Tire Height
Thread Starter
|
Freshman User
Joined: Feb 2011
Posts: 44
Likes: 0
Tire Height
I have a 86 f250 4x4, 4 speed with IH 6.9.The rear is 4.10. It has a sterling 10.25 My issue is that at highway speeds I’m maxed out at 60-65MPH. Can I (should I) go to a taller tire? Like a 19 inch or 20 inch? Where would I find such a wheel tire combo relatively cheap? I consider changing the rear pinion and ring gear but it’s beyond my skill level. BTW. It's a rat rod so looks are not important.
Hotshot
Joined: Jun 2015
Posts: 12,878
Likes: 2,446
According to Tire Rack the standard size was a 235/85R16 (30.4") and optional was a 235/85R16 (31.7"). How would a 19 or 20 inch tire be taller? Putting a taller tire on the 16" rims would be like putting a taller gear in the rear and increase top speed but sacrifice acceleration.
Lead Driver
Joined: Feb 2013
Posts: 5,006
Likes: 72
From: Roseville, MN
Yes you can put larger tires on to bring the engine rpm down. The 19" and 20" you're taking about is rim diameter. That doesn't matter (much), you need to look at OD (outside diameter). Figure out what size your stock tires are, then take the ratio change you want to achieve and multiply your current tire size by that. For instance, if you currently have 235/85-16 tires (31.7"), and you want to be running 75 mph at the same engine rpm you currently run at 65 mph (75/65 = 1.15) you need to go up to a 31.7 x 1.15 = 36.6" tire.
Alternatively you could do the same thing with gear ratios. 4.1 / 1.15 = 3.55, so swapping to 3.55 gears would get you the same thing as going to a 36.6" tire. You say a gear swap is beyond you, but you can have a shop do it (~$1000 including parts), which could also be an opportunity to upgrade to a traction-aiding differential if you want one. Or find a 10.25" with 3.55 gears in a junk yard and swap the axle.
Alternatively you could do the same thing with gear ratios. 4.1 / 1.15 = 3.55, so swapping to 3.55 gears would get you the same thing as going to a 36.6" tire. You say a gear swap is beyond you, but you can have a shop do it (~$1000 including parts), which could also be an opportunity to upgrade to a traction-aiding differential if you want one. Or find a 10.25" with 3.55 gears in a junk yard and swap the axle.
Thread Starter
|
Freshman User
Joined: Feb 2011
Posts: 44
Likes: 0
Thank you. Your post was most helpful. I think I’ll look for a “cheaper” fix by attempting to find a 10.25 axel in 3.55 ratio at a junk yard. It could save me a bunch although new ring and pinion in 3.55 plus a limited slip might be the more prudent direction.
Trending Topics
FTE Chapter Leader
Joined: May 2006
Posts: 12,342
Likes: 1,318
From: Central Mississippi
OK, unless I miss the point entirely the problem is you want to go faster than 60 in a cut down truck with a wood dashboard. 
I'm cool wid that.
I think there is another problem other than the 4.10 keeping you from speed....
But any way you look at it, you can put the biggest rim that will bolt on with the tallest you can find tire you can find. It's a rat rod.
Fire up the search engine & whip out the wallet.
(Edit... multiple responses above just as valid, good luck & more pics please)
I'm cool wid that.
I think there is another problem other than the 4.10 keeping you from speed....
But any way you look at it, you can put the biggest rim that will bolt on with the tallest you can find tire you can find. It's a rat rod.
Fire up the search engine & whip out the wallet.
(Edit... multiple responses above just as valid, good luck & more pics please)
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
3 Best / 3 Worst Parts of Modern Ford Ownership
Brett Foote
10 Amazing Upgrades That Solve Common Ford Truck Owner Headaches
Pouria Savadkouei
Every 2026 Ford Engine Explained
Brett Foote
10 Ugly Ford Trucks That We Still Kinda Love
Joe Kucinski
10 Things Every Truck Owner NEEDS (2026 Edition)
Michael S. Palmer
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Verdad Gallardo
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer
Joe Kucinski
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Top 10 Ford Truck Tragedies
Joe Kucinski
Cargo Master
Joined: Mar 2010
Posts: 3,340
Likes: 293
From: Laramie, WY
I think this is cleared up except the apparent confusion on what tire and wheel sizing is on the OP's part.
Do you have a tachometer? What is the engine speed at 65mph? A 4 speed manual in that truck is 1:1 in top gear, couple that with 4.10 gears and a 6.9idi (no turbo kit added I assume?) and top speed will not be very high. You can readily calculate rpm/speed for different tire sizes. There's a plethora of online calculators. How fast do you want to go?
Do you have a tachometer? What is the engine speed at 65mph? A 4 speed manual in that truck is 1:1 in top gear, couple that with 4.10 gears and a 6.9idi (no turbo kit added I assume?) and top speed will not be very high. You can readily calculate rpm/speed for different tire sizes. There's a plethora of online calculators. How fast do you want to go?
Lead Driver
Joined: Feb 2013
Posts: 5,006
Likes: 72
From: Roseville, MN
16" WHEELS were stock. The tire size is MUCH bigger. On a 4WD at least the stock tire size woud be 235/85-16 which is 31.7" (I'm not sure what size tires were stock on 2WD, but they'd be close)
Good question. In the first post the OP said a 4 speed. GoinBoarding and I were assuming a 4 speed manual in an '86, which would have a 1:1 top gear and 4.10 axle gears might be limiting the top speed with a naturally aspirated IDI.
But I'm not sure what year the 4 speed automatics came in. If this is an automatic, then it has overdrive and the axle gears aren't really limiting the top speed. Not to say that the OP might not want taller gears, even with an overdrive (I know I do). But if it won't go over 65 mph with an overdrive trans and 4.10 gears, then something else is wrong.
But I'm not sure what year the 4 speed automatics came in. If this is an automatic, then it has overdrive and the axle gears aren't really limiting the top speed. Not to say that the OP might not want taller gears, even with an overdrive (I know I do). But if it won't go over 65 mph with an overdrive trans and 4.10 gears, then something else is wrong.
Super Moderator
Joined: Apr 2004
Posts: 34,512
Likes: 2,824
With 31.7" tires, 1:1 hi, 4.10 axles you should run 2607 rpm at 60 MPH and 76 MPH at 3300 RPM, which is the max HP rpm for that motor. Some stuff I found says that is the governed max rpm as well. You shouldn't be done at 60 or even 65 but it may sound that way.
35 inch tires will give you 60 at 2361, and at 3300 you will be at an eye-ball splitting 84 mph.
Thread
Thread Starter
Forum
Replies
Last Post
bucks-heritage
Brakes, Steering, Suspension, Tires, & Wheels
4
Jan 24, 2005 11:35 PM