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Gas Mileage Drop With The Bigger Tires

Posted By: Anonymous

Gas Mileage Drop With The Bigger Tires - 11/22/10 07:11 PM

I have noticed a huge drop in fuel consumption since I put the 235's on.
It seems to have a lot of drag for some reason. Like it doesn't roll as freely as it used to with the stock wheels.
So obviously I am using more fuel to get it moving.

just wondering if this is just how it is and live with it kind of thing or not.
Posted By: promodmerc

Re: Gas Mileage Drop With The Bigger Tires - 11/22/10 07:38 PM

Bigger tires = more weight and also reqiures more power to move. You have basically decreased your gear ratio(numerically lowerd it)
Posted By: Anonymous

Re: Gas Mileage Drop With The Bigger Tires - 11/22/10 07:44 PM

Yeah I expected a drop in mileage but it has dropped in a big way.
Oh well...it's all good.

Thanks man
Posted By: Dryver

Re: Gas Mileage Drop With The Bigger Tires - 11/22/10 07:53 PM

Your gas mileage probably hasn't dropped as much as you think. Mine dropped about 2 mpg, and I put on 31x10.5x15 tires. Since I had gained about 2 mpg by doing the elctric fan conversion, I'm back to about where I used to be at stock.

You have to factor in that you are actually traveling more miles than your odometer says you are. 235/75/R15 tires have a diameter of about 29.1" versus about 27.1" for 205/75/R15. You have to multiply your odometer driven miles by 1.074 to get your true driven miles before figuring your mpg.
Posted By: DEFBOY35

Re: Gas Mileage Drop With The Bigger Tires - 11/23/10 04:52 AM

i believe i was getting around 320km to a tank with the 31's, its been a while since i have driven it though. so whatever that works out to after correction i am not sure.
Posted By: Anonymous

Re: Gas Mileage Drop With The Bigger Tires - 11/23/10 06:09 AM

Was getting about 440km a tank and I know I am not getting that now.
I will be filling it up tomorrow and will let ya know how far that tanks goes.
I am gonna take a guess at 375km.
Posted By: timd75

Re: Gas Mileage Drop With The Bigger Tires - 11/23/10 08:59 PM

After putting the 235's on my sportage, I have dropped to about 20-21 mpg instead of the 24 I was getting before. At miata.net there is a tire calculater that says your speedo is now 6.5% too slow. When your speedo says 60mph, you are actually going 63.9mph. Also, check your tire pressure. It will make more difference on the bigger tires. The tire shop put my tires @32psi, but the max psi is 50. I run them at about 46-48 and I can feel the difference, besides at the pump. The ride is a little rougher, but I drive a 50 mile commute, so less rolling resistance, the better.
Posted By: promodmerc

Re: Gas Mileage Drop With The Bigger Tires - 11/23/10 11:07 PM

50 PSI is for a loaded tire. Running too much PSI is just as dangerous was running too low of PSI.

Jsut because you can inflate the tire to 50 PSI doesn't mean you should.
Posted By: DamKia

Re: Gas Mileage Drop With The Bigger Tires - 11/24/10 01:00 AM

Quote
After putting the 235's on my sportage, I have dropped to about 20-21 mpg instead of the 24 I was getting before. At miata.net there is a tire calculater that says your speedo is now 6.5% too slow. When your speedo says 60mph, you are actually going 63.9mph. Also, check your tire pressure. It will make more difference on the bigger tires. The tire shop put my tires @32psi, but the max psi is 50. I run them at about 46-48 and I can feel the difference, besides at the pump. The ride is a little rougher, but I drive a 50 mile commute, so less rolling resistance, the better.


You will find that your speedo was already slow to begin with. adding the 235's brings it up to about right (+/- 2 or 3%).

Check the sidewall of the tire for the maximum inflation pressure and subtract 5 psi from that as your max inflation pressure.

Over inflating is poor economics anyway as the center of the tread wears out much quicker making replacement of all 4 tires more frequent (within 20K miles from new depending on the tire)
Posted By: timd75

Re: Gas Mileage Drop With The Bigger Tires - 11/24/10 02:03 AM

My Sporty came with 205-70- 15's and I switched to a 235-75-15 A/T. To figure out my actual speed, I Compare my odometer to mile markers. I found it was 10% off. I found my odometer was reading 200-215 miles @ my usual fill up, but it used to read 240-250. That's about right if there is a 10% difference. The tire calculater says there is 9.8% difference in the 205-70-15 and the 235. If the kia speedo is slow from factory, than maybe the 205-70 instead of 205-75 is the tire that runs true with the speedo.
Posted By: DennisThompson

Re: Gas Mileage Drop With The Bigger Tires - 11/24/10 11:43 PM

I checked mine with a GPS, with my 31s when my speedo reads 50 I am doing 58.9 MPH. When I calulate my MPGs I get about 19 MPGs, my speedo is about 18% off so when I add the miles I'm getting about 23 MPGs. When my Kia was stock I got about 24 to 25 MPGs and my wife got about 27 MPG. I don't think thats to bad of a drop for adding bigger tires, a lift, a heavy winch and a heavy rear bumper.

Dennis
Posted By: timd75

Re: Gas Mileage Drop With The Bigger Tires - 11/25/10 12:57 AM

I was wondering how accurate my equation was. I'm thinking it's probably pretty close. When my speedo reads 60, I'm doing 66. 20-21 mpg with 235's isn't too bad then, seeing I do 40 miles of my daily commute going 75-85mph. I would like to get a gps just to be sure, though.
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