Tire change. Am I crazy?
 

So I switched my LRR with new winter tires and the fuel economy has tanked I. The city. To be expected. But the highway readings are giving weird numbers. Even if I draft behind semis and hyper mile at 55-60 mph, I can barely get above 60 mph. If I speed up to 75 on the interstate, I only drop to 48-51 mpg. A small drop

When I had the ecopias tires, I got close to 90 mpg at 55 mph! Now it’s down to 60. A 30 mpg hit from a simple tire swap is brutal and doesn’t make sense to me

At 75 mph I was getting 55 plus give it take mpg

So low speed high way driving I drop from 90 to 60 mpg and then at fast interstate driving I only drop a little over 5 mpg

Might as well just drive fast since the numbers hit is so small

Can anyone explain this?

You must have purchased some soft sticky tires. Good winter tires are very soft to grab the road. They usually never last through a hot Summer. They also wear out very quickly.

Normal for new tires to be especially inefficient at first. Normal for winter tires to be less efficient than all-seasons.

There are no solutions; only trade-offs. Increase winter safety at the expense of economy.

As an aside, that might be the main reason why I struggle to get EPA economy on the Mazda. I choose a tire that performs well in winter conditions, but isn't quite a dedicated winter tire to run year-round. I don't pay for gas, and I occasionally drive in Montana in January, so I prioritized safety in winter conditions.

It’s just weird how they are more efficient at higher speeds than slower speeds. It’s almost like going slower gives worse mpg lol

What winter tires did you get? I've heard Nokians provide the best balance between winter performance and maintaining good fuel economy.

If you have high rolling resistance, the impact to fuel economy will be most apparent at lower speeds where rolling resistance accounts for the majority of energy loss. It will be less apparent at high speeds where wind resistance accounts for the majority of energy loss.

You should notice improved MPG as the miles accumulate on these tires (although maybe not since cold weather is coming).

My other question is if the circumference is the same? I believe it is not recommended to vary diameter more than something like 6%. Just going from bald tires to new ones with full tread can account for a significant difference in circumference. That will affect the accuracy of the odometer and MPG calculation since these do not account for changes in circumference.

For vehicles I own, my preference is to get cheap steel wheels to mount winter tires on, and swap them out myself as winter conditions set in. I've got a jack, stands, and various wrenches that allows me to get the whole thing done in something like 15min. Many tire shops will do free wheel swaps if you buy tires from them.

... this reminds me, Christmas tree hunting in T -3 days. I'll likely need snow capable tires up in the hunting grounds around Detroit (Oregon).

I got blizzaks ws90s. Horrible rolling resistance but the traction and grip is unmatched. I can literally go off roading in deep snow as long as I have ground clearance. Can even drive on solid ice. Great in the rain too.

I measured the diameter and it’s an inch more top to bottom. Also they added half an inch of ground clearance.

But I guess that means I’ll go faster versus trying to go slower on the interstate for better mpg

An inch is quite a lot, and your car is under-reporting your speed and distance compared to the previous tires. My guess is it's under-reporting by about 5%. When your speedo indicates 75 MPH, you might be going closer to 79 MPH. When the odometer says you've gone 100 miles, you might have done closer to 105.

Then you've got more frontal area due to the increased ground clearance.

Finally, if the width changed that affects fuel economy as well.

I also bought Blizzak WS90 this fall and noticed a sharp drop in fuel economy compared to my previous Michelin X-Ice xi3.

They do seem to perform a bit better in wet snow compared to the Michelin though.