Tire size vs. fuel milage

[fritzimax]

wrong, the 315's are 3" taller and about 1+1/2-2' wider than the stock 265"s

 

[Jeremiah]

Originally posted by 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.

The taller part I like, along with the corresponding drop in RPMs. Wider is not for me. The additional drag won't be worth it on the highway. Plus, the wider tire won't work as well in snow.

 

[jimnance]

I understand not wanting wide tires in the snow, but I just did the Calculations on RPM vs Speed with 315/70/17's:



RPM 1800 1900 2000 2100 2200 2300 2400



Fifth gear 49 52 55 58 60 63 66

Sixth Gear 68 71 75 79 83 86 90



If you could turn 3200 RPM in sixth gear you'd be at 120 MPH.

 

[Jeremiah]

Originally posted by jimnance

I understand not wanting wide tires in the snow, but I just did the Calculations on RPM vs Speed with 315/70/17's:



RPM 1800 1900 2000 2100 2200 2300 2400



Fifth gear 49 52 55 58 60 63 66

Sixth Gear 68 71 75 79 83 86 90



If you could turn 3200 RPM in sixth gear you'd be at 120 MPH.

Given the ease with which this truck runs up to the red line, I'd guess it might hit 120! Do you suppose the engine has a governor? More realistically, it appears that 2000 RPMs would net 75 mph, which is a 200-250 RPM drop from 265/70/17 factory tires. But I also wonder whether the larger tire you've got would give fewer miles per gallon? For me, the ideal might be a 265/70/19. 5 if there is such a thing. Then 75 mph might be achieved on 1800 RPMs or so.

 

[scbroder]

Here is the results of a few calculations I ran to determine the effect of tire size on engine RPM and their equivalent gear ratios.



Assuming NV5600,

315 w/ 3. 73 is same as 265 w/ 3. 43, 1900 RPM @ 70 mph in 6th gear

315 w/ 4. 10 is same as 265 w/ 3. 77, 2100 RPM @ 70 mph in 6th gear



There is a slight estimation on tire diameter or revolutions per mile but these numbers are very close to what you will see on the road.

 

[scbroder]

I am looking to purchase 315’s for my truck. As others stated before me I was just wondering if it would decrease my fuel mileage. Has anyone keep track of fuel mileage before (w/ 265’s) and after (w/315’s), with mileage corrections? As many have stated I realize driving habits, weather, and type of usage contribute greatly to overall fuel economy. I would just like to know if the decrease in RPM would compensate for the added resistance from the larger tires? The fuel mileage I am primarily concerned with is interstate driving at 70-75 MPH while empty. If anyone has this type of information I would be very interested to hear the results.

 

[Jeremiah]

Originally posted by scbroder

Here is the results of a few calculations I ran to determine the effect of tire size on engine RPM and their equivalent gear ratios.



Assuming NV5600,

315 w/ 3. 73 is same as 265 w/ 3. 43, 1900 RPM @ 70 mph in 6th gear

315 w/ 4. 10 is same as 265 w/ 3. 77, 2100 RPM @ 70 mph in 6th gear



There is a slight estimation on tire diameter or revolutions per mile but these numbers are very close to what you will see on the road.

If I am not mistaken, the 315 is a wider tire than the 265. I am looking for a tire no wider than 265, but as tall as the 315. Excuse my stupidity, but I assume this is on the 19. 5 wheel, right?

 

[Elite1]

Originally posted by Jeremiah

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.

With Taller tires you might actually be able to use 1st gear on the 6 spd. I was looking at the specs. and 1st seems like a granny low... with taller tires it may get it closer to a normal 1st gear. I believe 2nd gear on the 6spd. is still lower than 1st gear on the 48re auto.

 

[randyn]

Rim width on the stock aluminum spoke wheels is 8. 5" The rim width on the spare wheel (black steel) is 8". I noticed this because I thought rim width was 8" and when the balance guy entered 8. 5" I questioned him. He stuck the caliper back on & 8"!!!!! I liked that because the tire is about 12. 5" wide & needs more than an 8" rim I would think.



RandyN

 

[Mathew Clausen]

To answer your original question - there is a "calculated" loss in economy when switching to larger tires and not correction your odometer. You travel more miles than indicated, so your mileage will come out less - unless you correct for it.



The best numbers to use (besides GPS) is the rev's per mile shown on tire spec sheets. This is actually measured by the tire companies at speed and load for that tire (different brands and styles of tires of the same size will have different rev/mile) and accounts for tire flex and other factors. From the michelin and BFG web sites - here are the numbers for stock and 315/70 AT's.



Stock - LTX a/s 265/70-17 = 657 revs/mile

BFG AT 315/70-17 = 601 revs/mile



657/601 = 1. 09 times faster than indicated on the speedo (9% faster). So - for calculating mileage, take the odometer reading and multiply by 1. 09, then divide by gallons. This is a significant difference ... if you thought you are getting 18 - you are really getting 18 x 1. 09 = 19. 6!



Of course other factors change mileage - but this part is calculateable and fixed based on your tire size difference from stock.



Mathew

 

[Elite1]

Originally posted by randyn

Rim width on the stock aluminum spoke wheels is 8. 5" The rim width on the spare wheel (black steel) is 8". I noticed this because I thought rim width was 8" and when the balance guy entered 8. 5" I questioned him. He stuck the caliper back on & 8"!!!!! I liked that because the tire is about 12. 5" wide & needs more than an 8" rim I would think.



RandyN

Randy, Which is it... 8 or 8. 5? Seems like you contradict yourself... hmmm

 

[Jeremiah]

Originally posted by Mathew Clausen

To answer your original question - there is a "calculated" loss in economy when switching to larger tires and not correction your odometer. You travel more miles than indicated, so your mileage will come out less - unless you correct for it.



The best numbers to use (besides GPS) is the rev's per mile shown on tire spec sheets. This is actually measured by the tire companies at speed and load for that tire (different brands and styles of tires of the same size will have different rev/mile) and accounts for tire flex and other factors. From the michelin and BFG web sites - here are the numbers for stock and 315/70 AT's.



Stock - LTX a/s 265/70-17 = 657 revs/mile

BFG AT 315/70-17 = 601 revs/mile



657/601 = 1. 09 times faster than indicated on the speedo (9% faster). So - for calculating mileage, take the odometer reading and multiply by 1. 09, then divide by gallons. This is a significant difference ... if you thought you are getting 18 - you are really getting 18 x 1. 09 = 19. 6!



Of course other factors change mileage - but this part is calculateable and fixed based on your tire size difference from stock.



Mathew

Great advice, Mathew! Fortunately, Rickson sells a device that corrects the speedo. I also have GPS, just in case. My final concern is whether, with taller tires, it will have enough power in 6th.

 

[randyn]

Elite1



Oops! The stock aluminum spoke wheels on my '03 2500 QC SWB 4x4 HO 4. 10 w/ 315 BFG ATs measured 8. 5" w/ tire balancing machine calipers. The spare wheel measured 8". My fingers out ran my mind.

 

[dnitzel]

I ended up with getting BFG AT 285's with the white letters. They look REALLY sharp and handle GREAT, both solo and with two my trailers in tow tandem( 5th wheel camper and ATV trailer). We had 12" of wet, heavy snow last week, these tires and my new truck worked OUTSTANDING!! It was really fun plowing through the 3 to 4 foot plies of snow along the road with EASE! I know I shouldn't do that. Mileage, last tank with 2400 miles was 13. 1 in town lead foot miles, calculated. The overhead read 13. 3. Darn close. I eventually expect to get the same mileage as my '01 CTD 5 spd in town, 14 mpg as this baby break’s in.

 

[Jeremiah]

Originally posted by dnitzel

I ended up with getting BFG AT 285's with the white letters. They look REALLY sharp and handle GREAT, both solo and with two my trailers in tow tandem( 5th wheel camper and ATV trailer). We had 12" of wet, heavy snow last week, these tires and my new truck worked OUTSTANDING!! It was really fun plowing through the 3 to 4 foot plies of snow along the road with EASE! I know I shouldn't do that. Mileage, last tank with 2400 miles was 13. 1 in town lead foot miles, calculated. The overhead read 13. 3. Darn close. I eventually expect to get the same mileage as my '01 CTD 5 spd in town, 14 mpg as this baby break’s in.

When it breaks in you should do better than that if you have the 6-speed. I've got 12,000 miles on mine and am getting 20. 5 solo at 6-9,000 feet, and 18. 5 if I'm hauling 2500 lbs of water in my bed. However, I have the Access roll-up tonneau, which probably helps it a mile per gallon or so.

 

[crush]

BFG-315'S

I've had the BFG-315's since new, (6500mi. ), and love them so far. The ride is quite a bit better than stock with 45psi. I was thinking the gear ratio was somewhere close to the 3. 55 range without getting crazy with the calculator so the 3. 44 I read in a previous post sounds good. The speedometer is 5mph slow with the odometer . 08 off, (less) per 10 miles or 8 per 100 miles. With this figured in, the pen and paper mileage is 20. 1 unloaded at 65 in hilly terrain at 6500-7000. ' The RPM sits right at 1750. Pulling a 5-6k load in the same terrain at 75 and 2100RPM I get 12-14 which goes up to 16 when I get in the flats.

 

[Jeremiah]

Re: BFG-315'S

Originally posted by crush

I've had the BFG-315's since new, (6500mi. ), and love them so far. The ride is quite a bit better than stock with 45psi. I was thinking the gear ratio was somewhere close to the 3. 55 range without getting crazy with the calculator so the 3. 44 I read in a previous post sounds good. The speedometer is 5mph slow with the odometer . 08 off, (less) per 10 miles or 8 per 100 miles. With this figured in, the pen and paper mileage is 20. 1 unloaded at 65 in hilly terrain at 6500-7000. ' The RPM sits right at 1750. Pulling a 5-6k load in the same terrain at 75 and 2100RPM I get 12-14 which goes up to 16 when I get in the flats.

Your final figure tells me the 315s dropped your rpms 150 at 75 mph. With stock tires and wheels the rpm is 2250 at 75 mph.



Because I too believe in kicking the Cummins in the butt every few tanks full, I unloaded on mine the last tank and even hit 100 for a few miles going out across a long flat area where you can see a cottontail rabbit 5 miles away if he raises his ears. I also kept the truck in lower gears longer than I normally would for the rest of the tank. This only dropped my miles per gallon to 19. 5 instead of 20. 5. What a truck!