2006 Toyota Prius; decrease by 4 mpg with installation of #4 Cooper CS4 Tour 88T tires (48/49 to 43/44)
The new tires probably have higher rolling resistence. Same size as stock right?
You’re still getting better mileage than just about any other car on the road, I’d just live with it.
You Are Now Saving Over 50% In Gasoline Costs. Have You Had To Buy Any Gas, Lately?
It costs me less to cruise in my Bonneville now than it did in a Prius, a month or two ago. Gas near my home is less than 2 bucks/gallon and dropping. Gasoline is no longer an issue.
Gasoline in terms of one’s own pocket is less of and issue than it was a couple of months ago.
In terms of everything else it remains a huge issue - conserve for much bigger reasons than your own little pocket book.
The rolling resistance thing is probably the key as FoDaddy said. Was this tire size/model a recommended one? As I understand it the Prius requires special low resistance tires to keep it where it is supposed to be for fuel efficiency.
I Don’t Subscribe To Any Other Reasons Than The Cost Factor !
Should you choose to have other concerns, that’s fine. I have been around too long and studied (environmental science) this issue too much to be coerced into feeling selfish or whatever feeling it is you would wish upon me. It’s strictly a cost, government control, carbon dioxide tax, man-made global-warming (sorry, I mean climate change, we’re now cooling), thing. I will not be persuaded to the contrary.
You state, "In terms of everything else it remains a huge issue - conserve for much bigger reasons than your own little pocket book"
You do that if it concerns you.
Respectfully,
Common Sense Answer
P.S. By the way, I have been conserving and recycling since before it was popular and probably before most of the people pushing this philosopy, were born. I have never been wasteful and I won’t feel guilty about living conservatively. Just by my nature, my “carbon foot-print” is so small, I would probably make a tree-hugger blush.
America, what a country!
I meant no offense - it was just my 2cents. I have actually been an obsessed mpg junkie for a very long time going back to 99cent/gal gasoline, so its never been about cost to me. (I remember as far back as 40cents/gal, but I was just a kid then).
But I would add to environmental concerns and “carbon footprints” the geo-politics of oil supply as something else to throw into the mix.
Anyway, to the OP, if following your manufacturer’s recommendations results in noticeably improved efficiency of the car it would seem silly not to follow it.
No Problem ! I Am Concerned About MPG And Safety, Too !
I have owned and worn out two “old” Beetles, one used, one was new. Now I buy only American badged cars (I prefer GM and Dodge). My 3600 pound+ Pontiac Bonneville 3.8L gets as much as 34 MPG, actual (I live in a rural area). I try to get as much out of a gallon as possible, within reason, while enjoying safety and comfort.
America, what a country!
How do you get 34mpg out of a 3.8L? Are you doing some “hypermiling” types of driving? If you have some tips I’ll give them a shot. (I can only guess it isn’t a hydrogen booster 
I agree with FoDaddy that the new tires likely have a significantly higher rolling resistance than the original ones. Toyota puts VERY LOW rolling resistance tires on the Prius, in order to give it more of a gas mileage advantage.
When these are replaced with tires that have a higher-than-average rolling resistance (which is fairly typical for Cooper tires, unfortunately), there can be a substantial drop in mpg.
Nope, the GM 3.8L is usually coupled with a transmission that has very tall ratios. In high school a friend of mine had a Bonneville with the N/A 3.8L it was geared so tall that one could reach 80 MPH or better in 2nd gear, (4 speed automatic). Since the 3.8L produces a good amount of power, GM could get away with tall gearing. Whereas if Honda put something like 2.82 axle ratio in something like an Accord, it would bareley be able to move due to the lack of torque typical in 4 cyclinder engines. Anyawy the tall gearing means that the Bonnville as well as it’s siblings could get very good gas mileage on the open road because the engine isn’t turning very fast. Same thing with the Corvette ZO6. It has a 7.0L V8 but because it has very tall 6th gear it’s been know to get upwards of 30 MPG on the highway. When you look at a car like the Honda S2000 with it’s 2.0L I4, you would expected to get good mileage. But because it’s geared very short, it can only get maybe 23 MPG on the highway because at 75 MPH it’s engine is spinning at 4000 RPM, whilst the Corvette’s engine is loafing around at 1400 RPM. Obviously there’s more details to it, but that’s a basica rundown of how a larger displacement engine can get better-than-you-might-expect fuel mileage.
Rolling resistance is one answer, but it seems that new tires, even identical to the old ones, will often yield lower mpg, due to the change in tread depth. Folks on Prius forums have long noticed this. It’s not peculiar to the Prius, but it’s more noticeable due to the mpg reading always being visible.
I don’t understand. This was posted under “car questions,” but I don’t see a question.
dulaneyglen, are you asking if the tires are the cause? It would be hard to eliminate all the other possible variables.
dulaneyglen, if you are asking for advice, I think you should live with the new tires. The higher rolling resistance might translate into better traction, whould would make the car safer.
dulaneyglen, if you indeed have a question you would like answered, please ask it.
I am not buying a 10% decrease in fuel efficiency because of rolling resistance. Maybe the tires are a different size and are throwing off the odo.
Simply Eloquent, FoDaddy!
We were startled to see the major decrease in mpg and wondered if the new tires might be the cause. A 4-wheel alignment showed no improvement. The vehicle otherwise seems to operate without any stark change other than a sense that acceleration seems dampened. I don’t have available the prior tire type, but there were about 40,000 miles of wear.
Your tires may have a high(er) rolling resistance. The factory tires are spec’ed on your vehicle to have the least amount to bump your EPA figures as that is the sole selling aspect/strong point of a Prius.
Also verify the tire pressure they put in. Many tire places don’t pay attention and this can account for a few % if lower.
In that case, it could simply be that your old tires were so worn that they were actually smaller. It happened so gradually that you probably didn’t notice your fuel economy estimates creeping upward, or you attributed the increase to something else. Then you bought new tires and that gradual change suddenly reversed itself. The new tires are larger since they are not worn. So you are actually travelling a larger distance per tire rotation. The computer that tells you your fuel economy on the dashboard doesn’t know you have switched to tires with a slightly larger diameter than the old ones. Perhaps you were never really getting 48/49 MPG. Perhaps your car’s computer thought you were travelling farther than you actually traveled.
If I had to specualte, I would say it is probably a combination of tire size and rolling resistance. I would expect it to gradually get back to normal after the new tires are broken-in.
If tire wear is the cause, your actual fuel economy didn’t change that much. Only your calculation of your fuel economy changed because the car is travelling farther than it thinks it is traveling. If rollikng resistance was the only cause, your fuel economy did change. However, I think it is a combination of the two causes.
Keep in mind that measuring miles per gallon can give you deceptive results. You are really better off measuring gallons per mile, or gallons per 10,000 miles. An increase from 30 MPG to 40 MPG will save 83 gallons per 10,000 miles, while an increase from 10 MPG to 12 MPG will save 167 gallons per 10,000 miles. Now that you know a 2 MPG increase can save more gas than a 10 MPG increase, how does your slight decrease in MPGs look now? If your measurements are correct, you are using an average of 24 extra gallons per 10,000 miles. 24 extra gallons per 10,000 miles doesn’t seem like as much as 5 MPG, does it? It is probably within the margin of error of your car’s fuel economy calculations.
Here is an anecdote that corroborates your experience: My 1981 Toyota Starlet was EPA rated at 50 mpg, and (with care) would actually do it. The trouble was that the “high mileage” Toyo tires that Toyota put on it transmitted every crack in the road into the car and all the way to my teeth, and the tires had terrible grip in a panic stop on hot asphalt.
I replaced them with Michelin XZX tires one size wider. Much smoother, much quieter, much better grip, but lost 5 mpg. I was OK with the trade-off.
A high MPG vehicle will be more sensitive to extra energy loss than a low MPG vehicle.
Consider a vehicle that gets 50 MPG vs one that gets 10 MPG.
50 MPG = 2.56oz per mile, 10 MPG = 12.8oz per mile
Now, suppose on each vehicle an underinflated tire wastes an extra 0.5oz of gas per mile. The 50 MPG vehicle would drop 16% to 41.8 MPG whereas the 10 MPG vehicle would drop 4% to 9.6 MPG.
It’s strictly a cost, government control, carbon dioxide tax, man-made global-warming (sorry, I mean climate change, we’re now cooling),
Says who??? My daughter analyzes data from all over the world in her research project at MIT. Not only are we getting warmer…but it’s accelerating. Has been for the past 10 years.