35s?
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Freshman User
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35s?
I am thinking to perhaps replace the 33-12.5x15s on my 78 F150 4X4. I am getting about 10 mpg now and believe the swap might actually increase the mpg since I an running 4.11 gears. The 351M 400 has a mild cam, headers, Ebelbrock/Holly so I think it can pull the increased diameters with no issues.
Thoughts?
SS
Thoughts?
SS
Laughing Gas
Joined: Aug 2000
Posts: 1,008
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From: Eastern, Idaho
At best I can get 10 mpg but typically 8-9 mpg. That is with 35" tires, 3.50 gears, 460 and automatic. My BFG AT 35x12.5x15s are actually 33.5 diameter with rear truck weight on tires. I just measured and with the weight on my front tires it is 16" from the ground to the center of the wheel so actually a 32.
Keep in mind if your speedometer is off so is the odometer and you mileage calculation is affected. My speedo is off 5% so I add 5% of the mile on a tank when calculating. I cannot change my speedo gear any further without change ring and pinions.
If you do a lot of highway driving you would probably be better off especially with 4.10 gearing but the bigger tires will weigh a bit more depending on brand. You will need a 4" lift to clear 35s on an f150. If you keep highway speeds low (like 55 mph) you will maximize your highway mpg.
You have either a 351m or a 400 not both.
Keep in mind if your speedometer is off so is the odometer and you mileage calculation is affected. My speedo is off 5% so I add 5% of the mile on a tank when calculating. I cannot change my speedo gear any further without change ring and pinions.
If you do a lot of highway driving you would probably be better off especially with 4.10 gearing but the bigger tires will weigh a bit more depending on brand. You will need a 4" lift to clear 35s on an f150. If you keep highway speeds low (like 55 mph) you will maximize your highway mpg.
You have either a 351m or a 400 not both.
Lead Driver
Joined: Nov 2006
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From: Pa.
Is the motor a 351M or a 400 ? If the intake is a edelbrock performer I assume the egr plate and valve are removed .. If so has the distributor been recurved to compensate ? and has a non emissions timing set been added?
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I believe the proper designation for the motor is 351M 400.
That aside. the truck has more than adeqaute clearance as far as I can tell. The center fo the front axle is 22 inches from the arch of the fender and in the rea is is 23 inches.
I have not thought to check the calibration of the speedometer though.
Ss
That aside. the truck has more than adeqaute clearance as far as I can tell. The center fo the front axle is 22 inches from the arch of the fender and in the rea is is 23 inches.
I have not thought to check the calibration of the speedometer though.
Ss
Lead Driver
Joined: Nov 2006
Posts: 5,217
Likes: 506
From: Pa.
I believe the proper designation for the motor is 351M 400.
That aside. the truck has more than adeqaute clearance as far as I can tell. The center fo the front axle is 22 inches from the arch of the fender and in the rea is is 23 inches.
I have not thought to check the calibration of the speedometer though.
Ss
That aside. the truck has more than adeqaute clearance as far as I can tell. The center fo the front axle is 22 inches from the arch of the fender and in the rea is is 23 inches.
I have not thought to check the calibration of the speedometer though.
Ss
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Freshman User
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True it is either one or the other but I have never seen anyone refer to the 400 ci version of the 351M as a 351. 351M/400 Performance Build Up
Regardless, no egr valve/plate. It does have an electronic igntion that was installed at some time during it's life.
SS
Regardless, no egr valve/plate. It does have an electronic igntion that was installed at some time during it's life.
SS
Posting Guru
Joined: Aug 2010
Posts: 1,761
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From: Knoxville, TN
The reason you often see 351M/400 listed together is that the two engines are closely related (share the same block, heads, intake, etc.); but an individual engine is either one or the other - the crankshaft, rods, and pistons are different, and the 351M has a 351 ci displacement, while the 400 has a 400 ci displacement. 
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Laughing Gas
Joined: Aug 2000
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From: Eastern, Idaho
Whether you have a 351m or 400 doesn't matter much in regards to power to turn 35s. In stock form the 400 only gives roughly 10-15 ft-lbs and hp over the 351m. Source... M-Block 351M/400 Specifications
However it is confusing when people say 351m/400. Its like me saying I have a 429/460.
22" indicates you most likely have no lift. I will tell you from experience that 33"x12.5x15 with no lift will sometimes rub at the front tires when turning depending on wheel width and offset or if your fenders are out of adjustment. So that leads one to believe that 35s will not fit without a lift or trimming fenders.
However it is confusing when people say 351m/400. Its like me saying I have a 429/460.
22" indicates you most likely have no lift. I will tell you from experience that 33"x12.5x15 with no lift will sometimes rub at the front tires when turning depending on wheel width and offset or if your fenders are out of adjustment. So that leads one to believe that 35s will not fit without a lift or trimming fenders.
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Lead Driver
Joined: Nov 2006
Posts: 5,217
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From: Pa.
Whether you have a 351m or 400 doesn't matter much in regards to power to turn 35s. In stock form the 400 only gives roughly 10-15 ft-lbs and hp over the 351m. Source... M-Block 351M/400 Specifications
However it is confusing when people say 351m/400. Its like me saying I have a 429/460.
22" indicates you most likely have no lift. I will tell you from experience that 33"x12.5x15 with no lift will sometimes rub at the front tires when turning depending on wheel width and offset or if your fenders are out of adjustment. So that leads one to believe that 35s will not fit without a lift or trimming fenders.
However it is confusing when people say 351m/400. Its like me saying I have a 429/460.
22" indicates you most likely have no lift. I will tell you from experience that 33"x12.5x15 with no lift will sometimes rub at the front tires when turning depending on wheel width and offset or if your fenders are out of adjustment. So that leads one to believe that 35s will not fit without a lift or trimming fenders.
.. In other words they might as well just save their money
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Yes it has as far as I can tell no additonal lift but there is significant clearance in the fender wells. Not sure but I think the suspension height is the same as the so-called "High-boys". The shop here that built the motor (i have all the records from the original owner) said it is a performance motor. Not sure exactly what that means but I was told not to expect good mileage.
SS
SS
Lead Driver
Joined: Nov 2006
Posts: 5,217
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From: Pa.
Yes it has as far as I can tell no additonal lift but there is significant clearance in the fender wells. Not sure but I think the suspension height is the same as the so-called "High-boys". The shop here that built the motor (i have all the records from the original owner) said it is a performance motor. Not sure exactly what that means but I was told not to expect good mileage.
SS
SS
To check whether the motor is a 351M or a 400 locate the #1 spark plug and take it out then bring the motor to TDC (you may need to wire brush the surface clean to see the marks) on the harmonic balancer with a 1/2" breaker bar and a 15/16" socket rotating clockwise .. Now check the circumference of the harmonic balancer (HB) with a piece of string .. It should be 20.5" if not 1/2 what ever you get .. I use a 1" wide piece of white paper the length of 1/2 the circumference of the harmonic balancer .. You place one end of what ever you choose to use on TDC of the HB and wrap it around the HB and mark the HB with a sharpie .. Now with the wood dowel in the #1 cylinder and marked rotate the crank clockwise until you reach the sharpie mark you put on the HB .. Now mark your wood dowel again .. Now take and measure the distance between your two marks on the dowel .. 3.5"=351M .. 4'0 =400 .. Obviously having a helper is the way to go .. This is the only way I know to check the displacement on a 351M/400 motor ..
Fleet Mechanic
Joined: Jan 2012
Posts: 1,668
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From: Saskatchewan
If you already have 4:10s I say go for it. Also if they rub a little that's a good excuse to buy a lift kit. If your gonna buy a lift you might as well get a big kit like 6" or more. Boom now the tires are going to seem to small so you will need some 37"a at least, now you've maxed out your 1/2 ton axles so you might as well upgrade to 1 tons. If your going to do that you might as well put some 44"s on with full hydraulic steering. Now you've completely run out of power so it's time to start building a big block. Haha. Welcome to my brain. Sorry for rambling.
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