[AMM0029]

I bought a 2008 Touring about two weeks ago with 33k miles on it. I drove it 450 miles home and experienced 24 mph while cruising around 75 mph. The stock Michelins were almost worn to the wear bars and since I am Cleveland, I put new tires on it last week just in time for the snow. The stock tires were 235/60/R17 and I replaced them with Sumitomo 235/65/R17's as I have been impressed with Sumitomos on my CRV. The shop told me that increasing the aspect ratio by 5% would not really make a difference.

After putting on the new tires, I noticed that the mpg went from the low 20's to around 15. I first thought that it was because I had been driving it too hard and then I thought that it was because the speedometer was off, therefore giving the computer inaccurate information for the average mpg calculation. I filled it back up and drove it with my external gps. The speedometer is about 2 mph off when I am going 70 mph as the gps says that I am going 72 mph. I've really watched my driving and avoided any hard starts as well as avoided going over 70. Also, I have been coasting to stops and really trying to drive it like a minivan and not a sports car. I currently have around 17.8 mpg with 80% interstate driving.

Here are my questions:
1) Could increasing the aspect ratio on the tires 5% really have this dramatic effect on the mpg? I know that the tires are bigger but only by about 1/2 of an inch.
2) Has anyone had their speedometer calibrated? How much does it cost and is a 2 mph difference at 70 mph really enough to throw off the computer calculating the average mph as well as justify having the speedometer calibrated?
3) Has anyone experienced this when they increased the aspect ratio of their tires by 5% and/or has anyone else put 235/65R17's on their Odyssey? I can't be the first person who went with this tire size.

The ride is much smoother and quieter since the change and I am happy with the tires with the exception of the current gas milage.

Thanks in advance for any ideas.

 

[SVO LOU]

Like you, I don't the aspect ratio will make a difference, however, tire compounds can.
Michelin is now touting that gas-saver tire (X-one?) and it probably varies between different wear ratings also.
Yours is a big difference, though.

Lou

 

[kenny1773]

Here is what you did to the tires by changing the size

Tire Size Comparison

Specification - 235/60-17
Sidewall - 5.6in
Radius - 14.1in
Diameter - 28.1in
Circumference - 88.3in
Revs/Mile - 718
Difference - n/a


Specification - 235/65-17
Sidewall - 6.0in
Radius - 14.5in
Diameter - 29.0in
Circumference - 91.2in
Revs/Mile - 695
Difference - 3.3% (compared to stock)

Your odometer will read fewer miles than actually traveled, so trying to calculate MPG based on using your odometer will not work.

How about taking an odometer reading from your GPS and using that to calculate?

It's Monday, my brain can't spell out the math that is needed to calculate your real MPG using those tires


Since you have effectively changed the final drive ratio, your mpg will probably suffer in some cases (like city driving) and gain in others (like long highway where you are in overdrive most of the time)

 

[davedrivesody]

Good points above.

Also, the tread on your new tires is softer than the old ones (age makes it harder) and so will have a higher rolling resistance (i.e. better traction). Fuel mileage almost always drops with new tires for this reason.

 

[AMM0029]

I had the tires changed out yesterday with Michelin Primacy MXV4's that are 235/60/R17. The speedometer is matching up with my GPS and the instant MPG went from 18 mpg (which I know was not correct due to speedometer being incorrect) to 25 mpg. I called three Honda dealers to see if they could calibrate my speedometer and fix this without having to change the tires but all said that they could not calibrate the speedometer. The tire store (NTB) said that since I was close to being within 3% of the OEM aspect ratio then I would not see any difference but evidence is pretty hard to doubt. Thanks for all the comments.

 

[pvp]

Maybe I missed this, but just for my understanding did you notice as big a difference in the actual miles travelled on a full tank between the original tires and the Sumitomos? I realize the computer calculations were messed up by a lot, but what about the miles physcally travelled on a tank (or did it really affect mpg by that much)?

 

[AMM0029]

I don't know how much it actually affected my milage as I never tried to calculate it since I knew that the odometer was off. I do not think that it had that dramatic affect on my milage, but this is a guess because I only drove about 600 miles with the Sumitomos before I switched them out. One of the reasons that I bought the Touring was because of the computer that provided instant mpg. After the computer started acting up since the speedometer was off and I found out that the three dealerships in my area could not (or would not) re-calibrate my speedometer, I gave up and went back to oem spec tires which solved the problem. I considered keeping the Sumitomos and just always adding a percentage to any milage but it seemed like too much work for the next 65k to 70k miles. That and I would have to constantly remember that my speedometer was off, thus risking a ticket. I really don't want to spend the next few years driving a car with some of the critical systems not performing correctly. Finally, I started getting concerned about wearing the brakes sooner and if the van would perform differently (i.e. roll over) in the event of a crash since I had changed the height.

I have been impressed with the Sumitomos and the ride was great both on the Odyssey and on my wife's CRV. I also have recommended them on a friend's Avalon and they have been impressed with them. At first, I did not want to go with Michelin's since the factory Michelin LX4's only lasted 30k and I read nothing but complaints abouth them. The reviews for the Primacy MXV4's were better so we will see. I drove about 250 miles today on the Primacys and they seem to be just as quite and give a good ride. The Sumitomos gave good traction in the snow and I will find out tomorrow how the Primacys will perform in the snow.

 

[wolfort]

A common fact is that old tire, especially one almost wear out, has lower rolling resistance than the new one.

I bought a 2008 Touring about two weeks ago with 33k miles on it. I drove it 450 miles home and experienced 24 mph while cruising around 75 mph. The stock Michelins were almost worn to the wear bars and since I am Cleveland, I put new tires on it last week just in time for the snow. The stock tires were 235/60/R17 and I replaced them with Sumitomo 235/65/R17's as I have been impressed with Sumitomos on my CRV. The shop told me that increasing the aspect ratio by 5% would not really make a difference.

After putting on the new tires, I noticed that the mpg went from the low 20's to around 15. I first thought that it was because I had been driving it too hard and then I thought that it was because the speedometer was off, therefore giving the computer inaccurate information for the average mpg calculation. I filled it back up and drove it with my external gps. The speedometer is about 2 mph off when I am going 70 mph as the gps says that I am going 72 mph. I've really watched my driving and avoided any hard starts as well as avoided going over 70. Also, I have been coasting to stops and really trying to drive it like a minivan and not a sports car. I currently have around 17.8 mpg with 80% interstate driving.

Here are my questions:
1) Could increasing the aspect ratio on the tires 5% really have this dramatic effect on the mpg? I know that the tires are bigger but only by about 1/2 of an inch.
2) Has anyone had their speedometer calibrated? How much does it cost and is a 2 mph difference at 70 mph really enough to throw off the computer calculating the average mph as well as justify having the speedometer calibrated?
3) Has anyone experienced this when they increased the aspect ratio of their tires by 5% and/or has anyone else put 235/65R17's on their Odyssey? I can't be the first person who went with this tire size.

The ride is much smoother and quieter since the change and I am happy with the tires with the exception of the current gas milage.

Thanks in advance for any ideas.

 

[davedrivesody]

***snip*** After the computer started acting up since the speedometer was off and I found out that the three dealerships in my area could not (or would not) re-calibrate my speedometer, I gave up and went back to oem spec tires which solved the problem. ***snip***

The dealers are giving you a straight answer about recalibrating the speedometer. It's builtin to the powertrain control module and is not field modifiable.

 

[firemann1]

A few comments of points discussed earlier in this thread.

Changing the aspect ratio of 235 (235 mm wide) tires increases the tire diameter (and circumference) by 3.3%, per Kenny's info.

That decreases the vehicle's indicated speed by the same percentage, or conversely, increases the vehicle's speed above the indicted speed by 3.3%. So, assuming the Honda speedo is 100% accurate, a 3.3% increase from an indicated 70 mph means the vehicle should be traveling 72.3 mph (as indicated by a GPS system).

The 65% aspect ratio (as in 235/65/17), compared to 60%, adds almost 12 mm (1/2") to the radius of the tire (0.65-0.60)*(235mm)=11.75mm.

If the calculated mileage for the Sumitomo tires is increased by3.3%, we get 18.6 mpg (18*1.033).

It appears the combination of new tire, perhaps with higher rolling resistance than the Michelins, and perhaps interacting with a change in efficiency / power of the engine turning 3.3% slower for going the same actual miles, affected your fuel economy.

 

[gregersonke]

The heavier the tire, the more energy is needed to keep it rolling. It also could be that it raised your vehicle enough such that it screwed up the aerodynamics under the vehicle and hurt your gas mileage.