Tire size vs. fuel milage

[dnitzel]

Has anyone have an idea about the “calculated” fuel milage difference between, say the 265 stock tire standard tread and say a BFG 315R70? I ordered a 3. 73 gear ratio with mine and plan to buy new tires when it arrives. Thought it would be a better plan than ordering the factory off-road 265's. There are sure not very many 17 inch tires out there! For me, it’s between the Goodyear 285 or 305 MT/Rs, or the BFG TA/KOs 285 or 315. The price on the BFG 315's is outstanding at tirerack.com

 

[jimnance]

I don't know that a "calculated" fuel mileage improvement can be derived from tire diameter data.



My understanding is that at any given road speed peak economy is going to be near, but not beyond the RPM where peak torque occurs. I have seen several graphs depicting the torque curve on the ETH engine, but my best recollection is somewhere around 1800 RPM is where you want to be.



If you are always driving without a towed load I think I'd shoot for a tire diameter which yields that RPM at your normal freeway cruise speed.



With my six speed H. O. at seventy MPH I'm turning 2250 RPM which is higher than optimum. OTOH, it puts me at about 1950 RPM at 62MPH when pulling my fifth wheel. Still just a bit high for fuel economy, but right at the beginning of the sweet spot for towing. If you gear much taller you'll be in fifth gear at the smallest hill.



I guess I haven't really answered the question because there are so many variables in how each of us use our trucks.

 

[fritzimax]

I have the BFG 315/70-R17 AT and the 4:10 gears my around town 85% and highway 15% combo MPG is 17. 4 I have 2600 miles on her hope the MPG will go up as thing break in, but I am not complaining, I traded in my Dakota QC with 4. 7V8 and it was only getting around 16 MPG for the same combo driving and 1/2 the truck.

 

[dnitzel]

Originally posted by jimnance



I guess I haven't really answered the question because there are so many variables in how each of us use our trucks.

Actually it's a great answer! I forgot about the optimum RPM issue. My current '01 SO CTD 5 speed 3. 53 ratio with 285 tires gets 14 mpg empty stop and go highway and in town (all the lights here are timed, not sensor operated so most of them are red with no cross traffic #!?*) and about 15 highway 65-80 empty. I get about 10-12 with 5er pulling two trailers 8k to 10k at 60 mph.

 

[jimnance]

You also can't account for a heavy foot either. On my '99 with 3. 54's and an auto I generally got 17 MPG average, and 19 MPG on the Highway. I have a fairly conservative foot so I probably do better than some and worse than a few.



It's tempting to get into it, especially with the '99. With the auto and bombed it really scoots. Love to roll up next to a sedan and leave 'em in black smoke. Don't do it too often though, guess I'm getting old...

 

[lawdog]

Do not forget, when calculating your mileage, you can not go on what the odometer says when you have a larger tire. You will have to get into that math stuff and figure the difference in circumferance of the tires and figure what percent the difference there is in the tires then add that percent to your milage. I had 285/75r16's on my 2000 CTD Auto. I had an issue with mileage, and had a tech from JTE (Jeep - Truck Engineering) diesel lab and 2 from Cummins out to look at the truck at the dealer. They hooked their fancy (scary) computers to the truck and read the computer... ... This was real spooky as they said "Oh I see you have gone 100 mph in this truck :--) , They said that I had used X amout of gallons of fuel, that X % of the miles driven were between X rpm ranges, and that X % of the miles were towing. They said that by fuel flow they can tell when towing, or with weight heavier than just the truck alone. Anyways, they said ADD 4% to my milage figures for the increased size tires from 265's to the 285's... The larger tires were the FIrst thing they said when the approached the truck.....

 

[Jeremiah]

Originally posted by dnitzel

Has anyone have an idea about the “calculated” fuel milage difference between, say the 265 stock tire standard tread and say a BFG 315R70? I ordered a 3. 73 gear ratio with mine and plan to buy new tires when it arrives. Thought it would be a better plan than ordering the factory off-road 265's. There are sure not very many 17 inch tires out there! For me, it’s between the Goodyear 285 or 305 MT/Rs, or the BFG TA/KOs 285 or 315. The price on the BFG 315's is outstanding at tirerack.com

Seems like the question should be "what kind of result can you expect with TALLER WHEELS (as opposed to taller tires)? Rickson replaces the stock 17" wheels with 19. 5" wheels and includes a conversion kit that corrects the ODO and speedometer. I'm guessing the taller wheels will send the rear end ratio back to 3. 53 instead of 3. 73, and will allow highway cruise RMPs around 1800 instead of 2200. But bring your wallet when you switch 'cause they ain't cheap. OTOH, they say they run 100,000 miles or more.

 

[davidclark]

A java based Tire calculator tells me I am now off by 8. 02% going from stock to the 315s.

 

[jimnance]

Not cheap is an understatement Jeremiah. I downloaded the catalog yesterday and was shocked at the prices. Also, many of the tires they are selling are 70 series which makes the calculated diameter almost the same as the stock tires and rims.



To get the improved gearing ratio you need to get a higher profile 19. 5" tire as well.

 

[Jeremiah]

Originally posted by jimnance

Not cheap is an understatement Jeremiah. I downloaded the catalog yesterday and was shocked at the prices. Also, many of the tires they are selling are 70 series which makes the calculated diameter almost the same as the stock tires and rims.



To get the improved gearing ratio you need to get a higher profile 19. 5" tire as well.

That's worth knowing. I assumed that with a 19. 5" wheel (v. the factory 17" wheel) the tire would be correspondingly taller too. Not true?



I kinda figured the price of four wheels would be about $500 apiece, plus the price of the tire.

 

[Jeremiah]

Originally posted by lawdog

Do not forget, when calculating your mileage, you can not go on what the odometer says when you have a larger tire. You will have to get into that math stuff and figure the difference in circumferance of the tires and figure what percent the difference there is in the tires then add that percent to your milage. I had 285/75r16's on my 2000 CTD Auto. I had an issue with mileage, and had a tech from JTE (Jeep - Truck Engineering) diesel lab and 2 from Cummins out to look at the truck at the dealer. They hooked their fancy (scary) computers to the truck and read the computer... ... This was real spooky as they said "Oh I see you have gone 100 mph in this truck :--) , They said that I had used X amout of gallons of fuel, that X % of the miles driven were between X rpm ranges, and that X % of the miles were towing. They said that by fuel flow they can tell when towing, or with weight heavier than just the truck alone. Anyways, they said ADD 4% to my milage figures for the increased size tires from 265's to the 285's... The larger tires were the FIrst thing they said when the approached the truck.....

Brothers, I have a solution for everyone wanting to know how fast you're going, and you don't have to wait for the Highway Patrol to confirm it for you: go buy yourself a Garmin StreetPilot III GPS ($780 street price)! That sucker will tell you to the tenth of a mph how fast you're going and it won't care what kind of tires are under your vehicle!



Not only that, it will take you where you want to go, tell you when you're off-course, tell you your altitude and what streets you are passing, and lots of other interesting stuff.



Forget the tall tire math! This little jewel is the coolest thing I ever bought! Oo. Oo. Oo.

 

[kmiller]

For the frugal: you can check your odometer with with the milage markers on the interstate. I'm not sure how accurate they are, but they seem close. Eliminates concerns of different load ratings and inflation pressures for different size tires. A calculator has to make assumptions.

 

[Jeremiah]

Originally posted by kmiller

For the frugal: you can check your odometer with with the milage markers on the interstate. I'm not sure how accurate they are, but they seem close. Eliminates concerns of different load ratings and inflation pressures for different size tires. A calculator has to make assumptions.

That's close enough for government work, but a highway patrolman with an itchy fountain pen might quibble with using mile markers and a stop watch in place of a speedometer. I am amazed at how accurate the GPS is! But its real value is taking you from Point A to Point B without looking at a road map.



I have a friend who flies with one and hasn't looked at a chart in two years. In my opinion, he's pushing the envelope, but that's how good those things are.

 

[CATCRACKER]

I think that 315's will bring the 3. 73 to 3. 44. this will be slower off of the line but cruising on the interstate will greatly improve.

 

[Jeremiah]

Originally posted by CATCRACKER

I think that 315's will bring the 3. 73 to 3. 44. this will be slower off of the line but cruising on the interstate will greatly improve.

3. 44, huh? That's nice to know. It definitely will be slower off the line with a load. OTOH, the 3500 HO with the 6-spd might wind up faster with taller tires if it's running solo. ESPECIALLY, if EZ gets the go box online for the 2003s!



In addition, I have an idea the HO auto due sometime in the next month or two will smoke the 6-speed HO off the line. But we'll have to wait and see, I guess.

 

[jimnance]

Are we mounting these new BFG 315R70 tires on the stock wheel? What is the stock wheel width?



BFG recommends a wheel width of 8. 5" to 10. 0" for this tire.



Jim

 

[davidclark]

Are we mounting these new BFG 315R70 tires on the stock wheel?



Yes



What is the stock wheel width?



8 inches



BFG recommends a wheel width of 8. 5" to 10. 0" for this tire.



Oh that's just a recommendation...

 

[jimnance]

Thanks, just ordered a set from my tire guy at a cost of $120. 00 each. Special deal as we buy about $3000. 00 of tires a month for our business.



Looked into the 19. 5' rims and tires, but decided that the cost wasn't worth the improved load rating. Also, most of the 19. 5" tires had the same or smaller diameter than the BFG 315R70 AT's.

 

[Jeremiah]

Originally posted by jimnance

Thanks, just ordered a set from my tire guy at a cost of $120. 00 each. Special deal as we buy about $3000. 00 of tires a month for our business.



Looked into the 19. 5' rims and tires, but decided that the cost wasn't worth the improved load rating. Also, most of the 19. 5" tires had the same or smaller diameter than the BFG 315R70 AT's.

So what you're getting is a wider tire and not a taller tire, is that right, Jim?

 

[jimnance]

It is wider and taller. The 315 refers to width. The 70 is percentage of width that the sidewall is high. So a 315R70 is going to be about 3" greater around and about 1" taller.



It should bring my freeway RPM's at 70 MPH down about 250 RPM.