High RPMs @ low speed
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Joined: Nov 2012
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High RPMs @ low speed
Good evening ladies and gentlemen,
I own a 1985 f250, my first truck and i love everything about it,well almost. I was curious to see what type of gears i have because when i am on the highway and i'm going 55, i'm turning 3000 rpms. I have a 4 speed w/granny but i found this still very odd because it is only 55. I was wondering if this is common or there is something wrong. I was looking into re-gearing so i could at least go 65. I do a lot of work at the shore and it takes me a year to get there going 55
. Its a 351 4x4, not sure if that makes a difference.
Thanks a lot
Alex
I own a 1985 f250, my first truck and i love everything about it,well almost. I was curious to see what type of gears i have because when i am on the highway and i'm going 55, i'm turning 3000 rpms. I have a 4 speed w/granny but i found this still very odd because it is only 55. I was wondering if this is common or there is something wrong. I was looking into re-gearing so i could at least go 65. I do a lot of work at the shore and it takes me a year to get there going 55
. Its a 351 4x4, not sure if that makes a difference. Thanks a lot
Alex
Moderator
Joined: Nov 2007
Posts: 22,415
Likes: 92
From: Denver Metro Area, CO
This will happen if you have a 4.10 gear ratio.
Which means you have the torque and power to pull a house down, but highway gas mileage will bite.
What is your AXLE code on the door-jamb sticker?
Which means you have the torque and power to pull a house down, but highway gas mileage will bite.
What is your AXLE code on the door-jamb sticker?
Posting Guru
Joined: Apr 2011
Posts: 1,636
Likes: 5
From: Orlando area
Good evening ladies and gentlemen,
I own a 1985 f250, my first truck and i love everything about it,well almost. I was curious to see what type of gears i have because when i am on the highway and i'm going 55, i'm turning 3000 rpms. I have a 4 speed w/granny but i found this still very odd because it is only 55. I was wondering if this is common or there is something wrong. I was looking into re-gearing so i could at least go 65. I do a lot of work at the shore and it takes me a year to get there going 55. Its a 351 4x4, not sure if that makes a difference.
Thanks a lot
Alex
I own a 1985 f250, my first truck and i love everything about it,well almost. I was curious to see what type of gears i have because when i am on the highway and i'm going 55, i'm turning 3000 rpms. I have a 4 speed w/granny but i found this still very odd because it is only 55. I was wondering if this is common or there is something wrong. I was looking into re-gearing so i could at least go 65. I do a lot of work at the shore and it takes me a year to get there going 55
. Its a 351 4x4, not sure if that makes a difference. Thanks a lot
Alex
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i checked the axle code and it says 22. i have 245/75/16 tires are what i just put on it. Just got done with putting on a new fuel pump, now the male end of the fuel carb line is stripped so i got to pick up a new one of those tomorrow.
the adventure continues
the adventure continues
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so i have the lowest possible gear ratio they made that year, would large tires actually make that much of a difference or would it be better just to deal with it and use the money for gas. The stock tires on my truck are brandy new
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Post Fiend
Joined: Jan 2000
Posts: 5,330
Likes: 17
From: Dryden, ON, Canada
245's are not stock to that truck, it would have came with 235/85's, which are around the 30.5" mark in height, around 680 revs mile, with some variation with manufacturer and tread pattern. The stock 235/85's are a 31.5 more or less, and are closer to the 660 revs/mile mark.
However, there is no good reason for your truck to be spinning 3000 rpm at 55 if you have 4.10's, with the 245's it should be at just under 2500, with the proper 235's it should be right about 2400. A step up to a 33(285/75) and it drops to just under 2300, step up to a 35(315) and you are down to 2160.
Your numbers do point toward a highly unlikley ratio in the 5.1 range.
My vote goes more towards a wonky speedometer or an optimistic tach.
However, there is no good reason for your truck to be spinning 3000 rpm at 55 if you have 4.10's, with the 245's it should be at just under 2500, with the proper 235's it should be right about 2400. A step up to a 33(285/75) and it drops to just under 2300, step up to a 35(315) and you are down to 2160.
Your numbers do point toward a highly unlikley ratio in the 5.1 range.
My vote goes more towards a wonky speedometer or an optimistic tach.
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Cargo Master
Joined: Apr 2003
Posts: 2,408
Likes: 5
From: Parkesburg PA
My 3.55 geared F250 is at ~2500 @ 65mph IIRC.
I dusted off my old excel sheet and feed in your information...
With 245/75-16 tires and 4.10 gears you should be about:
2,500 rpm @ 55mph
3,000 rpm @ 65mph
3,400 rpm @ 75mph
A change to a 285/75-16 tire gives you about:
2,300 rpm @ 55mph
2,700 rpm @ 65mph
3,100 rpm @ 75mph
So a change in tires will give you 200 less rpms. Probably not going to do much for fuel savings. I plan on running 305/75's on my 86 F250 to replace the worn out 285's.
305/75's
2,200 rpm @ 55mph
2,600 rpm @ 65mph
3,000 rpm @ 75mph
A switch to 3.55 gears nets:
245's
2,200 rpm @ 55mph
2,600 rpm @ 65mph
3,000 rpm @ 75mph
285's
2,000 rpm @ 55mph
2,400 rpm @ 65mph
2,700 rpm @ 75mph
305's
1,900 rpm @ 55mph
2,300 rpm @ 65mph
2,600 rpm @ 75mph
And lastly a Zf Swap with it's approximate 25% overdrive with your 4.10 gears gets you...
245's
1,900 rpm @ 55mph
2,200 rpm @ 65mph
2,600 rpm @ 75mph
285's
1,800 rpm @ 55mph
2,100 rpm @ 65mph
2,400 rpm @ 75mph
305's
1,700 rpm @ 55mph
2,000 rpm @ 65mph
2,300 rpm @ 75mph
I dusted off my old excel sheet and feed in your information...
With 245/75-16 tires and 4.10 gears you should be about:
2,500 rpm @ 55mph
3,000 rpm @ 65mph
3,400 rpm @ 75mph
A change to a 285/75-16 tire gives you about:
2,300 rpm @ 55mph
2,700 rpm @ 65mph
3,100 rpm @ 75mph
So a change in tires will give you 200 less rpms. Probably not going to do much for fuel savings. I plan on running 305/75's on my 86 F250 to replace the worn out 285's.
305/75's
2,200 rpm @ 55mph
2,600 rpm @ 65mph
3,000 rpm @ 75mph
A switch to 3.55 gears nets:
245's
2,200 rpm @ 55mph
2,600 rpm @ 65mph
3,000 rpm @ 75mph
285's
2,000 rpm @ 55mph
2,400 rpm @ 65mph
2,700 rpm @ 75mph
305's
1,900 rpm @ 55mph
2,300 rpm @ 65mph
2,600 rpm @ 75mph
And lastly a Zf Swap with it's approximate 25% overdrive with your 4.10 gears gets you...
245's
1,900 rpm @ 55mph
2,200 rpm @ 65mph
2,600 rpm @ 75mph
285's
1,800 rpm @ 55mph
2,100 rpm @ 65mph
2,400 rpm @ 75mph
305's
1,700 rpm @ 55mph
2,000 rpm @ 65mph
2,300 rpm @ 75mph
pedant
Joined: Jan 2008
Posts: 23,576
Likes: 39
From: EXTREME southwest CT
Anyway,
You have new tires.
And 4X4, which means you would need to change the ratios in both axles.
My truck with 4.10's and stock 235/85 tires would indicate 3k @ 60-65.
Kind of inline with what you are saying.
I got a ZF5 gearbox from a fellow CT chapter member for $100. (transfer case, crossmember, driveshafts, inspection cover and all!)
This is about a 25% overdrive and made a world of difference on the highway.
You have new tires.
And 4X4, which means you would need to change the ratios in both axles.
My truck with 4.10's and stock 235/85 tires would indicate 3k @ 60-65.
Kind of inline with what you are saying.
I got a ZF5 gearbox from a fellow CT chapter member for $100. (transfer case, crossmember, driveshafts, inspection cover and all!)
This is about a 25% overdrive and made a world of difference on the highway.
Thread Starter
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New User
Joined: Nov 2012
Posts: 24
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My 3.55 geared F250 is at ~2500 @ 65mph IIRC.
I dusted off my old excel sheet and feed in your information...
With 245/75-16 tires and 4.10 gears you should be about:
2,500 rpm @ 55mph
3,000 rpm @ 65mph
3,400 rpm @ 75mph
A change to a 285/75-16 tire gives you about:
2,300 rpm @ 55mph
2,700 rpm @ 65mph
3,100 rpm @ 75mph
So a change in tires will give you 200 less rpms. Probably not going to do much for fuel savings. I plan on running 305/75's on my 86 F250 to replace the worn out 285's.
305/75's
2,200 rpm @ 55mph
2,600 rpm @ 65mph
3,000 rpm @ 75mph
A switch to 3.55 gears nets:
245's
2,200 rpm @ 55mph
2,600 rpm @ 65mph
3,000 rpm @ 75mph
285's
2,000 rpm @ 55mph
2,400 rpm @ 65mph
2,700 rpm @ 75mph
305's
1,900 rpm @ 55mph
2,300 rpm @ 65mph
2,600 rpm @ 75mph
And lastly a Zf Swap with it's approximate 25% overdrive with your 4.10 gears gets you...
245's
1,900 rpm @ 55mph
2,200 rpm @ 65mph
2,600 rpm @ 75mph
285's
1,800 rpm @ 55mph
2,100 rpm @ 65mph
2,400 rpm @ 75mph
305's
1,700 rpm @ 55mph
2,000 rpm @ 65mph
2,300 rpm @ 75mph
I dusted off my old excel sheet and feed in your information...
With 245/75-16 tires and 4.10 gears you should be about:
2,500 rpm @ 55mph
3,000 rpm @ 65mph
3,400 rpm @ 75mph
A change to a 285/75-16 tire gives you about:
2,300 rpm @ 55mph
2,700 rpm @ 65mph
3,100 rpm @ 75mph
So a change in tires will give you 200 less rpms. Probably not going to do much for fuel savings. I plan on running 305/75's on my 86 F250 to replace the worn out 285's.
305/75's
2,200 rpm @ 55mph
2,600 rpm @ 65mph
3,000 rpm @ 75mph
A switch to 3.55 gears nets:
245's
2,200 rpm @ 55mph
2,600 rpm @ 65mph
3,000 rpm @ 75mph
285's
2,000 rpm @ 55mph
2,400 rpm @ 65mph
2,700 rpm @ 75mph
305's
1,900 rpm @ 55mph
2,300 rpm @ 65mph
2,600 rpm @ 75mph
And lastly a Zf Swap with it's approximate 25% overdrive with your 4.10 gears gets you...
245's
1,900 rpm @ 55mph
2,200 rpm @ 65mph
2,600 rpm @ 75mph
285's
1,800 rpm @ 55mph
2,100 rpm @ 65mph
2,400 rpm @ 75mph
305's
1,700 rpm @ 55mph
2,000 rpm @ 65mph
2,300 rpm @ 75mph
also thank you very much for the chart
Cargo Master
Joined: Apr 2003
Posts: 2,408
Likes: 5
From: Parkesburg PA
Peak power is at around 4k IIRC. 5k-5,500 would probably be a safe Redline.