Tires & RPM Help

'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?

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.

Gary Lewis wrote

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.

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.

Gary Lewis wrote

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.

 

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.

Dyn Blin wrote

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.

Hi, sorry I am late to the party... I did get wrapped up in the math of gearing for my build, and I made pages and pages of calculations for different transmissions, axle gears and tire sizes in my quest for the perfect combination. There are some good online calculators if you don’t like to do the math yourself. Here is one:

http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

I am really enjoying the tires that I bought. The 235/85/R16 help the engine revs with the non-overdrive transmission. They are heavier, I guess, than stock rims and tires, but not drastically different. It is what I have always run on my heavier F250/F350’s. I wanted the load capacity and also ply rating for overlanding. I had to buy aftermarket rims which was sad since I have a really beautiful set of silver spoke wheels that I got from David, but it was the right choice. I am not a fan of wide tires. They are good for mud and soft sand I guess, but I went for the tall skinny for good mpg and snow traction.

Ford F834 wrote

 I had to buy aftermarket rims which was sad since I have a really beautiful set of silver spoke wheels that I got from David, but it was the right choice. I am not a fan of wide tires. They are good for mud and soft sand I guess, but I went for the tall skinny for good mpg and snow traction.

Bring the set of wheels to 2018 Garagemahal Ford Truck Show for the swap meet [selling of good used parts]!

Good idea David! I should have room for more parts this year!

Small world.  I grew up in Moscow, ID (30 miles north of Lewiston on the "Spiral Highway").  My brother and his family have since moved to Lolo.  I take a motorcycle joyride at least once a year in a long loop from NorCal to see family in Moscow and Pullman, then spend time with my brother and his family in Lolo.  When all the kids are out of school, the wife and I will head up to join them.  It really is beautiful country, and some great history, too.

It's funny, I spent my teen years and college working to get away, and now I count the years until I can get back.

If you ever head up there again, reach out to me and I'll give my .02 of routes and stops.

Thanks for that.  I'm looking to stay "skinny" too for the occasional romp up to Tahoe in the winter, and keep the road friction and rub to minimums.

Now I'm thinking about the possibilities with 16" wheels, but the outlay for new rims and tires will be too dear to budget for prior to the trip, unless I find something good at the yard.

For 15", it looks there there are a few choices at 31 or 33 and 10.5 that aren't so soft I'll be looking to buy in again in too short a time.

Just FYI, the 31x10.5’s are ~not 31” tall. They are more like 30” new, 29” when worn down... and wider than I like. I don’t know how they get away with calling them “31’s” but they might be a bit disappointing for rpm’s if you are expecting a true 31” diameter.

Boy, you're not kidding, that would be a disappointment after plugging all these numbers in and finding out the RPM savings isn't what I expected.  Thanks for the pointer.

Tire Rack gives you the rolling diameter on all of the many tires they sell.  And you can compare several of the same tire sizes in order to determine which one to buy.

Perfect Gary, & thanks for the additional resource.  

(...adds column to the spreadsheet)

I figured I'd come back to close the circle on the topic:

I got lucky and picked up a set of brand new "take-offs" from a local shop with less than 100 miles on them: 31x10.5x15 mud and snow tires with a mild tread.

On the long trip, it was hard to gauge their affect on mileage given the load in the bed and the winter formulation of gasoline that hits CA and NV in Oct & Nov, but the immediate noise & vibration improvement in the ~300 RPM reduction was a pleasure, especially at the 55-70MPH range.  Although the speed limit hits 85 on some parts of the long loop up to the panhandle and western MT, I'm usually in the left lane with the big rigs at an easier pace.

Since I got home, although I really don't notice much change in clutch feel getting it rolling unloaded.  City mileage has stayed the same- I expected a bit of drop with the increase in contact patch and resistance.  Highway has seen roughly a 10% improvement in mean MPG, and I hope to see a bit more all things equal when spring sees the return of the better gas.

The only other variable I can account for is an oil change back to 10w30 (Rotella T5) from full-dino 10w40.  However, given the mild CA weather this fall, I'm not sure that can account for any appreciable difference in the improvement percentage.

Overall, a worthwhile improvement over the 28" tires for me.