mpg up or down with 315's

[JARichard]

Has any one noticed an MPG increase or decrease with 315's?

I believe on my truck it wii drop RPM's at highway speed by 200 and was wondering what you guys that have calibrated the speedo were seeing?

TIA

 

[DougR]

You would think so,but mine always dropped a couple mpg.

 

[Moterhead]

Use this link,



http://www.csgnetwork.com/tireinfo4calc.html



plug in your original tire size and the 315 tire size, then calculate the percentage difference in RPM (revolutions per mile). This percentage will be your speedo error. See my post on this topic regarding 285 size.

 

[JARichard]

not miles per hour. miles per gallon does it increase or decrease?

 

[fencebuilders]

Originally posted by JARichard

not miles per hour. miles per gallon does it increase or decrease?

I have never seen a truck go from smaller to bigger tires and increase mileage. My . 02

 

[NDanecker]

Originally posted by fencebuilders

I have never seen a truck go from smaller to bigger tires and increase mileage. My . 02

I've heard over and over again that lowering the rpm's will increase your mileage. If you accomplish this by swapping gears or tires... . no difference as long as you have the torque to pull the truck at highway speeds. If you've done your numbers right and placed the motor at its peek torque at cruising speed, it theorectically should increase mileage.



True or not true... . good question!!!

 

[mgonske]

Originally posted by fencebuilders

I have never seen a truck go from smaller to bigger tires and increase mileage. My . 02

There's a special case for every rule



On my '98, when I switched from 295/75-16s back to the stock 245/70-16s before selling it, I saw a significant decrease. That was a V-10, with the 47RE and 4. 10 rear end, and on the same 4,700 mile cross-country trip towing the Jeep on the flatbed at 75 MPH, average mileage dropped from 8 MPG to 7 MPG. I don't remember the numbers (and am too lazy to do the math right now), but I found it easily explainable by the high RPMs the small tires forced me to turn at that speed.



That explains why I got the 3. 73s in the new truck--and I'm not worried about the CTD having enough torque to handle the higher gearing, even pulling my 14,000 lb 5ver and when I step up to 315s.



On edit: That same trip last Easter in the '03 netted 12. 9 MPG average for the 4,700 miles; this year, the 315s will go on at the halfway point--it will be interesting to see what happens to the fuel economy.

 

[XJSuperman]

peak torque does not always equal peak efficiency.



Our trucks peak at 1500-1600 RPM's... my mileage here is NOT good.

With my 315's I have not calculated any significant increase OR decrease. Under normal driving conditions and if I keep my size 15 out of it I am sitting on 15 city 19-21 hwy. This is exactly where I was with the 265's. With that said I have a worst tank of 12 MPG and this was with the 265's.

 

[klenger]

Increased rotating mass and less rolling resistance with bigger tires can offset the advantages of lower RPM's.

 

[mgonske]

Originally posted by klenger

Increased rotating mass and less rolling resistance with bigger tires can offset the advantages of lower RPM's.

I'd especially expect to see the effect of the increased rotating mass in accelerating and then wasting the energy as heat on braking in local stop and go driving. The 75-series tires were nice on the rolling resistance front, being a little narrower than a comparable height tire in a 70-series. No such luck for the '03, though (except for 19. 5s)--the 315/70-17s are W I D E.

 

[klenger]

mgonske:



The width of a 265 70 series tire is the same as a 265 75 series tire. The 265 is the width. The 70/75, etc number is the ratio of width to height from rim to tread or vice-versa (it's too late to figure it out now). In other words, a 265-70-17 is the same width and total diameter as a 265-76-16. Only the distance from rim to tread is different.

 

[mgonske]

Originally posted by klenger

mgonske:



The width of a 265 70 series tire is the same as a 265 75 series tire. The 265 is the width. The 70/75, etc number is the ratio of width to height from rim to tread or vice-versa (it's too late to figure it out now). In other words, a 265-70-17 is the same width and total diameter as a 265-76-16. Only the distance from rim to tread is different.

No argument here--I said for a "comparable height" tire. For example, when I was tire shopping for the '98, a Goodyear 305/70-16 was in the same ballpark for height as the BFG 295/70-16--but as you noted, wider.



The rolling resistance wasn't really what I was considering--I just like to keep tires under the fenders, to avoid winter (yeah, what's that, I live in Florida now) mess thrown up on the side windows. Other than in 19. 5s, there aren't any tall skinny tire options (that I like) for the '03 (due to limited availability for the 17" wheel).

 

[Duluth Diesel]

My mileage dropped 1 to 2 mpg with the 315 tires. Rolling resistance is an issue here. I am running a wider tire, at lower pressures, with much more aggressive tread. Guess what. They're harder to roll around under a 7000+ pound truck. But my gains in off road performance are SO worth it.