bigger tires = bad gas mileage?

[lostatlantis]

is it true??

[strobhen]

bigger tires = more rotating mass = more strain on engine = more power needed to make the car go = the possibility of lower gas mileage

but probably not much, you wouldn't really see it unless if you did something insane.

Btw, general rules seems to be tire questions in handling forum.

[zlightning]

Actually it depends on the wheel more then the tire. The tire doesn't weigh that much compared to the wheel. You can buy light weight wheels that are bigger then stock and weigh less I am sure.

[MTMike]

But, bigger tires have a longer circumfrence, which results in a slower rotating axle at the same speed, and a slower rotating axle results in a slower rotating tranny and engine, which may result in LESS gas consumption.

Something to think about.....

[Jack]

<blockquote>quote:</font><hr>Originally posted by MTMike:
But, bigger tires have a longer circumfrence, which results in a slower rotating axle at the same speed, and a slower rotating axle results in a slower rotating tranny and engine, which may result in LESS gas consumption.

Something to think about.....<hr></blockquote>

Mike is right. When I bought the Bird, the joker who had it before had some 225/60's on it. Highway mileage was awesome, got 34 as a best mpg. The drawback to that is it had no get up and go, and the engine had to work harder from a stop. I put some 235/55's on it and mileage dropped to about 30 on the highway, but acceleration is greatly improved, as was the appearance, it looked like it had truck tires on it when I got it.

[lostatlantis]

Jack, but those 225/60 were bigger than stock tires too... they can make that big a difference in gas mileage? what exactly is that "/60" mean... I remember it was some kind of ratio. I don't know, I've been getting about 19~20 miles per gallon in my '96 Y87 firebird. I'd say 70% or so is around town, 30 to 45MPH, and the rest is highway at 60~68MPH. The percentage is probably more than 70%, but I was expecting a bit better gas mileage.

BTW somehow people here give the thumbs down for the 235/55 tires in the Y87 package, which are the tires I have right now. I hope to get some new tires in the next few months or so, but I am a complete newbie when it comes to tires and I am trying to learn as much as possible [img]smile.gif[/img] if you got any tips let me know!

[V6Bob]

"those 225/60 were bigger than stock tires too... they can make that big a difference in gas mileage? what exactly is that "/60" mean... "

First the/60 means the sidewalls are about 0.60 times 225mm high (135mm high). It's not exact, tho.

Second, larger tires means the car travels farther per revolution than smaller. That means the odometer will read a little low, which could be misinterpreted as a change in gas mileage.

[ January 02, 2002: Message edited by: V6Bob ]</p>

[Jack]

<blockquote>quote:</font><hr>Originally posted by lostatlantis:
is it true??<hr></blockquote>

Getting back to your original question...on the highway slightly better mileage from the reduced rpm's but worse mileage in city driving from the engine working harder to pull the car from a stop. This has been my experience at any rate. I will never go back to a taller tire, 235/55 or 245/50's are better overall, in my opinion.

[lostatlantis]

ok... so, the 50 in 245/50 is essentially the height of the tire then. so if I go to a different height, that will throw my speedometer off. I guess my firebird is calibrated to use 235/55 tires, I wonder if there are any 245/55 that I can use so I won't throw off my speedo

[Jack]

<blockquote>quote:</font><hr>Originally posted by lostatlantis:
ok... so, the 50 in 245/50 is essentially the height of the tire then. so if I go to a different height, that will throw my speedometer off. I guess my firebird is calibrated to use 235/55 tires, I wonder if there are any 245/55 that I can use so I won't throw off my speedo<hr></blockquote>

yes...the heighth of the tire is approx. 50% of the tread width. if your speedo is calibrated for 235/55's and you go with a 245/50 the speedo will be off. Note however that it will not be off by more than a few miles per hours and I would be willing to bet that it was never 100% accurate from the factory. You can find out what the orginal tire size was by looking at the sticker on the drivers side door. I wouldn't worry too much about the speedo being off, they are not that accurate to begin with. Most police will not pull you over for being a few miles over the speedlimit...that is most, some will.

[V6Bob]

"the 50 in 245/50 is essentially the height of the tire then. so if I go to a different height, that will throw my speedometer off. I guess my firebird is calibrated to use 235/55 tires, I wonder if there are any 245/55 that I can use so I won't throw off my speedo"

No, that won't work. Sorry if I wasn't clear the first time. The height of the tire is _not_ the 50, it's .50 times the width. You need to include the width.

So 245/50s are about 245X.50 = 122.5mm high. 235/55 are 235X.55 = 129.25mm high. 245/55 would be 245X.55 = 134.75mm high. The speedo error would be about the same as the 245/50s, only the speedo would read low instead of high.

It's actually not that precise, but I wanted to be clearer this time, so I made the calculations more exact than they really are. You can get actual tire diameters off the manufacturers websites.