QuestionQUESTION: Hello Barry,
I just purchased new tires for my Jeep Grand Cherokee and since they were installed the gas mileage on my Jeep has dropped by 2.5 MPG. The new tires are slightly larger size 245/65-17 vs the old 235/65-17. The overall tire diameter is the same at 29.5 inches but each tire weighs 8 lbs more. Would this increase in weight, 32 lbs overall, cause such a detrimental effect on the mileage? I went with the larger size because I changed brands for what I thought was a better performing tire and I wanted to keep a tire with outline white letters which my old tires had but the other brand only offered OWLs in the 245 size.
ANSWER: Doug,
First I don't think the tires are the same diameter - and that is at least part of the problem. You are going further than your car is recording.
Try this calculator:
http://www.1010tires.com/TireSizeCalculator.asp
Second, new tires will have more RR than worn out tires - even if they are otherwise identical.
Third, if you replaced Original Equipment tires, then you took another hit. OE tires generally have relatively good RR, but wear isn't a strong point, where replacement tires are designed to wear well, but at the sacrifice of RR.
And lastly, the weight is somewhat a factor. The weight increase you mentioned is out of proportion for the size change - so I'll bet you went from all season tires to an all terrain - and that would be hit #4.
So overall there were a lot of reasons why your fuel economy suffered.
---------- FOLLOW-UP ----------
QUESTION: Thanks for the quick reply.
According to the tire manufacture's tech data, the 235 and 245 tires have the same diameter. That's where I am basing my information.
Also, both the tires I replaced (which were the second set of the same make/model as OE) and the new tires are all season highway. When the replacement set was new, I did not see a change in mpg.
That being said, would the weight of the tires be the factor in a 2.5 mpg drop? Let's say I drove around with a 100 lb of weight in the back of the Jeep, I wouldn't expect such a significant drop in mpg as I'm seeing here.
There may be some difference in sprung vs unsprung weight,however.
AnswerDoug,
"......According to the tire manufacture's tech data, the 235 and 245 tires have the same diameter. That's where I am basing my information...."
This just doesn't seem right. I'll bet someone made a typing error.
"......Also, both the tires I replaced (which were the second set of the same make/model as OE) and the new tires are all season highway. When the replacement set was new, I did not see a change in mpg......"
This type of change is kind of hard to detect, and could easily get lost in data variablility.
"....That being said, would the weight of the tires be the factor in a 2.5 mpg drop?......."
Again, I think the answer is bigger than this.
I believe there is a difference in the diameters between the old and new tires and that is part of the weight difference - as well as part of the issue with regard to the fuel economy.
"......Let's say I drove around with a 100 lb of weight in the back of the Jeep, I wouldn't expect such a significant drop in mpg as I'm seeing here....."
OK, you have some misconceptions which have to be cleared up.
Just today, I uploaded a new web page on a web site I am putting together. The web site is really early in development and looks a bit "funky" for my tastes, but here is the address.
http://www.geocities.com/barrystiretech/RRandFE.html
Revised 13 Oct 2010:
My new web site with the page on Rolling Resistance:
http://www.barrystiretech.com/rrandfe.html
