Increasing overall tire diameter 4 more MPG

The physical change would be to increase the Prius' effective gear ratio. It may or may not be more efficient after, at one speed-load combination or another. If it is, that would suggest that Toyota engineers have geared it wrongly in the first place. That seems possible, but not probable.

The apparent effects would be to make the speedometer and odometer read low, because the system counts wheel revolutions, not distance per se. Therefore the MPG gauge would read high by the same proportion.

 

It will in effect allow you to operate the vehicle at slightly higher speeds without getting into modes where the engine must be running/turning. The 41 mph limit is a function of rpm. Moving up to say 45 mph would be nice on many of my local drives. (Of course the indication would still be 41.) There is also a sweet spot around 62 mph if memory serves and it would be nice if that was 65+ (although this might be related to engine load and not wheel rpm.)

Practically speaking, one would need to find skinny tires with relatively low rolling resistance or else there would be some additional friction loss by the tires. The car would also sit higher increasing aero drag slightly.

Have any of the hypermilers tried this and documented the results? It would probably require comparing two sets of new tires of the same series for apples-to-apples. It would also require some testing over known distances and such, correcting indicated mileage for odometer differences.

I guess where I'm coming out is that it has some hypothetical benefits, but that there are several problems--not the least of which is getting a tire/rim combo that will fit with sufficiently larger diameter to make a difference.

 

Almost forgot, the land-speed Prius used oversized tires and reduced the final drive gear ratio. That and a bucket of ice got them almost to 131 mph They also enlarged the wheel wells, which you may be considering?

I agree that moving the 41-mph limitation to a higher speed would probably help fuel economy on many drives. Getting it up to 45 mph actual means increasing the tire diameter by almost 10%.

 

I neglected to mention another factor: larger diameter tires means more weight farther from the center of the wheel hub. This means that there is greater rotational inertia in the wheel & tire. That will have a negative impact on mileage because of some efficiency loss in spinning the wheel up and down. One might be able to minimize this somewhat with a relatively lightweight rim when going up in rim and tire size.

When I did a quick search on tires the options seemed limited and the most likely diameter changes were only about 4% larger. That would move the magic speeds up from around 41 and 62 to almost 43 and a little over 64 respectively.

I doubt that there is much fuel savings benefit available this way and that the conversion costs would outweigh the economic benefits, but I'm curious to find out what folks might have tried. Some might even have done something like this to get a little more ground clearance for example.

 

The NHW11 tire size:

• 175/65R14 - standard tire size, I just finished 50k miles on set of Sumitomo T4s that suffered an unfortunately, sidewall puncture.
• 175/70R14 - my current Sumitomo T4s (TireRack specifications)

These are the same manufacture and style of tires running at 50 psi by the same driver. The driver is well trained in engineering practices continues to use the same brand of gasoline, Shell 87, and consistent maintenance practices including near perfect alignment. However, I did reduce the front right camber in early September from 0.8 to 0.3 degrees.

I drove to HybridFest, 750 miles, and Sunday morning, swapped the front, 175/65R14s with my rear 175/70R14s:

• 175/65R14 (front) - baseline drive, ordinary performance and mileage. Trip meter was reading less than 0.5% off from the mile markers.
• 175/70R14 (front) - steering seemed more stable, less flighty. Trip meter reading was 5% under the mile marker.

I now have 175/70R14s on all four wheels and the spare is the best of the old 175/65R14s. So I drive my indicated, best MPG speeds but am running 5% faster:

• 38 mph -> 1.94 + 38 = 39.94 mph, perfect for 40 mph streets
• 63 mph -> 3.15 + 63 = 66.15 mph, perfect for 65 mph highways
• 68 mph -> 3.40 + 68 = 71.40 mph, perfect for 70 mph highways

Over at GreenHybrid, you are welcome to look over my mileage record:

Hybrid Mileage Database - GreenHybrid
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Currently I adjust up my distance by the 5% additional distance covered from the indicated trip meter record. I don't see any evidence of a significant increase in mileage. Rather, I do seem to flow with traffic a little better now without suffering a mileage penalty. Of course your mileage may vary.

BACKGROUND

The bulk of the tire rolling resistance at non-highway speeds comes from bending hysterisis. One SAE paper suggests about 30% is from sidewall flexing the the rest from tread flex. A larger diameter tire with the same tread width, inflated to the same pressure will have a smaller flex angle and less hysterisis.

Hysterisis converts some the rolling drag into heat, making the tires warmer than the roadway and air. But the warmer tire reduces the flex loss. Sad to say but I don't have comparative metrics between the two size tires ... although I do have one 175/65R14, my spare. I have a plan for measuring tire rolling temperature, actually individual tread temperature, and the parts. But this will have to wait on some of my other projects.

When I commuted on bicycle, I had occasion to go from 50 psi 'clincher' to 100 psi 'sew-ups.' The difference in rolling resistance was remarkable even though the tires were the same diameter. There was less flex with the 100 psi 'sew-ups' and I could tell when the pressure was off, it was harder to maintain speed. Topping off the tires made the bike roll with almost no rolling drag.

Today, there are mountain bikes with smaller diameter, fat tires. They are great for unprepared surfaces but a lot more work to pedal. For bicycle commuting, I prefer a touring bike with the hardest, narrowest tires I can find.

The failures of the Ford SUV tires back in the 90s occurred because they were at too low of pressure. This caused excessive heat build-up and in a hot climate, they would fail and the Ford would roll-over.

GOOD LUCK!
Bob Wilson

 

Also, don't forget that increased tire diameter requires increased torque for any speed change. Such as at standing start from stop lights. Merges onto highways etcetera. So, if you are not doing a lot of same speed cruising, I'd bet you lose MPG.

 

After three years of Prius driving, my routes and driving practices minimize unexpected, traffic driven speed changes. The real test will be after a year when I can compare like-to-like mileage.

Bob Wilson

 

I'm very interested in the out come of this. I've been considering a the same for my 2007. From what I've learned so far I think a small change larger in tires, would help with my driving FE. Of course you can easily over gear - Audi had a 4000e in about 1982 that got better FE in 4th gear than 5th gear.

 

Exactly! Changing the tire diameter does not mean we can ignore running the ICE in fuel inefficient ranges. This is not a 'bolt on' performance improvement since the change in indicated speed can lead to operating in speed ranges that have higher fuel consumption. But it does mean we can avoid some 'inflection points' that are well know mileage hit areas.

Bob Wilson

 

If you do this then get one of these > Jaycar Electronics=

 

The Prius has potentially multiple, independent speed sensors:

1. MG2 rpm - fixed by the gearing
2. wheel encoders - each has their own sensors

Although one might spoof the wheel encoders, it would be impossible to spoof MG2 since the HV ECU has to know exactly where the rotor is relative to the stators to command the inverter power signals.

What I'll do is drop by my local Toyota service center and ask if they can program a different tire size. All they can do is say, "No," and I'm in the same boat I'm in now.

Bob Wilson

 

I don't agree that it was overgeared and I don't believe it got better FE in 4th - I had one and the gearing was about 26.5 mph/1000rpm in 5th (i.e. about 2250 RPM at 60mph).

It was very tall gearing for a 1700cc/76HP engine but it did very well in fuel consumption. About 37mpg @60-70mph. I did over 560 miles on one tank once. It had still had power to accelerate in 5th gear at 70mph on the level.

It would only have been overgeared if the efficiency of the engine was worse at the lower RPM/higher throttle in 5th relative to 4th. In general the lower the RPM the better and it is only at close to full throttle that the any richening occurs.

It used a Bosch mechanical continuous fuel injection system that had a mass airflow sensor to control the fuel flow - I don't believe it had any enrichment at high-throttle.

kevin

 

been there, done that.

here's the bottom line, let's say the "width" of the contact patch is the same on the OE tires versus the new tires with larger diameter. in theory, this SHOULD get better gas mileage on the TOP end because the RPM's will drop in a higher speed...............BUT it doesnt achieve that. it actually loses gas mileage.

if you want to go larger diameter, make sure that you use rims that are VERY light, if possible, lighter than OEM rims. i believe the OEM rims and tires = 32 lbs. you want to stay UNDER 32 lbs. this might be a rather expensive endeavor.

good luck.