Could 17" after market tires lower MPG more than 20%?

Yeeouch! Not a wheel expert myself, but from what I've read here even the best aftermarket 17" wheels and tires will rob you of a few mpg's, part of the price of looking good. If these are heavy wheels and the tires aren't low rolling resistance (LLR) then yeah, it's possible. Also, maybe they weren't balanced properly?

 

First, you need to check the calibration of your tripmeter distance with the new tires. If you have a GPS, set the car on cruise control at a given speed and check against the GPS. Then for good measure, find a road or highway with mile markers. Reset the trip meter as you approach a mile marker and then drive at least 10 miles and record the tripmeter mileage and 10ths of miles as you approach a mile marker.

I put larger diameter tires on my 14" wheels, a 2003 Prius, and there is a 3% error because it takes fewer rotations per mile. So when I record my mileage, I have to 'adjust' the distance before I enter the record. I also have to drive using an indicated speed 1-2 mph slower to match the posted speed limit.

Bob Wilson

 

That's true that calibration could be an issue, but shouldn't the diameter of the 17" wheel and tire be the same as the 15"?

 

Check the tire specifications at 'Tire Rack' and you'll find there can be as much as a 10% variance in revolutions per mile. A larger diameter, fewer revs per mile, tricks you into driving faster because the speedometer is reading low, the car will be in a higher drag region, compounding the mileage problem. Worse, the indicate MPG will be lower than true.

I didn't ask about tire width but this also increases the aerodynamic and often rolling drag. Actually rolling drag is a complex function including tire material, tread, and alignment.

Our Prius is a well tuned car and it is very easy to make what might seem like minor tweaks that send mileage the wrong way. Don't give up hope but just assume you're in 'test and debug.'

My recommendations:

• tripmeter calibration - we need to know if you're seeing a lower indicated mph and smaller than actual miles.
• alignment - this can rapidly result in a permanent increase in rolling drag and worse, higher tire wear. Fortunately, it can be fixed, even rear wheels.

FYI, I've long thought about getting four 'donut' tires for some stunt driving. The smaller diameter means the indicated speed will read higher than actual and the indicated mileage will be higher too. <GRINS>

Bob Wilson

 

It is entirely within the realm of reason if you chose a very heavy wheel and put a tire with a very high rolling resistance rating. I regularly see about about 5mpg avg. drop when swapping between my 15" combo and my 17" combo and I have very light wheels with well worn tires. You also have to factor in the new tire effect which will produce lower MPG even if you had kept the tire size and model the same as original. It takes time for tires to break in. Additionally a new tire has more tread so you will see less revs per mile like Bob already stated.

 

1. Are your rims open spoke design? Could make a difference if you normally drive highway speeds due to increased drag. The V's wheels are more aerodynamics than most.
2. Are your tires low rolling resistance? The V has LLR tires.
3. What's the weight of your wheel/tire combo? Unsprung weight can make a difference.

 

How does the tp in the 15's compare to the 17's? The max sidewall pressure on the stock 17's is 51psi.

 

55psi on mine... i keep them low though.. really low. 55 psi was fun for a while.. i think i'm running almost as low as 30 right now.

expect mileage to drop.. in my GenII it did.. i'm 36+mpg. i used to be no lower then 42...

 

Ouch! My mileage went from 49 to 43. Scion TC Rims with 215 45 17 Z Rated tires which are probably high rolling resistance. I just recently switched back to stock. It is hard to hypermile with 17's since the car slows down so quickly from the thick rubber. Just like most things both options have their pros and cons.

John

 

I am a newb here and I am also a (former) auto tech.
Looking at this post, I see a few good thoughts about the cause for the mileage drop.
Most of them (tire pressure, calibration, rolling resistance) woud have an effect on mileage but not 20%.
I would suspect that the wheels that were installed are causing the brakes to drag.
This can happen if the brakes rorotrs/drums are being warped because of the engineering of them, or possibly overtightening of the lug nuts, or both.
Just my thoughts.
Good luck.