Last post on Apr 30, 2010 at 11:00 AM
You are in the Chrysler Town & Country/Dodge Grand Caravan
What is this discussion about?
Chrysler Town and Country, Wheels, Van
#9 of 12 Re: T & C "99 16 inch wheel to 17 [timbodey]
Apr 28, 2010 (11:07 am)
I need a little more clarification of your goal.... you want to increase fuel economy by doing what, increasing the circumference of the tire? In other words, you're thinking that with fewer revolutions per mile the engine will be running at slightly lower RPM and therefore use less fuel? If so, I'm not sure your results are going to net you gain, especially if it involves purchasing separate rims (and tires) and recalibrating your speedometer.
More effective may be to reduce rolling resistance of the tire by reducing the friction with the road. You can do this by going with a skinnier tire (say a 205 vs. a 215) or increasing the pressure in the tire.
The '99, as with all similar and more recent model years uses a 5x114.3 bolt pattern with a 71.5 mm center bore. For alloy rims, an offset of 40mm is, I believe, stock. The last I looked, Tire rack showed compatible rims with offsets of 35-40mm, but I never ended up putting alloys on my van and have stock steel rims (15") on it.
If the stock tire size is 215/65/R16, a smaller tire of 205/70/R16 will keep your calibration within 1.1%, a larger size of 215/70/R16 will put it off by 3%. If you go with 215/65/R17, you will be off by 3.6% and will likely increase your rolling mass because the rims are typically the heaviest part of the wheel/tire equation (which means more effort to get it moving and more stress on the suspension). But, that is the largest tire amongst these mentioned and it is only 1" larger in diameter than the stock tire, so any of them are going to physically fit on the van.
215/65/R16 rotations/mile: 747 calibration 100% speedometer reading at 60mph: 60
215/65/R17 rotations/mile: 720 calibration 96.4% speedometer reading at 60mph: 57.86
215/70/R16 rotations/mile: 724 calibration 97.0% speedometer reading at 60mph: 58.18
205/70/R16 rotations/mile: 739 calibration 98.9% speedometer reading at 60mph: 59.35
Now, if you want to go with a 17" rim for the sake of it, a 215/60/R17 tire is an excellent match, at 99.4% calibration.
#10 of 12 Re: T & C "99 16 inch wheel to 17 [xwesx]
Apr 29, 2010 (6:57 pm)
capriracer, xwesx ; thanks so much for your input. Am still unclear on a couple things. 1) what is the mass issue ? Is it the volume, (total area , size of the wheel ) , is it the weight of the wheel ? If stock size is 215/65 / 16 can 205/65 or 70/ 16 be safe ? I notice many SUV s have 17" or 18" wheels and 235/65 so I was wondering how much dammage can occur by going up one rim size 1" Seems like so many vehicles have such oversize tires that have been added after market. Your explanation of what I'm trying to accomplish is right on. But how is it that a larger rim size, with fewer rotations of the wheel does not improve gas mileage.? Is it as so say there is more mass which means ??? Of course there is the stress on other componets ...transmission, suspension,... and all this taken into account can amount to big trouble tha t is just not worth the bother and bottom line simply will not provide better gas mileage. thanks
#11 of 12 Re: T & C "99 16 inch wheel to 17 [timbodey]
Apr 30, 2010 (4:12 am)
Going up in rim diameter, and making the proportional changes so that the load carrying capacity and overall diameter are the same - a process commonly called "Plus Sizing" - makes for a wider, heavier tire and rim assembly.
The loss in fuel economy is partially due to the extra weight that has to be accelerated (and slowed down) and partially because of the larger amount of tread rubber being deflected.
BTW, it is NOT the friction with the road surface that causes rolling resistance in a tire. It's a property called hysteresis. Another way to look at it is the internal friction of the rubber - and since most of the tire - particularly the part the is most involved with deflection - is the tread rubber, this dominates the rolling resistance picture. So more would be worse - meaning wider, deeper, etc.
But the single most dominant factor in rolling resistance is the rubber itself. You can get HUGE differences between tires that are otherwise identical just by changing the tread rubber. Unfortunately, this comes at the sacrifice of treadwear and/or traction.
Plus upsizing tends to put you into lower profiles - and this direction tends to go for traction by sacrificng treadwear (and RR).
So your best bet is to stick with the current size and search out tires that have the best combination of the properties you want.
#12 of 12 Re: T & C "99 16 inch wheel to 17 [timbodey]
Apr 30, 2010 (11:00 am)
But how is it that a larger rim size, with fewer rotations of the wheel does not improve gas mileage.?
After capriceracer's fantastic post, this is likely a moot point, but keep in mind that it is not the rim size that determines the number of rotations, it is the final diameter of the assembly, which is primarily determined by the tire size. For example, I can have a 17" rim and a 16" rim, with 225/55 tires on the first and 225/60 tires on the second, and the rotations per mile are nearly identical (754 versus 757). With a difference of 0.006%, do you really expect noticeable change in fuel economy if all other factors are equal? If I wanted to change the diameter significantly, I could simply put 225/75 tires on the 16" rims and I would have nine percent fewer rotations per mile (689 vs 757); the rim size remains the same - it is the tires that change the overall diameter.