Need advice on wheels

In case you haven't run across it yet look in preformance mods at "What wheels are on your Prius" there are some pictures of the 17" Scion wheels on prii. Don't know what effect it would have on mileage but you could PM some of the posters and find out.

BTW if you check tire sizes for 15" wheels at some various tire manufacturers you may notice that 195/60-15 is a little smaller, i. e. rotations per mile of 862 or so, compared to 855 revs/mi for the stock 185/65-15. Where as a 205/60-15 is just a shade larger, aprox 847 rev/mi. Those numbers are for the Toyo Proxis TPT as an example. Both the 195/60 and the 205/60 are well within 1% of the stock 185/65 and both are recommended for 6 inch wheels, which we have.

OK so what difference would it make? Well if we used the exact same tires except for size and assumng the odometer was correct at 855 rev/mi the 195/60 would indicate slightly higher mpg and the 205 would indicate slightly lower than what it really was, but in both cases not enough to make any difference. With the same inflation pressure the 205 would put a little bit more rubber on the road and therefore have slightly higher rolling resistance, the extra cm in width might make the CD a trifle higher. But a larger contact patch should make it stop and corner a little better. The 205 would most likely hydroplane at a slightly lower speed and would not be quite as good in the snow, but in deep water or snow the narrower stock tires would be better than either of the larger sizes anyway. Unsprung weight is important as previously mentioned, the TPT weight in the stock 185/65 is 17.2 lbs, the 195/60 is 17.9 lbs, and the 205/60 is 19.8 lbs. The stock Goodyears weigh 17 lbs each.

I wouldn't make a recommendtion for anyone else. But if I were going to keep the 15" wheels, and I may do that, I would think the 205/60 size tires would be better under almost all driving conditions that I will encounter. The thing that concerns me the most is the extra unsprung weight. As several people are already using 215/45-17 tires with no apparent problems I assume there would be no clearance problems with the 205/60-15 size.

Any additional thoughts would be welcome.

 

Windstrings,

I notice that you have two threads on the same topic running concurrently. This board had a crash a while back, where many threads were lost, and there was quite a bit of info on the topic of aftermarket wheels/tires. The bottom line was that everyone who went to 17" (even the TRD and Scion wheels which are pretty light) took a significant mpg hit. And not just the computer being wrong - a real decrease. One reason for this is that there are few (if any) 17" tires in a 185 or 195 width - most are running 205 or 215, which puts a lot more rubber on the road. Second, there are very few 17" wheel tire combos that aren't heavier than the stock 15" set-up. Third, most tires in that size/profile are in the "performance" category, where low rolling resistance isn't really a design concern.

You could go to 16" wheels like the OEM Japanese and European models and a 195/55 like the stock tire in those markets. From all reports, their mileage is (on the whole) the same as ours. The problem with that is that there are very few 195/55/16 tires available in North America, and the few light weight wheels in 16x7 available here that would fit (I'm not aware of any 16x6 like the OEM wheels), are very expensive. And after spending all of that money, the best you could hope for would be no decrease in mpg. As for as I know, nobody on this board - living in North America - is running 16" wheels.

To hit some of the points you've raised in this thread...

Plus sizing will reduce mpg. That may seem counter-intuitive because the bigger tire makes fewer rotations for a given distance, but the extra energy required to move the extra pounds of rotational mass per wheel is substantial.

As for just getting wider rims, all that will do is add weight, so same as above.

Wheel aerodynamics were mentioned in this thread, but weight is a bigger factor. Any increase in unsprung weight will make the ride rougher (ignoring tire sidewall differences). Any increase in rotational mass will always require more energy to accelerate. That energy will not be captured by regen braking because there is already more energy available than the max that regen can capture. Adding energy will not increase the car's ability regen.

As for getting the speedo/odo/mfd mpg recalibrated, so far nobody here has done that. I'm not even sure it's possible.

Ultimately it's up to you how much of a trade-off you are willing to make between mileage and traction. If it were me, I'd try to stick to a circumference/weight that is similar to the OEM tire, while getting a tread design/compound that improves traction over the OEM tire. Any resultant reduction in mpg might be worth it (to some) for safety reasons.

 

I went to the 17" Scion TC wheels w/215's. After a few thousand miles I can confirm that it does significantly decrease fuel efficiency. I noticed a decrease from 47 MPG average to about 42 MPG average, and that is not "computer error" due to different sizing. The biggest factor is probably due to the increased width, which adds to rolling resistance and wind drag. My driving is 80% highway, so I don't do much accelerating... hence the added inertia isn't an issue for me. The car looks much better, but I hate to dig into my efficiency numbers that badly.

 

How about the weather there in Las Vegas. It's much colder right now (43 degrees). That also can have a significant effect on your mpg. You may have to wait until spring/summer to make a final determination on the effect of the wheels.

 

I upgraded my wheels and tires on my BMW 540iA from
225/55x16 (front rear) 16x7 wheels
to
235/45x17 front, (17x7.5)
255/40x17 rear, (17x8)
Same driving style, same commute range.
I used to be getting 300 miles out of a full tank.
Now I am getting about 260-270mile.

Sorry that I don't have the data on wheel weight.
Factors that contribute to the lower MPG
- extra weight (tires, wheels)
- wide profile, more drag
- larger contact patch, more friction (rolling resistance)

Of course, if you simply size up by one, then the impact is not as drastic.

 

:huh:
If you're referring to bringing the tire material 1/2 rotation around from the road to the tire's highest point, that is exactly balanced by the force of gravity bringin the tire material from the top to the road.

 

No, the inertia only comes into play when changing the speed of rotation. If you are cruising, the effects of a heavier wheel/tire combo are reduced to near inconsequential. Stop and go traffic is where the weight will hurt you.

Starting in January, I will be commuting 60 miles each way every day. Of that 120 miles, only about 15 miles is regular streets. On the interstate, if I use cruise control to maintain a constant speed, the effects on milage due to the weight will be minimal. the increase surface area and corresponding increase in static friction will have an effect, but the overall mpg loss will be reduced.

Anyhow, once I put the 17 inchers on and drive the 120 miles a day, we should ahve some numbers pretty quickly. At that distance daily, I'll still fill up with the same frequency as my 15 mpg SUV. I'll post once I start to get some good numbers.

 

Yes, the wider profile will indeed give more resistance, both from the air and the increased contact with the ground. Both will work against you.

As for the inertia, keep the old saying in mind, "A body in motion tends to stay in motion, a body at rest tends tostay at rest."

A spinning wheel, in the absence of any othe forces, will continue to spin. Of course in the real world you have friction from the ground, friction from the air, friction fromthe actual mechanical connection to the car (bearings) and so on. All these willslow you down. But giving a little gas (or electricity) will balance out those forces and help maintaina constant speed. So in the absence of any force bias in any one way, the inertia will not be an issue, and the weight will not be an issue. But as stated, the increased width causes an increase in friction frm ground and air, thu requiring a greater input of power to balance those forces. That is why mpg is reduced.

 

I've used the Stock tires with the stock rims. I've used low profile with some "pretty light" rims. First off, nothing really compaires to those stock rims. They are light. Very light.

What i recommend is keeping the stock rims. Changing the stock tires to a better tire. I went with the Goodyear Assurance Confortthreads. Awesome tire in the snow, rain, sun.. doesn't matter. Even on dirt roads for miles and miles of driving. They are awesome with an 80K mile warranty. They are also very durable. That whole dirt road thing.. yeah.. the roads i drive, you usually only see large 4x4s on it it because it's bumpy, and you have rocks. No damage was done to the tires. They are good.

Impact on MPG was minimal.

 

Of course, all those factors apply. The body in motion thing is pure theory. It states inthe absence of any other forces. In the real world, your car is constantly affected by forces in all different directions and magnitudes.

Yes, weight has a significant impact. What I said was that at highway speeds while cruising, those effects are minimized. The Prius has EPA ratings of higher inthe city than on highway. With larger/heavier wheels and tires, that would likely be reversed, with your city milage dropping greatly. Highway milage may drop also, but not quite so much as those factors affect it less at teh RELATIVELY constant speeds. You will never be able to go the exact same speed for a long distance, but if you average it out, it's pretty constant. Anyway, your highway milage may drop by only 1-2 mpg, while your city milage may drop by 10 mpg. It all depends. How this affects your OVERALL mpg depends on how much you drive on each.

For me, I'll be commuting 120 miles each day, of which less than 10 is city. The rest is pure highway, all of which has 65-70 mph speed limits. So for me, I may not notice a sharp drop in overall milage from the larger wheels/tires becuase of hte percentage of highway vs. city. Now someone else who drives only 15 miles each day, all in the city, would likely see a much larger drop.

 

Nah, my Prius is pavement only. I'm keeping my monster truck for the off-road stuff. Actually, I just gave it to my dad today, but I'm sure he'll still let me use it.

 

Here is a pic I just posted on another thread. It's my 2005 prius with upgraded wheels. I bought Primax Sportmax 920 17" rims with Kumho 215/50/17 tires.

I just bought the car so I can't tell you much about mpg except that the day before I put the new rims and tires I got a all time high of 53 mpg going to work. The day after I put the rims and tires I got 46 mpg going to work.

I did notice it takes a bit more power to get the vehicle going from a stopped position. Maybe when these tires where out I will go down to a 205 width tire.



Hope that helps.

Nick