Mileage, ? Figuring out 77
Thread Starter
|
Lead Driver
Joined: Jun 2006
Posts: 5,663
Likes: 985
From: Near Yosemite CA
Mileage, ? Figuring out 77
Ok, my 77 F-150 4x, NP435, 9" w/3.50, I originally had 31's on it, from what I can tell, factory tires were about 31's. I went with 255/85/16's which measure out to a 33" tire (tire calc says 32.5, tape measure says 33), @ 2000 RPM speedo says 50, about 2200rpm = 55
So I used one of the calculators, I think it was off about 5mph, so when I figure the mileage.... If I went 300 miles, would I add 5 miles for every 50 driven, so I would actually have gone 330 miles? and use that for my math?!
So I used one of the calculators, I think it was off about 5mph, so when I figure the mileage.... If I went 300 miles, would I add 5 miles for every 50 driven, so I would actually have gone 330 miles? and use that for my math?!
Senior User
Joined: Feb 2002
Posts: 378
Likes: 0
From: USA
Sorry I am not good enough w/math to help you. After putting taller tires on mine, the mileage came out to about 7mpg. I decided I no longer wanted to know. I just put gas in it. You have a granny gear, big tires & an M motor, you prolly don't want to know either. If you are worried about distance from here to there, pick up a cheap GPS. Brian
Laughing Gas
Joined: Jul 2002
Posts: 843
Likes: 1
From: USA
Origional factory tires on your '77 were L78-15's, about 29" tall. Comparable tire today is a 235/75R-15. Your math sounds about right, however you're not going to like the answer. These trucks don't get good milage in their origional configuration. With your 33's you will get about 8 or 9 mpg and that's about it. I know it looks funky but for better milage you need to go with the stokc size tire. I am aware that by increasing your tire diameter you effectively lower the number of your gear ratio but the cost in energy to turn the bigger tires is higher then the gain.
Thread Starter
|
Lead Driver
Joined: Jun 2006
Posts: 5,663
Likes: 985
From: Near Yosemite CA
Thanks Guys, when I was figuring it, it came out to about 9.5 - 10, so I am guessing I am close... I think at some point in time I will be building another 300 I6 and put that in. I get 14-16 out of the one in my 66. Or I might stroke it to a 400, it is already punched .040. And the good thing is that it is a NON_Catalyst, no air pump or cat converter. I didn't realize the stock tires were so small!! I might regear then
Posting Legend
Joined: Sep 2001
Posts: 30,255
Likes: 37
Check the Read First: FAQ thread at the top of the 73-9 forum for speedometer calibration info.
Trending Topics
New User
Joined: Jan 2008
Posts: 0
Likes: 24
A tire with a larger diameter will a greater diatance in revolution than a lessor diameter tire .
so a 29 " tall tire will turn more revolutions in a set distance than a 33" tall tire.
So given that info and the gearing being the same..a taller tire will register a lower actual distance than a shorter tire.
If the 33" tire measured a distance of 2 miles...the shorter tire (29") will register a distance of (for example only) 2.5 miles but the tires actually went the same actual distance.
This is where the calibration figures come in so they will both read the same distance with alterations.
Therefore to answer your question.
With 31" tires you actually registered less distance than the 29" tires.
The shorter tire makes the axles and transmission turn faster and increase mileage indication.
The fuel mileage would read better for the taller tire than the shorter one...... therefore in your case... you decreased the distance travelled and decreased the mileage to fuel calculation making it appear worse than it actually was.
so a 29 " tall tire will turn more revolutions in a set distance than a 33" tall tire.
So given that info and the gearing being the same..a taller tire will register a lower actual distance than a shorter tire.
If the 33" tire measured a distance of 2 miles...the shorter tire (29") will register a distance of (for example only) 2.5 miles but the tires actually went the same actual distance.
This is where the calibration figures come in so they will both read the same distance with alterations.
Therefore to answer your question.
With 31" tires you actually registered less distance than the 29" tires.
The shorter tire makes the axles and transmission turn faster and increase mileage indication.
The fuel mileage would read better for the taller tire than the shorter one...... therefore in your case... you decreased the distance travelled and decreased the mileage to fuel calculation making it appear worse than it actually was.
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
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
AEV FXL Super Duty - the Super Duty Raptor Ford Doesn't Make
Brett Foote
Lobo Vs Lobo: Proof the F-150 Lobo Should Be Even Lower!
Michael S. Palmer
Ford's 2001 Explorer Sportsman Concept Looks For a New Home
Verdad Gallardo
10 Best Ford Truck Engines We Miss the Most!
Joe Kucinski
2026 Shelby F-150 Off-Road: Better Than a Raptor R?
Brett Foote
2027 Super Duty Carhartt Package First Look: 12 Things You NEED to Know!
Michael S. Palmer
Posting Legend
Joined: Sep 2001
Posts: 30,255
Likes: 37
There are a ton of automotive calculators here:
https://www.ford-trucks.com/calculators/index.php
https://www.ford-trucks.com/calculators/index.php
Senior User
Joined: Jul 2004
Posts: 216
Likes: 0
From: North Branch, MI
Take a look at this web page http://www.offroaders.com/info/tech-...ng/mph-rpm.htm
It gives formulas to figure out correct speed based on tire size, rearend ration etc.
It gives formulas to figure out correct speed based on tire size, rearend ration etc.
