Oh No!!! Not another mileage thread about tires...

I was going to add this to another thread, but I'm so surprised at the difference, I decided to start a new thread.

I've had my Cooper S for more than two years. With the stock tires (17" 205 Dunlop runflats) I got pretty good mileage, but a lousy ride. I decided to swap them out for a set of Bridgestone Potenza 215 series tires when I did my Spring maintenance (oil change, wheel swap). My mileage dropped quite a bit from easily getting 28mpg all around miileage to struggling to get that with the Bridgestones. I'd get about 32-34 on the highway at 65-70mph. I thought it was partially due to the oil level being higher as well as the wider tread.

When I put my winter wheels on, my mileage went up to where it had been initially 26 city 34 hwy with 16" wheels and Dunlop Winter Sport tires.

When I switched back to the Bridgestones and did my oil change, mileage got even worse. So I sucked some of the oil out of the car (to get to 1/2 qt from full to the top mark) and mileage improved somewhat.

I'm heading fotr Omaha next week so I decided to get a new set of wheels and tires. I wanted to get as much mileage and comfort as I could. So I found something that would just fit. I got 15" wheels that weighed 12.5 pounds each (vs. the 25lb Mini 17" wheels) and 195/65/15 Bridgestone Ecopia tires that weigh 19lbs each (vs about 25lbs of the Potenzas).

This configuration gave me a larger diameter, but now my speedo is closer to being accurate than with stock. And they just fit in the wheel wells.

But my mileage has jumped to 40mpg highway at 65mph, and if I drive at 50mph on flat ground, I can squeeze almost 50mpg out of them. Now, this is using the trip computer calculations, so in actuality mileage is better than that due to the increased circumference of the tire.

And they're scads more comfortable than the 17". Granted, performance is not as good as with low profile tires, but it isn't bad, and I'm getting older so I don't go bombing around like I used to.

The Ecopia tires are supposed to be designed for lower rolling resistance, and I'm not sure how much that contributes to the increase; but these tires are a lot less noisy than the Potenzas too.

So I'm delighted with these wheels. They look pretty nice too.

 

Interesting, what wheels in 15 did you find to fit the 07 MCS brakes? Source? Cost (if you want to share)?

Have to ask, are you a hyper-miler?

 

wider wheels = more friction on the road = worse gas mileage.

I am surprised that your mileage dropped so much though. ...I am running 17/205's and still get 34-35mpg (I do have a FMIC + CAI which increases gas mileage though).

 

Interesting result...thanks for sharing!

 

i swapped out my stock 205 17 for 215 17 (also on new wheels which were a slightly lighter) and I get about 35mpg on the freeway doing steady 75.

the stock tires were bridgestone and the new tires are toyo t1-r which are about 1.5-2lbs lighter than the stockers.

 

Don't forget, different circumferences will affect the odometer as well as the speedometer. If you did not calibrate the odometer to the different tire size circumferences then your numbers are not real.

 

I was wondering if the hard sidewalls on the runflats would effect mileage. Looks like there is something good to be said about them.

But like Browser said, you can't compare the mileage with the large circumference tires without making adjustments for the different miles traveled being reported.

 

NM, somebody already said it...

 

+1

50 mpg sounds little high to me....

Grab a tape measure, and measure the circumference of each wheel tire set up if possible. More than likely your odometer is registering more miles than actually travelled.

If you think about it, the auto industry has spent tens of millions of dollars each year on R&D to shave off pounds, wipers, antennas, door handles, window trim, ect increasing aeordynamics. If it were as simple as swapping tire/rims to increase fuel mileage by 30%, I would have hoped.....hoped they would have done that.

 

Of course tires have some effect on MPG because of many factors, but to a much smaller degree than the data your presenting. As stated the size of the wheel is whats effecting your math more than the actual. If you went from a 205/45 17 to a 215/45 17 your speedo is reading a 1.5% difference so when you do 60 mph your actually doing 61mph, and thats going to effect your mpg calculation.

http://www.miata.net/garage/tirecalc.html

 

That's why Eco box like Prius comes with narrow low rolling resistance tires(aka. low grip). The trade-off is always there .

 

Bingo. There's no way the car gets 40-50 mpg, nada nohow. You can pretend though

 

I forgot what brand and type my stock tires were... but the size was 195/55/16 on the stock 16x6.5. I changed to Volk Racing TE37 16x7 with Hankook Ventus RS2 205/50/16. Noticeable and consistent mpg change. I'm getting about 30-40 miles more per fill-up now. Then again, the change was probably more due to the very light TE37s compared to stock than the tire size...

 

I think it's really kool that dropping half a quart of oil can boost mpg. Now, to what should we attribute that?

 

Hey, oil weighs a lot, don't you know that?

 

Yeah. I've heard of heavy weight oils. Never thought about it, but suppose that's why the filler tube is only at waist level.

 

Also, even if you're replacing worn tires with identical new tires, the difference in tread depth can make as much as a 2% difference in tire diameter (and calculated speed), giving you falsely-low economy readings whether you use the OBC or do the calculations by hand using the odometer.

Plus, new tires can weigh as much as two pounds more than worn tires because of the extra tread, and all that weight is at the worst possible place (far away from the center of rotation), causing an actual loss of fuel economy.

I lost 2-3 MPG in my calculations after replacing my tires recently. Some of the loss is real, and some is just from the odometer inaccuracy due to the fresh tires being taller than the worn tires.

 

Since the new tires are larger in circumference, the mileage is actually BETTER than it appears, not worse. I didn't believe the increase, but it is correct. I'm going out to Omaha next week, so I'll have 2,200 miles of driving to confirm or refute what I've gotten in the last 600.

Whatever it is, if you like the look of the 15" wheels, and you want to increase your mileage dramatically, look into a set. Be careful selecting wheels because these just fit, and not because of brake clearance. They are <1/8" from the hubs in the back. The tires almost touch the wheel liners at full turn lock in the front.

 

Wow they do look great, didn't know there was a 15" rim that fit the MCS brakes.

The AB looks so blue/purple in your pics.

 

...not so sure about that. Cars are way larger and heavier than they've ever been. Look at most cars from the 60-70's they were much smaller and weighed a lot less than our 5,000 lb typical car on the road today. They key is to make smaller cars not SUV's. It takes a lot of energy to move a heavier car and less to move a light one. If auto manufacturers wanted to make more fuel efficient cars they'd make them smaller.

 

I lived the 70's,,,,, and sat in the Carter era gas lines too.

In the mid 70s they were making hidden wipers, headlights, removed window trim/moulding, stream lined door handles, bumpers, grilles, new tire/tread compounds, underbody streamlining, and by the 80's removed roof racks, and car antenna's and started using lots and lots of light weight plastics......I'd say that cost them a bundle in R&D......

Holy cow, can't be too many CAR's being manufactured today weighing 5000 lbs ? Are there

"CARS" were actually bigger in the 70's- : Check out the 71 Chevy wagon, Ford Grand Torino Wagon, Oldsmobile Vista Wagon, and ANY one of the Cadillac's being made then.....every car in the early 70's was built like a Tank, and the doors and trunk lids were two feet thick and built of raw steel and not of aluminum like today's cars. It wasn't until the late 70's a just a few small cars came onto the scene and a very small percentage when compared to the rest on the road. Ford Pinto, Gremilin, Ford Mustang II......

Which cars in the 60's and 70's were lighter ?

 

Passenger cars are generally lighter today than they were in the late 60s/early 70s, when almost everything on the road was American. But compared to the mid/late 80s, they've gotten significantly heavier. Look at the weight/size progression over the last two decades for popular cars like the Accord, Civic and Camry - they've all gained about 1,000 pounds.

I think part of it is that manufacturers are having a hard time keeping the weight down with all the safety/emissions requirements and added electronics in newer cars. Look at the Lotus Elise. It's a tiny little pocket rocket, and weight was obviously a major design consideration (even the carpet is optional). But it weighs more than the 1984-1987 Honda CRX - as much as 280 pounds more, depending on which CRX you're talking about.

 

What are you smoking? Cars from the 60s and 70s were huge and were all well over 2 tons, as a result of the oil crisis in the late 70's, the 80s and 90s saw a movement toward smaller cars, but the rise of the SUV has brought the average car weight back closer to 60's and 70s avgs. Still your average car (excluding SUVs) today are much much smaller than those of the 60's or 70's. Again SUV's aside, there are very few cars over 4000lbs on the road.

 

Unfortunately, you can't discount SUVs when they make up almost 30% of the vehicles on the road.

In 1975, cars (of all sizes) made up 81% of the market, and SUVs of all sizes made up 2%. In 2007, the market share for cars was down to 45% and the share for SUVs was up to 29%. (The rest of the market is made up of pickup trucks and vans, whose numbers were about the same in 2007 as they were in 1975).

Source

Clearly, SUVs are being purchased and used in much the same capacity as cars were used in decades past, so it's time to end the emissions loopholes currently afforded to SUVs (which were originally intended for things like farm vehicles and construction vehicles) and start treating them like what they are - big cars.

 

+1

Also, (slightly off-topic) the law that let's SUVs have darker tint in the rear and cars can't (at least that's the way it is here in Hawaii).