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Alignment/New Tires adversely affecting gas mileage

This is a puzzler of a different sort for me.

I recently had an alignment job done on my 2004 Toyota Prius, as well as a replacement of all 4 tires. Since then, I am consistently experiencing 2-3 fewer MPG on my gas mileage readout, despite employing the same route to work and driving habits. I am on my third tank, now, since the work was done and the mileage has been consistently 50-52 MPG, where I normally am able to get 53-55 in this type of weather.

Does anyone have any insight as to why this might be happening?

DO NOT…repeat DO NOT go by the readout to determine gas mileage. The ONLY accurate way is to fill up and determine how many gallons you added. Then divide that into the number of miles you’ve driven since the last fill-up. Do this several times and take the average for more accuracy. Those MPG readouts are notoriously inaccurate.

Tires have different levels of rolling resistance. This translates to MPG loss if prior tires were better. Typically Hybrids and other “eco” labeled cars are equipped from the factory with tires with very little rolling resistance.

You likely chose or were given tires that did not offer lower rolling resistance. Rolling resistance is one of many factors(compromises) that make up a car tire. Many (older design) low rolling resistance tires offer less than stellular winter traction for example.

What tires(make/model) did you have installed?

@MikeInNH: The Mileage total at which I need to fill up has dropped somewhat as well, so I’m pretty sure it is, at best, not just the readout.

@raj: If I recall correctly, they were Goodyear Integritys.

It does not take very much to drop mpg by 4%. A single extra stop on a regular route will do it. Traffic congestion, an extra errand – all can account for your perceived change. This is true whether you use the questionable computer readout or compute fuel economy the right way.

It is possible your new tires may contribute to the perceived change. Some tires have low rolling resistance, others sacrifice mpg for other desired qualities such as performance or treadlife. You go look up the properties of your old and new tires. Maybe that will put your mind at rest.

A good read for you below. Goodyear integrity’s are a dated design btw.

http://www.tirerack.com/tires/tests/testDisplay.jsp?ttid=121

Like MikeinHH, I would suggest that you ignore the mileage gizmo and also ignore “mileage at which you need to fill up.” A correct MPG can only be calculated by dividing the fuel usage by the miles driven and averaging after several fill ups. When we look for problems we usually find them. Quit looking.

@raj: Dang. Live and learn. I went for the cheaper tire because I thought tire life was the determining factor and my Prius is already over 135,000 miles. If I had known they would result in poorer mileage, I would have sprung for the better tires.

Thanks for the response.

You should also be aware that new tires consume more fuel than worn out tires - all other things being equal.

The alignment can also affect mileage, although in your case I do think it is the tires. Why did you get an alignment? Did your previous tires wear out too fast or show an irregular wear pattern?

You could easily be using the air conditioning more lately. That would do it.

@CapriRacer: New tires consume more fuel than worn-out tires? Why would that be the case? I would have expected the opposite–that newer rubber would mean better traction and, thus, less effort required by the vehicle’s engine.

@keith: The dealer recommended it after performing scheduled maintenance. They said it was off by a couple of degrees.

To get the same mpg you would want to replace the tires with the same tire brand and model fitted on the car when it was new. If that tire is no longer available you need to specify the you want a low rolling resistance tire such as tires that would come on a new Prius or another hybrid car.

Your new tires are simply less efficient than your old ones. While you are noting a 2-3 mpg difference you are getting 50+ mpg so the difference in cost of fuel over a year is not too big. TireRack.com has a good evaluation of currently available low rolling resistance tires. Some are more efficient than others, some less noise, some better traction, etc.

In the case of the Prius, you’ll definitely see that size mileage drop from installing ti8res other than the “low rolling resistance” tires that it came with.

I’ve even seen that large a drop in switching types. I bought some Cooper all-season tires once and I could feel the drag. And my miileage dropped. And they didn;t last worth a darn. They were the best winter tires I ever drove…but definitely the antithesis of “low rolling resistance”. I switched to Hankooks at the nest tire change.

Regarding rolling resistance, over the years I have pushed hundreds of vehicles into the shop using my truck or my back and there is no rhyme nor reason to the rolling resistance of tires on vehicles. I have pushed heavy duty pickups and seen them roll away and require me to run ahead to prevent crashing into a vehicle. And, on the other hand, some small coupes have required me to get help to struggle and move them foot by foot. And no, the brakes were not dragging. Go figure.

I thought the Integrity was the OEM tire for the Prius, despite its dated design. If so, OP shouldn’t have seen a drop in MPG from installing it. Any chance this just LOOKS like a drop in gas mileage? Newer (less worn) tire has a slightly larger diameter, so turns less times per mile. Odometer now underestimates mileage driven by a slight fraction and computer accordingly underestimates gas mileage.

Scrabbler

Have you checked the pressure on the new tires? Were the older tires a tad overinflated?

I agree that the gizmo is not to be trusted. On my Honda the mileage is delusional and gives me 2-4 miles over what I really get.

StuckInNeutral,

Why more RR for new tires?

Long version: http://www.barrystiretech.com/rrandfe.html

Short Version: RR is mostly about internal friction caused by movement, and inversely related to road friction (traction). More mass = more internal friction.

RR is about

  1. the movement of the tread against the pavement, which is a result of tire construction, air pressure, and tread durometer, and tread design
  2. internal friction caused by the internal movement of the materials of which the tiire is constructed,
  3. load (weight), which affects 1 & 2 above.

Mileage changes can occur with a differnt set of tires for the above reasons PLUS the tire weight, which while it has no effect on rolling resistance DOES affect rotating mass. More mass requires more energy to get spinning.

One can also experience a measured change in mileage if the new tires have a different diameter, although this should be miniscule unless one has changed tire sizes.

Yes, a different tire can definitely affect mileage both real and measured. Been there, done that. I’ve personally experienced a 2 mpg difference.