I am having a problem with my 2008 Prius purchased in Sept of 2007. Through January of this year (over 3 years), I was averaging approx. 51 MPG and refilling my 11.4 gallon tank at the 450-75 mile mark. In January I had my 55K check up, and 10 days later I put new tires on my car (Capitol/Sport is the brand/style). Ever since my 55K checkup my mileage has fallen to approx. 47 MPG and I am now refilling my tank at the 375-400 mile mark. This mileage issue did start right after my service… prior to the new tires. My local Toyata dealership where I get my services has looked at the car and said that they cannot locate any physical or systemic issues. They are blaming the new tires because they are not Low Rolling Resistance (LRR) tires. The Toyota manual does not mention anything about LRR tires being necessary for the Prius, however the service people at Toyota say that it’s common knowledge in the tire industry that LRR tires are needed for a Prius, making the MPG issue the tire dealer’s fault since they didnt recommend them to me.
The tire dealer states that there is no reason for the tires to impact the mileage, LRR or not. The footprint for the Capitol Sports tire is actually smaller than the Michelins that were on the car, so there is less ground contact… less resistance/drag, therefore equal or better MPG than the original tires.
In all of the articles I have read, underinflated tires should affect MPG by approx. 3%. My MPG has been reduced by almost 10%. I believe that this argues against the tires being the issue and in favor of an issue with the car itself.
In addition the the actual MPG statistical reduction, and the need to fill my tank more often, here are my symptoms:
when I take my foot off of the gas pedal at an intersection, my car does not roll forward as is normal with an automatic transmission / when I take my foot off of the gas pedal at speed I immediately begin to decelerate (even when going down a hill… as if the Prius’ transmission was in the “B” regenerative braking postion, however it is 100% in “D” drive) / I can feel sluggishness in the car as it is moving… whether I am accelerating or have removed my foot from the gas pedal it feels as if the wheels are not spinning freely… as if I were driving through several inches of standing water / when I do accelerate I can hear the engine working harder than prior to my 55K check up in January / the realtime MPG calculator on my dashboard will plummet to 12.5 MPG whenever I accelerate… whereas before the 55K checkup it would drop to the 25 MPG mark.
Now here is the kicker:
About 6 weeks ago this entire problem disappeared. My car ran 100% normal; 51 MPG averages and 450 mile gas tank fill ups. About 2 weeks ago the problem returned, except now it varies from day to day. Monday I might get 53 MPG, Tuesday/Wednesday 44 MPG, Thursday 51 MPG. I drive the same route to work every day (95% highway miles), and have ever since I have owned this car. Same start and end points for my commute, aggregate weather and traffic conditions for the past 3.75 years
As noted, my primary Toyota people have visually inspected my car, and plugged it in to the monitoring system more than once, and they state that there is nothing at all wrong with the car. I took my car in for my 60K check up a week ago to a separate Toyota dealership. I explained the issues I was having, the offered no additional insight other than to say that my fuel injectors needed to be cleaned. It doesnt seem likely that dirty fuel injectors would cause my car to slow down when I take my foot off of the gas rolling down a hill, so…
…anyone out there have any suggestions or thoughts about what might be wrong? Tom or Ray, or any CarTalk people…? Please assist…!
You’ll have better luck posting this on a Prius forum. This variation, while different than you’ve had, is still within a pretty narrow range. And 47 vs. 51 mpg is a very small reduction over all. Any possibility you’re now getting gas with 10% ethanol? And the close proximity of the change to the tire change also makes me wonder.
Footprint alone does not determine rolling resistance. There is more to a LRR tire than being skinny. The rubber compound and tread pattern have a larger affect.
But all that said, was any work done on the brakes, even an inspection? If so, they should look there again, especially the parking brake.
Keith’s right, the tire dealer is way wrong, LRR tires are LRR because of many details in the design. I bet LOTS of the reduced mpgs is the tires, no-name tires (at least I’ve never heard of them) are not the way to go to get maximum mpgs.
“Toyota say that it’s common knowledge in the tire industry that LRR tires are needed for a Prius, making the MPG issue the tire dealer’s fault since they didnt recommend them to me. The tire dealer states that there is no reason for the tires to impact the mileage, LRR or not.”
While it may not necessarily be “common knowledge” that LRR tires are needed for a Prius, it is fairly apparent to me that the OP did not do sufficient due diligence prior to buying those “off brand” Capital tires.
Yes, a tire that does not have a LRR characteristic is going to take a toll on your mileage–even if the tire dealership lies and tells you otherwise. You get what you pay for, in essence.
Whether the tire dealer (who wanted to make a sale regardless of the consequences) made the OP aware of the importance of LRR tires or not, the OP owed it to himself/herself to find out what type of tires are best suited to that car. Just because a tire is the same size, and just because it is cheap, that does not necessarily mean that it will provide high mpg figures. Just because the tire dealer does not tell you that a particular tire will negatively impact your gas mileage, that does not automatically mean that the tire dealer is at fault.
While I empathize with the OP, this incident serves to prove the principle of Caveat Emptor. If you don’t perform your due diligence prior to making a purchase, you are likely to suffer the consequences.
You bought the wrong type of tires for the car, and whether you can blame the tire dealer or yourself for this mistake is an issue that you can debate from now until Doomsday. I am of the opinion that any consumer who does not educate himself/herself prior to making a purchase is being VERY foolish.
One tire dealer classifies Capitol Sport tires as high performance sport tires. High performance sport to me indicates that the tires have a sticky rubber compound. These don’t sound like LRR tires (OEM) to me. It is not surprising that your mileage decreased.
high performance tires…on a prius… 
I seem to recall something about Toyota dealers reprogramming the transmissions in these cars to take a load off the brakes without the customers knowledge.