Tire size expectations?

[euromodder]

Originally Posted by SentraSE-R

Ouch. The truth hurts. I'm now reduced to thinking that if I go through with this, the best I can expect is no net loss in mpg - if I'm lucky.

You're getting some pretty good mileage already with this set up

It'll get tough to improve upon that.

[SentraSE-R]

Thanks. It's difficult to keep the mpgs up. I drove 200 miles to and from a car meet yesterday. I fought a 35-40 mph headwind going up, and averaged only ~37 mpg going out. I had to run lights on the way home, and averaged only 41+ mpg on the return.

[zonker]

Originally Posted by SentraSE-R

Ouch. The truth hurts. I'm now reduced to thinking that if I go through with this, the best I can expect is no net loss in mpg - if I'm lucky.

I'm in a deal for the wheels. It looks like my best option is to sell them & stick with the stockers.

On a small 4 cyl car of less than 2.0 liters in engine size, 17" rims and 45 series tires are nothing more than boat anchors. You'll gain steering response from the low sidewall, but ride quality, straight line performance and fuel economy will suffer.

[Vekke]

Originally Posted by arcosine

Going from 175/70R14(stock) to 185/65R15 to 165/80R15 to 205/70R15 all positive for MPG. SC1 RPM now 2200 at 70 mph. Bought used 205/70R15 tires for $5 each.

Here you have only mildly increased the rim diameter and went to taller tires. That is the way to go but not from 15" to 17" wheels.

When I put 10% taller summer tires on I see instant 10% better mileage.

[SentraSE-R]

One reason I'd like to try the larger diameter wheels/tires is the obscene 3000 rpm my engine needs to turn to go 60 mph. If I could drop that 3.5%, that moves it <2900 rpm @ 60 mph.

[Frank Lee]

.815 fifth gear x 4.312 diff = 3.514:1 overall top gear ratio.

876.1 tire revs/mile x 3.514 = 3078.86 engine revs/mile;
@ 60 mph = 3078.86 engine rpm.

84.6mm (3.33") stroke @ 3080 rpm = 1709 ft/mn piston speed.

It is said that many engines achieve peak efficiency at 1000-1200 ft/mn piston speed.

1800-2200 engine rpm = 1000-1200 ft/mn piston speed.

That also equals 36-44 mph.

If that piston speed theory applies to your engine, it looks like you could benefit from a "gear-up"... or get used to cruising at 44 mph.

[Bill in Houston]

Sometimes if you don't increase the WIDTH of the tire, you can get a LOT taller tire on the car. It depends where the rub point is.

[SentraSE-R]

In the interest of science, and because I have a curious mind, I bought taller tires today. I was browsing Craigslist yesterday, and found four used Goodyear Eagle NCT 5s with 70% tread for $30 each, size 205/50-17. I've saved >$1700 hypermiling, so I don't feel guilty spending some of my savings on the frivolity of pretty wheels.

The new tire/wheel combo is 5.31% taller, 25.07" to the stocker 185/60-15s' 23.74". They turn 829.6 rpmile v. 876.1 rpmile for the stockers. So when the engine used to spin ~3000 rpm to go 60 mph, it will now spin ~2840. They rub a bit on full left turns, but so did the stock tire/wheel combo.

The stockers had a 5% speedometer error (60 mph indicated was about an actual 57 mph), but no odometer error. The new tires should correct my optimistic speedometer, with their 5.6% slower factor, and I'll need to incorporate a 5.6% odometer error.

I hope to do some ABA testing in the future.

[redpoint5]

Originally Posted by SentraSE-R

In the interest of science, and because I have a curious mind, I bought taller tires today.

If you do a lot of hwy driving I would expect an increase in efficiency. I'm VERY interested to see your ABA results, or anything you can share on the subject.

I have been considering a larger diameter tire when it comes time for new meats on my car, but only for reasons of increased MPGs.

[mcrews]

Originally Posted by SentraSE-R

In the interest of science, and because I have a curious mind, I bought taller tires today. I was browsing Craigslist yesterday, and found four used Goodyear Eagle NCT 5s with 70% tread for $30 each, size 205/50-17. I've saved >$1700 hypermiling, so I don't feel guilty spending some of my savings on the frivolity of pretty wheels.

The new tire/wheel combo is 5.31% taller, 25.07" to the stocker 185/60-15s' 23.74". They turn 829.6 rpmile v. 876.1 rpmile for the stockers. So when the engine used to spin ~3000 rpm to go 60 mph, it will now spin ~2840. They rub a bit on full left turns, but so did the stock tire/wheel combo.

The stockers had a 5% speedometer error (60 mph indicated was about an actual 57 mph), but no odometer error. The new tires should correct my optimistic speedometer, with their 5.6% slower factor, and I'll need to incorporate a 5.6% odometer error.

I hope to do some ABA testing in the future.

Weight.......weight..........weight............:co nfused:
TIRERACK says that an 185 weights 18 lb and the 205 weights 24lbs
15 inch alloy rim 15.4 lbs 17 inch same brand is 17.6lbs.
your vehivcle weights 2395lbs

while moving you are slinging an additional 40lbs at each wheel or all 4 wheels moving have added 5% to the curb weight.........

you will be buying rotors and pads.......