Mileage loss with 265's

[chrisinkanata]

Never having professed to be the brightest bulb on the tree, I'm confused. I was reading a thread here about changing from 245's to 265's on my truck. One or two members alluded to experiencing a loss of about 1 mpg. I've installed these on my truck now. I have yet to calculate the new mileage but I do know the engine is turning about 200 rpm less at 110km/h (about 70 mph). 1900 vs 2100. How can there be loss of mileage if the engine is closer to the torque peak and turning 200 rpm less?

Any thoughts?

Chris

[DieselDixon]

Well one thing you have to remember is along with your speedometer, your odometer is also moving slower, so your actual miles traveled per fill-up is less as well. In order to calculate your MPG correctly you would have to add approximately 3.9% to your miles traveled, which is about the percentage your speedometer is off with the 265's according to this wesbite. Tire Calculator (http://www.miata.net/garage/tirecalc.html)

[JimWilson]

Yea, what he said. ;)

[403turbo]

Well I have a few thoughts.

Your speedo is calibrated for the 245 tire size so... your speedo/odo is off by about 4-5% now. So for every 400 miles your clock says you travel you are really traveling 420 miles. Hence if you burn 20 gal and your odo says you have gone 400 miles you think you are getting 20mpg, but really you traveled 420 miles, so you are really getting 21 mpg.

So for the same distance on the ODO and the same fuel burned it looks like you lost 1 mpg.

Did that make any sense.....my head hurts....need to lay down! :D

For 265's fill your tank....do your math and add 1 mpg.

[mackey_62]

Been running 265's close to a year now.
After figuring in the 3.9% I can't really say I see a difference.
Still get 18.0 - 18.5 mpg 75% of the time.
Occasional 17.5 or 19, but not too often.
But I feel better knowing my motor make 200 less revolutions every minute I drive :D

[MAV]

I have 285's on and this last trip to Ramsey, MN, I calculated a total of 458 miles round trip. I add 7 miles for every 100. Toatal mileage plus 3 dyno runs and a bunch of horse play to warm the motor before the runs, I got 20.9 mpg with the 285's. I ran back and forth on the Quad 70 tune.

[Jelisfc]

With 315's and adjusting for the 13% difference I get about 1 mpg less on the highway partly due to sitting 4 1/2" higher. In town it goes down a lot more because the lower effective gear ratio makes the motor work harder. 265's shouldn't do much.

[Dawg]

I have 265's on mine. I multiply the number of miles travelled by 1.036 which gives me the actual number of miles travelled. Then divide by number of gallons.

[Colorado Kid]

I don't want to sound mean, but I'm certain that you can't put a tire that is 3.6% larger in diameter and reduce RPMs by 10% at the same speed.

2000 RPM gives 67 MPH in my truck, and if I changed to LT265/75R16s then 2000 RPM will give me 69 MPH, while the speedo will still indicate 67 MPH. Or I can continue to cruise at 67 MPH at 1930 RPM while indicating 65 MPH (Assuming the speedo was accurate with the stock tires in the first place, which is not a very safe assumption).

[chrisinkanata]

Thanks for all the input gents, but I'm still confused. In mid Nov, I have a road from Ottawa to Bethesda MD for a soccer tournament. Taking into account the difference in the Imperial/US gallon, I'll actually figure out what the difference in mileage is. Admitedly not scientific, a 1300 or 1400 mi. round trip may give some rough indication. Of course, it'll be beautiful on the way down and snow like there's no tomorrow on the way home. Why do I bother?

I find the comments from Colorado Kid particularly interesting. The RPM numbers I quoted were with the correct/uncorrected speedo. So with the 265's, at an indicated speed of 110, in fact I'm closer to 114 or so km/h, I guess thereby reducing the actual RPM difference a bit.

I'm with 403turbo......my mellon aches! I give!

Tks,

Chris

[jtegland]

If you didn't recalibrate your speedo for the bigger tires, I would think that an indicated 70 mph, no matter what your true speed is, would always be at the same rpm.

John

[Lone Eagle]

3.6% is the correction factor for everything. I figure the RPM difference is 75. Later! Frank