How fast am I going??
Thread Starter
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Junior User
Joined: Oct 2009
Posts: 60
Likes: 0
From: Southern MD
How fast am I going??
I am driving a 79 F250SC fromTampa to Maryland tomorrow. This is going to be a long drive as it has a 6" lift with 37" tires. I am pretty sure that the speedometer has not been calibrated after the 37's were put on so my question is: is there an equation to figure how fast I am going at a certain RPM? In other words if I run 3000 RPM I will be traveling at 65 MPH??
It has a 4.10 gear set with 37" tires. Oh and a 460
Mark
It has a 4.10 gear set with 37" tires. Oh and a 460
Mark
Cargo Master
Joined: Jul 2005
Posts: 2,311
Likes: 367
From: Kansas
you need a calculator and fairly accurate way to measure seconds (stopwatch, etc).
Start the stopwatch at a milemarker, drive a constant speed, stop the stopwatch at the next milemarker. The stop watch will read the number of seconds to go 1 mile.
with the calculator, divide 1 by the number of seconds then multiply by 3600 to get MPH.
Here is a quick reference chart that will save you doing the calculation...you will just need to measure seconds to go one mile...
Seconds MPH
40 90.00
41 87.80
42 85.71
43 83.72
44 81.82
45 80.00
46 78.26
47 76.60
48 75.00
49 73.47
50 72.00
51 70.59
52 69.23
53 67.92
54 66.67
55 65.45
56 64.29
57 63.16
58 62.07
59 61.02
60 60.00
61 59.02
62 58.06
63 57.14
64 56.25
65 55.38
66 54.55
67 53.73
68 52.94
69 52.17
70 51.43
Start the stopwatch at a milemarker, drive a constant speed, stop the stopwatch at the next milemarker. The stop watch will read the number of seconds to go 1 mile.
with the calculator, divide 1 by the number of seconds then multiply by 3600 to get MPH.
Here is a quick reference chart that will save you doing the calculation...you will just need to measure seconds to go one mile...
Seconds MPH
40 90.00
41 87.80
42 85.71
43 83.72
44 81.82
45 80.00
46 78.26
47 76.60
48 75.00
49 73.47
50 72.00
51 70.59
52 69.23
53 67.92
54 66.67
55 65.45
56 64.29
57 63.16
58 62.07
59 61.02
60 60.00
61 59.02
62 58.06
63 57.14
64 56.25
65 55.38
66 54.55
67 53.73
68 52.94
69 52.17
70 51.43
Postmaster
Joined: Dec 2008
Posts: 3,940
Likes: 8
From: Southern Oregon Coast
If you have mile markers on your route, use those to calculate your speed - just figure that, at 60mph, you're traveling 1 mile every minute - speed=distance/time.
Heh, looks like dlburch beat me to it...
Heh, looks like dlburch beat me to it...
Elder User
Joined: Jul 2005
Posts: 525
Likes: 1
do you have a gps? I have a hand held unit I use for hiking and you can map and mark yourself so when you want to go back it will tell you if you wander off coarse. It will also tell you how fast you are moving. I never know that but I have a teen aged son that showed me that in the car going down the highway. Got to love the kids as they know all of the electronic gismos. Hell, I still cant text on my phone.
Elder User
Joined: Aug 2004
Posts: 796
Likes: 1
Many years ago I made a spreadsheet to tell me just that. Here you go
The first and second gear speeds are calculated for the gear ratio's in my ford 3-spd, so if your trans is different, so too will the speeds. 3rd gear though is 1:1 and should be correct, unless of course you have overdrive. If you do, it will be wrong but can easily be fixed with the proper ratio.
The first and second gear speeds are calculated for the gear ratio's in my ford 3-spd, so if your trans is different, so too will the speeds. 3rd gear though is 1:1 and should be correct, unless of course you have overdrive. If you do, it will be wrong but can easily be fixed with the proper ratio.
Thread Starter
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Junior User
Joined: Oct 2009
Posts: 60
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From: Southern MD
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Elder User
Joined: Aug 2004
Posts: 796
Likes: 1
I made the whole spreadsheet years ago. Lets me figure out top speeds in any given gear, the impact of different tires / gears, etc. Great for racing.
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