New wider tires decreasing MPG's?

[nospamprl]

My stock tires are 245-50-20, but I broke one tire and found a good deal on Toyo's 255-50-20, so I went for them and put 2 of them on the front, keeping the originals on the rear.

Since I put them, I noticed an significant decrease in my fuel economy, both measured by the trip computer and the actual miles I get from a full tank.

I did the math to calculate the diameter of the new tyres vs the old ones and the difference is not that large, maybe 1%. But I'm seeing my full tank now lasts around 420 km instead of 500 before (City). This weekend I drove on the highway so I pulled my GPS and I noticed a very small difference between the GPS and the odometer, the odometer lost about 0.5 kilometers out of 60 vs. the GPS. So I don't think the MPG decrease is due to the odometer-trip computer not being calibrated for this new tyres.

I'm wondering if the extra push that the engine has to put in order to move the car with larger front wheels is causing it. If thats's the case, will putting the larger tyres on the back will correct it?

Thanks for your comments.

 

[VQ Dawg]

Well this is a late response, but I figure I will go ahead with it. It looks like you did a thorough job ensuring the size difference is not introducing errors in your mileage calculation. Your measured 1% in diameter and the GPS vs. odometer reading supports this measurement as well. This also means that the difference is not due to a gearing change either. Different tires do have different rolling resistances, but there is no way that this will cause a nearly 15% drop in efficiency. The only thing I can think of is maybe the air pressure. Check to sure this is OK. If it is, maybe the new tires need a little extra pressure due to differences in size and side wall stiffness. Other than that, all I can think of is a coincidental mechanical problem elsewhere (i.e. sticking brakes or something similar) or you are unknowingly driving more aggressively.

 

[table5511]

Well, a 255 not only has a slightly larger diameter, but will also have a slightly wider tread. My guess is that your fuel economy has been affected by not only the larger tire, but the tread must have more rolling resistance than your prior tires as well. Rear tires likely would not make much of a difference as they are just there for the ride, since they don't do any accelerating, and most of the weight of the car goes into the front end anyhow. (even with awd, majority of acceleration still is in the front)

 

[ceric]

Note that he is using 245s at the rear and 255s at the front.
That is not a big problem, but I can't think of anything good about it, either.
What is the model name of the Toyo tires?

 

[danz75]

Is it a FWD or AWD?