Tire Height
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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.
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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.
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Joined: Feb 2013
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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.
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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.
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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)
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Joined: Mar 2010
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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?
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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.
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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.
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