I need help with math
Thread Starter
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Fleet Mechanic
Joined: Jun 2019
Posts: 1,898
Likes: 234
From: Southern California
I need help with math
Hey guys, I'm driving a 65 F100 with the 352. I have changed the transmission, rear gears, and tires to the point where my speedometer is off by ten MPH @ 80.
Therefore my odometer is also off which makes it difficult for me to get a realistic MPG figure. I know it's not much more than 10 MPG if at all because of the nature of the beast, and the way I drive it. The only reason I even care about mileage is that I am switching transmissions, again, but I'm getting overdrive! And I am interested in any changes in fuel usage. Partly to justify the purchase price of a new 5-speed overdrive tranny, and partly because everyone will ask if I get better gas mileage.
So, does anyone know how to factor in the inaccuracy into the MPG equation?
Therefore my odometer is also off which makes it difficult for me to get a realistic MPG figure. I know it's not much more than 10 MPG if at all because of the nature of the beast, and the way I drive it. The only reason I even care about mileage is that I am switching transmissions, again, but I'm getting overdrive! And I am interested in any changes in fuel usage. Partly to justify the purchase price of a new 5-speed overdrive tranny, and partly because everyone will ask if I get better gas mileage.
So, does anyone know how to factor in the inaccuracy into the MPG equation?
Thread Starter
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Fleet Mechanic
Joined: Jun 2019
Posts: 1,898
Likes: 234
From: Southern California
Just push the button on the Roku and the remote will make noise.
Cargo Master
Joined: Jul 2015
Posts: 2,063
Likes: 137
From: tucson
Find some roads with the radar speed signs and compare what they display against your speedometer. You could also follow a friend and have them drive at different speeds, in they're own car, and tell you when they reach different speeds. Both of my trucks are off. At about 40mph, one reads close to 5mph fast and the other reads 5mph slow. I just have to remember which truck I'm driving.
Tuned
Joined: Nov 2013
Posts: 424
Likes: 16
I don’t understand the question, basically you fill the tank check your mileage, run it however you wish till your at a half tank, fill the tank again and the number of gallons divided into the number of miles traveled will give you mpg. Do it twice if you wish just to get a rough estimate.....
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Mountain Pass
Joined: Nov 2004
Posts: 242
Likes: 54
From: Central Texas
Get a friend to follow you and cross check your speed with theirs, based on that you can figure out first your actual speed and secondly your mpg's.
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Tuned
Joined: May 2008
Posts: 252
Likes: 48
From: Somerset, OH
I don’t understand the question, basically you fill the tank check your mileage, run it however you wish till your at a half tank, fill the tank again and the number of gallons divided into the number of miles traveled will give you mpg. Do it twice if you wish just to get a rough estimate.....
Mountain Pass
Joined: Jul 2017
Posts: 169
Likes: 5
From: Ahwatukee
Download a speedometer GPS app to your phone, there should still be some free ones. Then compare your actual speed as read by the app at several points,
say 30, 40, 50, 60 and 70, to make sure the error is consistent (as a percent). If it is, then the math is easy. If your speedometer is reading
12% over, just multiply your miles times 0.88%
So if your odometer claims you went 200 miles on 16 gallons: 200 X .88 = 176 actual miles. 176 / 16= 11 mpg.
say 30, 40, 50, 60 and 70, to make sure the error is consistent (as a percent). If it is, then the math is easy. If your speedometer is reading
12% over, just multiply your miles times 0.88%
So if your odometer claims you went 200 miles on 16 gallons: 200 X .88 = 176 actual miles. 176 / 16= 11 mpg.
Laughing Gas
Joined: Nov 2001
Posts: 1,062
Likes: 78
From: Edmonton, Alberta
MPH = (TD x RPM)/(336 x GR)
MPH = Miles per Hour
TD=Tire Diameter (in inches)
GR=Gear Ratio
**For MPH in Overdrive, take MPH and divide by OD ratio
eg: 60 MPH, OD ratio of 0.68
OD Corrected MPH =88.23
I would just go ahead and install the 5 speed, then get it all sorted afterwards, no point messing with speedo gears now, only to have to do it again after the swap.
**Probably multiple apps out there that will use your phone as a Speedometer/Odometer
MPH = Miles per Hour
TD=Tire Diameter (in inches)
GR=Gear Ratio
**For MPH in Overdrive, take MPH and divide by OD ratio
eg: 60 MPH, OD ratio of 0.68
OD Corrected MPH =88.23
I would just go ahead and install the 5 speed, then get it all sorted afterwards, no point messing with speedo gears now, only to have to do it again after the swap.
**Probably multiple apps out there that will use your phone as a Speedometer/Odometer
Thread Starter
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Fleet Mechanic
Joined: Jun 2019
Posts: 1,898
Likes: 234
From: Southern California
To those asking, when I am doing 80, the speedo reads 70. At 40 it reads 35 or so. checked against those radars they put up by the schools.
Thanks guys, I'm okay at math, but figuring out what equation to use, Might as well be speaking greek.
Dan, TK65