What should wheel alignment be for best MPG?

Front: 0 toe, -0.58 degrees camber (which is the center of the spec range)

Rear: 0.30 degrees total toe-in and -1.5 degrees camber is the center of the spec range. The minimum total toe-in is 0.05 degrees total.

If you set the rear to the minimum of 0.05 degrees, that may improve your mpg but the car may be less stable at highway speeds.

I would set the camber at the middle of the spec range if possible, as that will improve handling when you take a turn at speed while promoting good tire tread life.

I assume that shims will be used to adjust the rear wheel alignment? You may find that it is not possible to simultaneously adjust toe-in and camber to hit the desired spec. If a trade-off has to be made I suggest that you prioritize achieving the desired toe-in adjustment. Good luck.

 

Since the number is positive, is the rear toe-ed IN or toe-ed OUT?
On most cars a (very) little toe OUT at the rear makes it more stable. If your car is pushed sideways a bit (crosswind, rain grooves, minor steering correction, etc), this shifts weight to the side, and then the rear toe-out brings the rear around slightly, aiming you back to the center. Lack of this effect causes highway wandering and constant steering correction, which is very tiring on long drives.
My 2005 lacks this, and will need a shim kit to make it so. Next alignment I will bring the kit and let them have at it.

 

Since the measurement is total toe-in, a positive number means the wheels are toe-in, not out.

I disagree that toe-out is helpful in improving stability. Note that the Toyota spec range for the rear axle toe-in (0.05 to 0.55 degrees) does not allow any toe-out on the rear wheels. The minimum toe-in of 0.05 is still positive.

 

In seeking the best mileage, Toyota specs seek minimum friction. Lack of front caster is another contributor to low friction (under dynamic conditions) but lack of high speed stability.

Trial and error may be best, but for example, with the spec toe front and rear the car is very tiring to drive on the open highway at speed.

What I've found is that every perturbation from a straight line sets up a series of reactions in a car. Say the car is steering to the right in a curve. Weight is shifted to the left tires. If the car is toe-ed out in front, the front of the car will steer to the left, since the left-aimed tire is taking more downforce than the right-aimed right tire, due to weight shift. The car will require further corrective steering to brig it to the intended line. Toe-in is intended to avoid this dynamic.

Now think of the left rear tire under this scenario. If it is toe-ed out slightly, the rear will tend to seek a wider turning radius than the front, bringing the rear around a bit, restoring you to your intended line.

Fractions matter, but it is a difference that can be felt as increased precision of hitting the apex of the turn, or in maintaining your line without repeated steering corrections.

This is counter-intuitive, but if you picture that the wheel with the greatest weight will do more of the steering duties, it starts to click.

With its weird rear suspension, an old VW Bug taught me about rear wheel toe and camber effects, which were exaggerated by its rear-engine (and therefore rear weight bias), and I found out how crucial rear toe adjustment is.

But the Prius is not adjustable for rear toe without using shims. Nevertheless, I'm finding out it is key to stable handling and directional control.

My next alignment will be four wheel, with rear shims, and I will be able to verify the use of rear toe as a means of making the car more stable on the highway, in undulating corners, and in crosswinds. Next to the uncomfortable driver's seat, this is my biggest complaint abotu the car, but the solution is near.

 

This is not universal. Manuals for other cars always use negative numbers for negative values of camber and toe.

 

How are they aligning the rear? It can be done with shims or a Toyota custom prybar, I am told. Given my local dealer's lousy alignment skills (they could never get the steering wheel straight!), I'd stick with shims and an outside shop. But yours may be better at it.

 

I went to Toyota thinking they would know the best way to align the car for optimal milage. I was told they would not be able to align the rear. I figured if there is some deeper problem with my car that is causing the lower mpg that I am receiving, and it is best to have the alignment performed by Toyota. I called and asked them to check my wheel bearings while they were at it, they said they would have to charge me $90 more to put the car up. In the conversation I found out my car was now at another facility. I asked if it was a Toyota shop the service rep said it is T2 an affiliated shop.

I now think they outsource thier alignment to a regular shop and I'm dissapointed since I already have an alignment guy that would have aligned all 4 wheels for me for less. The printoutI was given by Toyota shows all 4 wheels are now in spec, but the rears are not near perfect and I may pay to have it done right elsewhere. The invoice states in the exact words "The customer requests a 2 wheel alignment." I was told a two wheel is all that could be done. Based on the numbers you guys see here should I go elsewhere and have all 4 done, just the rears, or is it good enough?

 

The rear alignment seems OK. However I am surprised that no effort seems to have been made to correct the RF camber which is too negative.

The front toe-in was excessive before, so perhaps you may notice a slight mpg improvement now.

 

The person who took your appointment request may not realize that Toyota repair policy is that the rear suspension is not adjustable.

See if they charge you for a four-wheel alignment, but only perform a front-wheel alignment.

 

Patrick, is the front camber adjustable on the 2007 Prius? You leave me wondering the same thing. Should I call them and ask?

 

The normal way to adjust the rear with shims is to use a pair of tapered shims per side. By rotating the two shims relative to each other in the shim pack, then rotating the shimpack as a unit, any combination of toe and camber can be achieved within the limit of the shims. IIRC, the total range is +/- 1.5 degrees from the no shim condition.

 

Do you have a source for metal shims? I am not confident the plastic ones will stay torqued down properly.

 

Yes, within limits. The adjustment process is to loosen the two bolts that hold the bottom of the strut to the hub assembly. To make the camber less negative, pull the strut towards you, then tighten the bolts and recheck the measurement.

If the camber cannot be properly adjusted within the existing range, then special bolts can be installed that will modify the available range.

I think it is reasonable to make a query regarding what had already been done to adjust camber, and why the special bolts were not offered to you (at an additional charge) as a means to get the measurement into spec.

 

I have had some concern about that too, but the plastic ones have been around for years without developing a bad reputation. Talk to some people who have used them.

They are a "filled" plastic so the load path isn't just plastic.

That said, if I use some I will still recheck the torque after a couple k miles.

 

Patrick,

Notice my "SAI" and "INCLUDED ANGLE" do you think those would have much of a negative effect on MPG? My feeling is the alignment is probably not causing most of my low MPG, do you think that's reasonable?

I got a Prius with 16,000 miles on it, I've been trying to figure out why I can't seem to break 37-38 average.

Based on what you see could it be because of the alignment?

 

SAI is steering axis inclination. That is not adjustable, and if that spec is off this may be due to accident damage or excessive suspension part wear. Since your car only has 16K miles, unless it was in a front-end accident, that measurement should not be so far off.

I don't know what "included angle" is.

In any event, I think that toe is the major alignment parameter that potentially could affect mpg, since that has the potential to add rolling resistance. Incorrect camber might cause uneven tire wear but I don't think it would have much mpg impact.

 

"Included Angle" is the sum of the Camber and SAI.

I think the most likely explanation for the strange SAI numbers is alignment "tech" error.