Speedo/odo calibrated for OE tire size?

That seems to be the standard on cars not sold in the USA. It is the same in European cars.
Well, it is not 7 km/h always, I measured it to be 6 km/h from 55 km/h to 100 km/h and 7 km/h above that.

The odometer (distance) is correct, so changing tyre size could get the speedometer better aligned to the correct speed, but then the distance will be off.

Just have to learn to live with it. I always drive 5 km/h faster than the posted speed limit, that way I am driving almost the posted speed limit.

 

I've recently fitted much larger all terrain tyres to my car and I want to get the odometer and speedometer back within regular accuracy margins.

On other cars in the past I have installed an electronic module behind the dash to slow down or speed up the pulse from the ECU to the gauges, is this something that's possible in the Prius? And if so where would I find the computer\wire that reads vehicle speed?

My new tyres are 10% larger which obviously throws my speedo, odo, and mpg reading way out.

 

I read an article from a Subaru owner who measured height to center of wheel hub as he varied tire pressure. His tire effective radius changed about 1/32 of an inch for each 2 psi of tire pressure. Not much of a change but it would enter into this discussion because many on here run tire pressure higher than the factory specs.

For a 205/60-16 with an advertised tire height of 25.69 I did the calculations based on the Subaru owners tire radius measurement changes and the results:

25 psi 794 revs/mile
35 psi 785 revs/mile
45 psi 775 revs/mile

My calculations treated this as a linear function when it is probably not. There is likely a smaller tire radius increase as pressure increases and a larger tire radius reduction as pressure drops. If you really wanted to know the effect for your particular tires, you would have to do the empirical testing yourself.

There are 63360 inches per mile.

Formula for revs/mile = 63360 / (tire radius *2 * pi)

 

Did you try to match this up with a specific tire brand and model?

When I pull up this tire size on TireRack.com, the first handful of models checked found revs/mile specs of 810, 812, 814, 808, and 808. I.e. a span not even overlapping your computations.

I've tried some of these explorations myself, including changes with pressure and load, and run into a mess of conflicts between computations, tire RPM (revs per mile) specs, and real world observations. Real world was much closer to RPM spec than to my computations.

For a short summary answer, I must now say that while the tire is mostly round, the rolling surface contact patch is flat, so normal circle equation derivations don't match up well to the real world, or to the tire manufacturer's RPM specs.

So now, I'm not going to believe any computation-only 'effective radius' stuff until it is backed up with actually rolling distance measurements.

 

I was just trying to make the point that revs/mile will change with inflation pressure.

If the Subaru guy's measurements are in the ballpark, the change is pretty small, about 1 rev per mile for each psi change.

I got the tire diameter of 25.69" (205/60-16) from one of the tire websites but those measurements are with the tire off the ground. Their revs/mile values are probably with the tire loaded which would be a shorter radius and higher revs/mile.

Many enthusiasts on this site run higher pressures. If you increase the fronts to 42 psi then that is about 7 revs/mile lower than with the recommended pressure.

That said, even with the higher recommended pressure (35 front and 33 rear) in the front, the front tires have more weight on them than the rears and so they may have a slightly shorter effective radius so revs/mile on the front and rear are probably slightly different. It got me thinking about how big the difference needs to be for ABS to kick in but that question is for another day.

I agree that you would have to do empirical testing on your tires to get the actual revs/mile. The torque app has a PID for wheel speed. If I am ever allowed to go out of the house again, I could try to add the "wheel RPM" PID to my torque app and set the cruise at GPS 60 mph and observe wheel speeds at several different tire pressures. Revs/minute at 60 mph should be revs/mile.

 

I think it could be very misleading to measure the height of the hub off the ground and think that the rolling circumference changes with 2*pi*r.

Think in the extreme case that instead of a (mostly) rubber tire on your wheel, you use a flexible steel circumferential band that does not stretch perceptibly, so the rolling circumference never changes. The hub height would similarly reduce as air is let out, but there would be zero change in revs per mile. Modern tires have circumferential steel belts (cables / wires) that similarly do not significantly change in length.

In our tires that are a composite of rubber and steel cables, the change will be somewhere between the two extremes predicted by the hub height measurement and the idealized steel band concept. I think it will be closer to the steel band though (i.e., much less of a change than predicted by the hub height).

The change is definitely not zero though. And that fact is used (or at least used to be) by some cars to detect low tire pressure without needing TPMS sensors.

A far better test than measuring the change in hub height would be to directly measure change in rolling circumference. I've done this on one of my 4wd vehicles when measuring the change when my collapsible spare is installed, vs. my aftermarket wheels+tires. I had been concerned about a significantly different rolling diameter stressing the differential or transfer case, so I wanted to quantify the difference.

The street in front of my house is flat and very lightly traveled, so I did it there. I lined up the car, with the spare tire installed, put masking tape on the the 4 wheels, all indicating the point on the wheel contacting the ground (i.e., lowest point on each wheel). I then drove straight and level for a couple of hundred feet, a few dozen rotations. After that I was amazed when the 3 regular tires were all virtually exact (they should be the same, but I expected some variation; the exactness confirmed the accuracy of the test procedure). And I could quantitatively measure the difference in rolling diameter based on where the tape on the 4th wheel ended up. As we were doing the test, my daughter was driving, and I was walking along side, taking intermediate measurements, etc., and everything made sense.

Very easy test to do. For this application, you could set all 4 tires to the same pressure, get a baseline confirmation that all are the same rolling diameter (or not, and by how much). Then let some air out of one tire and see what changes.

I really expect if anyone cares enough to do it, they will find that the change in rolling diameter with air pressure exists, but is much less than that predicted by the hub height change with inflation pressure.

 

I was trying to make the point that the described approach has failed to match up with the real world, at least for me. As 2012_Prius_v_wagon points out, rolling circumference matters a lot, and those steel belts under the tread don't significantly change length with inflation pressure.

Yes, there is some change in effective rolling circumference with inflation pressure. No, it isn't as much as the computation in post #43 suggests. And the mismatch between the RPMs in that post, vs. tire maker's specified RPMs for that size, is an additional clue that those computations are not realistic.

That is why I insist on actual measurements of Revs Per Mile, not back of the envelope calculations based on simple circle equations.

 

Yes, depending on the accuracy you're aiming for.

In the test I mentioned above, where I wanted to know how much smaller my collapsible spare was vs. my 22" wheels, wheel/tire math said there would be a 1.1% difference, and my measurements gave the answer as 2.3%.

 

I had toyo hp 205 45r 15. Noticed a large increase in acceleration, way better handling and compromised ride comfort and ground clearance. I enjoyed it for my 8 mile commute but won't be taking any road trips with them. Recently switched back Michelin premier all seasons.the ride comfort is back and the handling is not that bad but I can tell a big decrease acceleration off the line.

Posted via the PriusChat mobile app.

 

Did you mean to say 215/45?

 

OK, I'll bite.

'How large was it' , he asked?