might not be ideal, if they're adjusting suspension for expected tyre thickness. Could be excessively harsh.

 

It is true that the suspension components are different between the 17" and 19" setups. Namely the shocks/struts, stabilizer bars, and front stabilizer bar bushings. I'm going to guess that the differences are because of the combined weight differentials between the whole car and the wheel+tire combo. IOW, it might be less about the differences in wheels and more about the heavier curb weight of the higher trims.

That being said, Toyota seriously overbuilds their suspension components and I wouldn't let that stop me from changing wheel/tire setups. But I would also accept that those changes might have a very slight negative impact on handling/comfort and/or long-term longevity of the suspension components. A person just needs to balance their priorities and not think they can get something for nothing.

 

@daisy555 here are the 2023 - 2025 Prius Prime wheel specs from the wheel-size.com database, so you don't have to take anyone elses word on it.

• Generation: XW60 [2023 .. 2025]
• Production: [2023 .. 2024]
• Sales regions: USA+, Canada
• Power: 220 hp | 164.1 kW | 223 PS
• Engine: 2.0 L, M20A-FXS, I4 , Hybrid
• Trim Levels: SE, XSE, XSE Premium
• Center Bore / Hub Bore: 60 mm
• Bolt Pattern (PCD): 5x114.3 (5x4.5)
• Wheel Fasteners: Lug nuts
• Thread Size: M12 x 1.5
• Wheel Tightening Torque: 103 Nm

Code:

TIRE                 RIM           OFFSET   OFFSET range (mm)       
E195/60R17          6.5Jx17         ET40         38 - 42
OE195/50R19         6.5Jx19         ET40         38 - 42

tire size comparions tool at tiresize.com and get a 17" tire that is close to if not exactly the same diameter and width as the 195/50R19 tire

 

Thanks so much! On my walk today I was obsessing about whether I should be switching to an LE instead of XLE but I really want the comfort features of the XLE. I don’t have anyone to potentially switch tires out for me so I would be relying on a tire store or dealership. Maybe not do anything unless I have an issue with the OEM size. I guess selling the 19” wheels would be easier if they were new but I don’t have a place to store them while trying to sell them.

I really wish Toyota had just made all levels 17”. What a PIA.

I did find this: