Tires vs MPG

stevem5000 · Jun 26th, 2006, 09:59 PM

My 2000 5sp 4 banger with 129,000 finally needed some new tires...I was currently running a set of Michlen 235/75R15 LT tires...

Last mileage test about 3 months ago I got was 21.3 MPG...I have never gotten over 21.6 on this truck...

Last week I put on a set of Michlen 215/70R15 X Radial from Sams Club...just ran a mileage test and I got 22.7 MPG...

Not a lot of improvement, but at close to $3.00 a gallon, I'll take every little bit I can get...

And actually, I really wasn't expecting the mileage to go up...smaller wheels, faster engine RPM to get the same speed...I figgured it might drop slightly...but it didn;t...

jerryp58 · Jun 26th, 2006, 10:14 PM

Quote:

Originally Posted by stevem5000

My 2000 5sp 4 banger with 129,000 finally needed some new tires...I was currently running a set of Michlen 235/75R15 LT tires...

Last mileage test about 3 months ago I got was 21.3 MPG...I have never gotten over 21.6 on this truck...

Last week I put on a set of Michlen 215/70R15 X Radial from Sams Club...just ran a mileage test and I got 22.7 MPG...

Not a lot of improvement, but at close to $3.00 a gallon, I'll take every little bit I can get...

And actually, I really wasn't expecting the mileage to go up...smaller wheels, faster engine RPM to get the same speed...I figgured it might drop slightly...but it didn;t...

Did you take into account the effect the smaller diameter of your new tires would have on your odometer? Travelling the same distance, your 235/75s would show less miles travelled than the 215/70s. So, the smaller diameter tires would yield a higher MPG.

I went the other way (225/70 to 235/75) and I'm trying to compensate for that on my Fuel Mileage page, but I'm not sure I've gotten it quite under control yet.

There are other factors to consider. You've lowered the height of the truck (less air flow underneath), you've probably reduced the weight of the tire(s), and you've reduced the frontal area of the tire. All things that should improve MPG.

Anyway, you might actually be getting better MPG. If so, congrats!

Reverendbiker · Jun 27th, 2006, 08:27 AM

Quote:

Originally Posted by stevem5000

My 2000 5sp 4 banger with 129,000 finally needed some new tires...I was currently running a set of Michlen 235/75R15 LT tires...

Last mileage test about 3 months ago I got was 21.3 MPG...I have never gotten over 21.6 on this truck...

Last week I put on a set of Michlen 215/70R15 X Radial from Sams Club...just ran a mileage test and I got 22.7 MPG...

Not a lot of improvement, but at close to $3.00 a gallon, I'll take every little bit I can get...

And actually, I really wasn't expecting the mileage to go up...smaller wheels, faster engine RPM to get the same speed...I figgured it might drop slightly...but it didn;t...

You're actually getting worse gas mileage. The old tires were 97.7 inches in circumference, while the new ones are only 84.4--a difference of about 11.5%. What you've done is effectively changed your final drive ratio by going to a shorter tire; this will make a number of changes:
1) Your speedometer will be off by about 11.5%
2) You should pick up just a bit of acceleretion, as you now have a slightly lower gear.
3) Your fuel economy will be lower.

Using your own numbers, your observed gas mileage went up by 6.5%, but you were traveling 11.5% less miles than you thought you were.

05 NISMO 4X4 · Jun 27th, 2006, 08:38 AM

I recently purchased a ScanGaugeII (www.scangauge.com) and it has the capability of adjusting for changes in tire size, rear gear ratios, etc. It has a lot of other useful functions as well, but it's ability to monitor gas mileage accurately has made it a very useful tool/toy!

azrocketman · Jun 27th, 2006, 10:35 AM

To me 21 mpg and change sounds low for a 5 speed 4 cylinder. I'd expect to see around 25 mpg or better in mixed driving for that combination. I see mid to high 20's on my 4 cylinder auto this time of year dropping to lower 20's in the winter. Are you "flogging" the truck?

Steve

Markcuda · Jun 27th, 2006, 11:04 AM

I get 21 MPG on my Frontier, glad I did not go with a 4 banger

Z_Rated · Jun 27th, 2006, 11:21 AM

Quote:

Originally Posted by Reverendbiker

You're actually getting worse gas mileage. The old tires were 97.7 inches in circumference, while the new ones are only 84.4--a difference of about 11.5%. What you've done is effectively changed your final drive ratio by going to a shorter tire; this will make a number of changes:
1) Your speedometer will be off by about 11.5%
2) You should pick up just a bit of acceleretion, as you now have a slightly lower gear.
3) Your fuel economy will be lower.

Using your own numbers, your observed gas mileage went up by 6.5%, but you were traveling 11.5% less miles than you thought you were.

I tend to disagree, my belief is that the latest replacement tire size is closer to OEM and still larger. Thus mpg calculations could be skewed from the start. Noting lower than advertised fuel economy for a 4 banger also makes one wonder?

Reverendbiker · Jun 27th, 2006, 04:22 PM

Quote:

Originally Posted by Z_Rated

I tend to disagree, my belief is that the latest replacement tire size is closer to OEM and still larger. Thus mpg calculations could be skewed from the start. Noting lower than advertised fuel economy for a 4 banger also makes one wonder?

Why would you disagree? It' simple math. Use a tire size calculator and you'll see that the new tires are significantly smaller that the last set. His mileage calculations may well have been skewed from the start, but his premise was that the recent change of tires improved his fuel economy. Not a chance.

avenger · Jun 27th, 2006, 05:53 PM

Quote:

Originally Posted by 05 NISMO 4X4

I recently purchased a ScanGaugeII (www.scangauge.com) and it has the capability of adjusting for changes in tire size, rear gear ratios, etc. It has a lot of other useful functions as well, but it's ability to monitor gas mileage accurately has made it a very useful tool/toy!

I own a scangauge, the original one... I don't see on the website where you can program different tires with a scangauge. I know you can pass codes on to the ECU, but these are raw codes...

05 NISMO 4X4 · Jun 27th, 2006, 08:06 PM

Maybe it's a function of the ScanGaugeII. You can down load the manual from their website, and it shows the formula to use for calculating an adjustment for a different sived tire or if you were to change the final drive ratio. SGIIManual.pdf

Z_Rated · Jun 28th, 2006, 10:52 AM

Quote:

Originally Posted by Reverendbiker

Why would you disagree? It' simple math. Use a tire size calculator and you'll see that the new tires are significantly smaller that the last set. His mileage calculations may well have been skewed from the start, but his premise was that the recent change of tires improved his fuel economy. Not a chance.

Less rolling resistance equates to better fuel economy, smaller contact patch equates to less rolling resistance. My difference in opinion is based on the limited facts given and my belief that the wheels and tires used prior were too large and that the smaller replacement tires are still larger overall both in diameter and contact width as compared to what I believe would be the calibrated OEM tire/wheel combo. Thus recorded distance travelled and rolling resistance decreased, he now is closer to actual and yet still less than actual recorded. In which case his speedo would be off by less than what you quoted going from the larger to the smaller tire.

My disagreement is not in your logic, it is based on your application assuming that his distance travelled was accurate to begin with and his statement about never getting "over 21.6 on this truck", which makes me believe his milage was better than he thought all along. BTW- Thats horrible for that generation four cylinder. Then again, I'll admit I'm assuming more than I should given the facts. It just makes me wonder. Enjoy - Z