Tire Size, general opinions welcome.

[4banger]

What I am about to say most likely goes against every aspect of buying new tires for an ecomodder but im 18 years old and my car came with a set of 17" rims in the trunk, so theyre going on it. The tires on my cavalier are 195/65-15 (stock) but theyre dry rotting. So im putting the 17x8 wheels on with 225/45-17 tires. What I am hoping for are opinions on how much this will effect my mileage. There is a chance I will go with LRR tires. Will that possibly make up for the new tire size?

Thanks in adavance.

[mcrews]

how about if you read one of the many, many threads...........

[bikin' Ed]

Sell all of the tires and wheels that you have. go to a salvage yard and get a set of 13" wheels and some 175/80 or 185/80 tires. Then you can easily attach smooth aero hubcaps. I think that'd be your best bang for the buck.

Just a note... you may need to go 14" if the brakes are upgraded.

[gone-ot]

...ignore the actual equations for the moment and just follow the "what goes UP" and what goes DOWN" discussion:

MPH = ( 60 / (G × A))( RPM / rpm )

where:
MPH = vehicle speed, miles-per-hour
RPM = engine speed, revolutions-per-minute
rpm = tire speed, revolutions-per-mile
60 = conversion constant, minutes-per-hour
G = transmission GEAR ratio, ie: 3.21:1, etc.
A = AXLE ratio, ie: 3.08:1, etc.

Any values above the vinculum (divisor line "/") are GREEN values; any values below the vinculum (divisor line "/") are RED values:

• increasing any GREEN value increases MPH.
• increasing any RED value decreases MPH.
• decreasing any RED value increases MPH.
• decreasing any GREEN value decreases MPH.

...and, tire/wheel diameter affects rpm:

rpm = 5280' / ((dia" × PI)/12)

The larger the tire diameter, the fewer rpm's needed per mile and, conversely, the smaller the tire diameter, the more rpm's needed per mile. Thus, tire/wheel diameter is a GREEN value (see here: http://www.gordon-glasgow.org/tirecalc.html ):

dia" = (5280' × 12)/(rpm × PI)

Until the point where you "lug" the engine, anything that enables you to go faster (increasing MPH) enhances fuel economy. This is why "race" cars have 'deep' (higher numbers) transmission gears and axle-ratios while "economy" cars have 'lighter' (lower numbers) transmission gears and axle-ratios.

[4banger]

Pardon me if i missed anything but according to the calculator the stock tires are .01 inches bigger. So the diameter is the same really. So does that leave me with just the added weight and contact area of a wider tire to worry about? Are those big areas of concern or is the diameter the main area of concern?

[gone-ot]

...basically, yes, width = more Crr, tall & narrow is better for MPG.


ADDENDUM - for instance, look at the difference between 'racing' bicycle tires (tall narrow) and 'racing' drag slicks (big wide).

[CapriRacer]

Originally Posted by Old Tele man

...basically, yes, width = more Crr, tall & narrow is better for MPG.

Actually the opposite is true. Wider = more load carrying capacity = better RRC.

But the issue at hand is if a 195/65R15 would be better or worse than a 225/45R17. That means that not only is the width changing, but so is the aspect ratio and the rim diameter.

According to the results of the Smithers study on tire size, the 17" will have about a 7% better RRC than a comparable 195/65R15.

But the key word here is COMPARABLE. Smithers also reports up to a 60% difference in RRC between tires of the same size - so careful selection is extremely important - more improtant than tire size.

[mcrews]

weight(as discussed NURMEROUS TIMES (TRY SEARCHING) is multipled times 4 for a moving mass.
Understand that the wiegt is all on the uotside of the circumferance.(sp)
so the suspension and brakes will have to work much harder than they were designed to.
thus creating more wear.

[ATaylorRacing]

I put 195/50/15s on my 96 Geo Metro to replace the wornout 155/80/13s. Only on flat land with nearly no stops did I get the same mpgs, ANY areas that variable speeds were involved cost more fuel usage, especially in the hilly southern In areas. I was using the car to deliver rims/tires that I sold. I really need to update my sig...this is the first time I have been on this forum in nearly a year and the Geo (The White Gnat) was sold last fall. As of Saturday we bought a 2011 Prius that seems to be getting better than the best 60 mpg that the Geo got. I later bought a new set of 13s and NEW tires that had a max of 41 psi compared to the old 13s 36 psi.

Not only is the Geo sold, but now I have 38 NHRA/IHRA Championships and the 95 Neon gets a solid 46 mpg at 55-60 mph AND the Voyager was traded for the 2011 Prius.

[4banger]

Thank you for your input, How much more fuel are you talking about when driving at variable speeds/hilly areas?