I have two pairs of different size Sumitomo tires:

T4 175/65TR14 (worn) - 919 rpm/mile (specs)
T4 175/70TR14 (new) - 886 rpm/mile (specs)

During the trip, I would find sections of highway with mile markers and measure the differences between indicated and mile marker distances:

175/65TR14 (front) - 1.3% low indicated miles
175/70TR14 (front) - 5.8% low indicated miles

4.5% difference of worn 175/65TR14 vs new 175/70TR14
3.6% difference of specs 175/65TR14 vs 175/70TR14
----
0.9% difference due to wear from new vs old tires

With the larger diameter tires on the front, there was more and welcome bumper clearance. Bumps and dips that in the past would scrape the air dam were no longer a problem. Also, the steering seemed more stable perhaps due to the larger angular momentum. However, my indicated 70 miles per hour was actually 74 miles per hour and the indicated 49.2 MPG was closer to 52.1 MPG. Before heading back to Huntsville, I swapped the front and rear pairs.

With the smaller diameter tires on the front, the steering seemed a little less stable, more wander. The indicated 70 miles per hour was closer to 71 miles per hour. Unfortunately, I didn't make a record of the MPG (I was pretty tired) but remember it being in the low 50s. Given I have the same four tires, the actual MPG should not change from swapping the front and rear tires.

I like the handling and extra ground clearance of the larger diameter tires. They are a little noisy on the yankee concrete highways but rolling up the windows mutes it out.

I will be ordering another pair of 175/70TR14s so the headlights will be level, increased ground clearance (0.5 cm) and improved stability. The same size tread width means there is no risk of interference with the wheel wells. When I swapped the front and back tires, the larger tires "looked good" as if they were made for the car ... their outer diameter seemed to fit the curve of the wheel well better than the smaller tires.

Bob Wilson

 

We had a problem at Volvo when the new XC came out in 2000. If one tire was worn more than the others, or if one tire was under inflated the circumference would shrink relative to the others. Shortly after that the viscous coupler box the provided all wheel drive would self-destruct. Fun times.

I wonder if a similar condition would put stress on driveline components in these cars as well.

 

A good question but I suspect not.

The Prius transaxle is pretty simple and the control laws have to deal with the 'donut' spare. I think the worst is the idler gear in the differential that would get more rotations than otherwise. To the best of my knowledge, it has no slip-limiting mechanism that might be at risk. I think the ABS handles wheel slip by modulating the brake but I could be wrong ... don't take this speculation as gospel. But I suspect the differential idler gear probably has a basic bushing so I wouldn't want to run say 100,000 miles that way.

Bob Wilson

 

Were I planning to drive any substantial distance on a donut spare (unlikely), I would not run it on any front-wheel drive gearbox. Perhaps an abundance of caution...

DAS