Tires & RPM Help

[Dyn Blin]

'Looking for experience & advice from the group-

I’m taking the truck on an 1800 mile trip to visit family in Lolo, MT in October, bringing a load of (empty) wine barrels. I’ll be buying new tires prior to the trip, and figured it would be a good time to review options to lower highway RPMs. I’m not expecting huge gains in MPG, but would like to reduce the buzz and noise, and saving a few bucks on gas will be a nice side benefit.

I currently run 15” wheels, with the stock 235/75/15 (28.9 inch tall) tires and no lift. I have a code “16” (not limited-slip) rear axle, which according to most sources, is a 3.50 ratio. My NP435 manual transmission has a 4th gear of 1.0. When I plug these into the online RPM calculators, it calculates approx. 2600 RPM at 65 MPH and approx 2850 RPM at 70 MPH. These figures almost match my dash tach, with the tach reading a little higher in “real life” with the older tires.

Getting through NV, South ID, and MT will see higher speed limits, so the bigger RPM reduction, the better for the trip. With the "granny" low 1st gear and 351, I’m not so concerned with starting to roll with the load I’ll be taking, and the truck doesn’t see anything more rugged than dirt roads in its regular use.

To the question: I’ve seen the forums list everything from 33" to 35(?!)" tall tires fitting the stock suspension set up, with width targeted under 11" (which seems to limit choices to a 33" max). The load of the barrels will be 550-650 lbs in the bed.

How tall a tire would you be comfortable in using?

 

[Gary Lewis]

The factory spec's show a "16" rear diff to be a 3.50. Big Blue is running 3.55's and 33" tires and still seems to be 2300 R's at 65 MPH. So that's what you'd be looking at as well with 33's.

The tires on Big Blue is wide and it is pretty easy to rub on a tight turn. So I don't think I'd want to go bigger than that. But I had 31's on Rusty and they didn't rub very often, although I don't know how the offset on the wheels compared.

Bottom Line: I think you can go to 33's, but I'd keep the width as narrow as you can. That keeps the rubbing down and minimizes the friction with the road. But 31's are a sure thing and may be the best compromise.

[1986F150Six]

The factory spec's show a "16" rear diff to be a 3.50. Big Blue is running 3.55's and 33" tires and still seems to be 2300 R's at 65 MPH. So that's what you'd be looking at as well with 33's.

The tires on Big Blue is wide and it is pretty easy to rub on a tight turn. So I don't think I'd want to go bigger than that. But I had 31's on Rusty and they didn't rub very often, although I don't know how the offset on the wheels compared.

Bottom Line: I think you can go to 33's, but I'd keep the width as narrow as you can. That keeps the rubbing down and minimizes the friction with the road. But 31's are a sure thing and may be the best compromise.

Get in touch with Jonathan [Ford F834]... he installed 16" wheels [after market] and something like 265/85 X 16 tires on his F150. You can see them on the picture of his truck at the top of the Forum page.

[Rembrant]

But 31's are a sure thing and may be the best compromise.

Bingo. If it was me, I'd be installing 31's. The only 33's I've ever owned were mud tires, and I'd guess that the extra weight and height of them would negate any savings of lower RPM's. (Then again, I had them on a Toyota, not a full size Ford).

I think 31's look nice, and they'd give a little extra circumference, while still being mild mannered on the road.

The other guys may have completely different experiences, so this is only my 2 cents;).

 

[Gary Lewis]

But 31's are a sure thing and may be the best compromise.

Bingo. If it was me, I'd be installing 31's. The only 33's I've ever owned were mud tires, and I'd guess that the extra weight and height of them would negate any savings of lower RPM's. (Then again, I had them on a Toyota, not a full size Ford).

I think 31's look nice, and they'd give a little extra circumference, while still being mild mannered on the road.

The other guys may have completely different experiences, so this is only my 2 cents;).

Weight is certainly an issue, and one I forgot to mention. I remember comparing Dad's truck's tires, which were 29's, to those on Rusty, the 31's. And Rusty's were significantly heavier. Part of the difference was in the aluminum wheels on Dad's compared to the steel "saw blades" on Rusty. But the larger tires were heavier as well. And, when I put Dad's tires on Rusty the ride quality and the steering precision improved.

Fast forward to the trip to FL to pick up Big Blue. My brother, who is stronger than I, came rolling the spare out. As I bent down to pick it up he was saying "No can do!" Yeah, right. Well, he was sorta right. I could have picked it up, but it was more than I should pick up. In fact, it was HEAVY!

Granted those are BIG tires. But they are far heavier than they look. And they surely play a part in the poor ride quality of the truck.

So, there's another factor or three going on as you go up in tire size. One is the weight of all of the un-sprung mass of the tire & wheel, which upsets the ride and the steering. Another is the inertia of the heavier tire, which reduces acceleration. Another is the width of the tire, which causes more friction and reduces MPG.

So, it is a balancing act. 33's may be a bridge to far.

[1986F150Six]

Weight is certainly an issue, and one I forgot to mention. I remember comparing Dad's truck's tires, which were 29's, to those on Rusty, the 31's. And Rusty's were significantly heavier. Part of the difference was in the aluminum wheels on Dad's compared to the steel "saw blades" on Rusty. But the larger tires were heavier as well. And, when I put Dad's tires on Rusty the ride quality and the steering precision improved.

Fast forward to the trip to FL to pick up Big Blue. My brother, who is stronger than I, came rolling the spare out. As I bent down to pick it up he was saying "No can do!" Yeah, right. Well, he was sorta right. I could have picked it up, but it was more than I should pick up. In fact, it was HEAVY!

Granted those are BIG tires. But they are far heavier than they look. And they surely play a part in the poor ride quality of the truck.

So, there's another factor or three going on as you go up in tire size. One is the weight of all of the un-sprung mass of the tire & wheel, which upsets the ride and the steering. Another is the inertia of the heavier tire, which reduces acceleration. Another is the width of the tire, which causes more friction and reduces MPG.

So, it is a balancing act. 33's may be a bridge to far.

I mentioned that Ford F834 was using 16" wheels and tires. I found his thread and the size is 235/85 R16. If I remember correctly, he already had these tires, but he also was looking for weight carrying capacity and best gas mileage [more narrow tread].

According to an online calculator, the following sizes will give food for thought:

235/75 R15 = 28.9" tall, 9.3" width, 699 rev./mile

31.0 X 11.0 R15 = 31" tall, 11.0" width, 651 rev./mile

235/85 R16 = 31.7" tall, 9.3" wide, 636 rev./mile

33.0 X 11.0 R15 = 33" tall, 11.0" wide. 611 rev./mile

Ford F834 works 12 hour shifts and is in the middle or rebuilding a house. I look forward to his input regarding this matter.

[Gary Lewis]

I mentioned that Ford F834 was using 16" wheels and tires. I found his thread and the size is 235/85 R16. If I remember correctly, he already had these tires, but he also was looking for weight carrying capacity and best gas mileage [more narrow tread].

According to an online calculator, the following sizes will give food for thought:

235/75 R15 = 28.9" tall, 9.3" width, 699 rev./mile

31.0 X 11.0 R15 = 31" tall, 11.0" width, 651 rev./mile

235/85 R16 = 31.7" tall, 9.3" wide, 636 rev./mile

33.0 X 11.0 R15 = 33" tall, 11.0" wide. 611 rev./mile

Ford F834 works 12 hour shifts and is in the middle or rebuilding a house. I look forward to his input regarding this matter.

Good find, David.

If it was me I'd put together a table (spreadsheet) with the sizes and the various factors, including weight & cost. And, I'd consider new wheels as part of the package if I didn't have some I loved.

To get those factors I would go to tirerack.com.

[Rembrant]

So, there's another factor or three going on as you go up in tire size. One is the weight of all of the un-sprung mass of the tire & wheel, which upsets the ride and the steering. Another is the inertia of the heavier tire, which reduces acceleration. Another is the width of the tire, which causes more friction and reduces MPG.

So, it is a balancing act. 33's may be a bridge to far.

I was thinking of the added weight of the 33's, and the height increase...every little bit you lift the truck, you make it harder to push through the air. I can't remember comparing my 31's and later the 33's, but I remember those 33" tires being really heavy. No idea how much any of it matters in the grand scheme of things, but my money is still on the 31's, assuming he wants to stick with the same 15" rims.

 

 

[Dyn Blin]

I appreciate all the posts gentlemen. Good points to consider as I start shopping.

Gary: yep. I'm tracking all of the options on a spreadsheet, out of necessity. Somewhere after the 2nd kid, I wasn't able to keep more than 3 items in my head without writing them down.

I welcome more experience and advice if anyone has some to offer.

[Rembrant]

I’m taking the truck on an 1800 mile trip to visit family in Lolo, MT in October...

Slightly off topic, but my wife and I did a coast to coast motorcycle trip back in 2011. We traveled west all the way across Canada, and then east all the way across the northern USA. We stayed in Lewiston Idaho, and then took hwy #12 east from there and then over the Lolo Pass into Montana. I've often told people, that the drive east from Lewiston and into Montana has to be one of the nicest drives in the country. The way the road meanders along following the river is really very pretty.

Anyway, sorry to veer away from tires here...but when you mentioned Lolo MT, you reminded me of a very beautiful part of the country, and somewhere I'd love to have had more time to explore.