Tach / RPM help
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Tach / RPM help
Greetings,
I just installed a tachometer in my 1966 F100 and was surprised by the results. I have a 352 3spd on the tree with an 3.54 (A8) rear end. I have 225 70/15 on the front and 255 70/15 on the back. Is there a calculator that can tell me how far off my speedo is from the actual speed?
The truck sounds like it is working really hard at 3000. What is the typical RPM operating range for similar trucks? I would hate to take this truck to 4400 which is listed at max power!
It sounds like most of my time will be spent between 1000-2000rpm. Am I in the ballpark or is my tach playing tricks on me?
Many thanks.
Couldn't resist adding a pic of the project. Still a long way to go, but it is now back on the road.
I just installed a tachometer in my 1966 F100 and was surprised by the results. I have a 352 3spd on the tree with an 3.54 (A8) rear end. I have 225 70/15 on the front and 255 70/15 on the back. Is there a calculator that can tell me how far off my speedo is from the actual speed?
The truck sounds like it is working really hard at 3000. What is the typical RPM operating range for similar trucks? I would hate to take this truck to 4400 which is listed at max power!
It sounds like most of my time will be spent between 1000-2000rpm. Am I in the ballpark or is my tach playing tricks on me?
Many thanks.
Couldn't resist adding a pic of the project. Still a long way to go, but it is now back on the road.
Fleet Mechanic
Joined: Sep 2016
Posts: 1,459
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From: Southern Maryland
A lot of aftermarket tachs can be switched for either a 6 cyl or 8 (and some even for a 4). If it is switched for a 6, it will be reading high. I have a 3.25 gear in my F150 and it's not too hard to hit 3K on the tach on the freeway, and it doesn't sound like it's racing, just a bit higher than normal (2200 - 2500 is the typical range).
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Thanks 66V8. Right you are, if the tach was set for a 4 or 6 it would be reading very high RPM. That is the opposite of my situation. If I may ask, what RPM are you running at 40 or even 50mph? Maybe I just need to recalibrate my ears and expectations to 50yr old Fe sounds. Thanks for your help.
TheMonson, thanks for the kind words. The paint in the box is POR15. It looks very good at the moment but that will change as the paint is affected by UV rays. I'm going to cover it up with a spray on bedliner (also POR15) this summer.
TheMonson, thanks for the kind words. The paint in the box is POR15. It looks very good at the moment but that will change as the paint is affected by UV rays. I'm going to cover it up with a spray on bedliner (also POR15) this summer.
Fleet Mechanic
Joined: Aug 2016
Posts: 1,480
Likes: 6
From: Great White North
There are several websites that have spots you plug your info into and it spits out your actual speed. I also have posted an old school equation into another thread titled 1965 Mercury M250 Build on page 25. I am on a phone right now and searching and pasting is not easy for me to do. I can do it later tonight from a real computer.
Cargo Master
Joined: Apr 2003
Posts: 2,389
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45mph should be about 1900rpm. 70mph should be about 2950rpm. So 2000 to 3000 rpm is about right for most situations.
Use GPS and you will see how fast you are going. Use that to compare to your speedometer indication.
Use GPS and you will see how fast you are going. Use that to compare to your speedometer indication.
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Went out for a quick run this evening. 50mph (according to my speedo which is bouncing like crazy under 40) is about 2200rpm. Looks like I'm in the ball park.
I'm not a phone guy Charlie, one of the last on the planet without a cell it seems. Great idea re. GPS though. Next time someone is in the passenger seat I'll ask them to put their iPhone to good use.
I'm not a phone guy Charlie, one of the last on the planet without a cell it seems. Great idea re. GPS though. Next time someone is in the passenger seat I'll ask them to put their iPhone to good use.
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Fleet Mechanic
Joined: Aug 2016
Posts: 1,480
Likes: 6
From: Great White North
This wonderful site won't let me cut and paste my own handy work 
Here is the magic formula:
RPM x Tire Diameter
------------------------------------------------------------ = Speed
Axle Ratio x Tranny Ratio (1 for direct drive) x 336.13
To get the actual tire diameter you should park on level ground as in your beauty of a pic , and put a level on the top of the tread on the rear wheel. Next measure from the bottom of the level to the ground. Do not use what it says on the sidewall!!
If you want your speedo inaccuracy divide what you read on the speedo by what you calculated and multiply by 100:
actual
----------- x 100
calculated
I might have the actual and calculated backwards, I am not a math wizard.
You can also try calculating what your RPM is at a predetermined speed such as 45 mph or 70 mph. That way you use your tach as the speedo assuming it is accurate itself (it should be close enough). New equation:
speed x tranny ( use 1) x axle ratio x 336.13
--------------------------------------------------- = RPM
tire diametre
Hope this helps
Here is the magic formula:RPM x Tire Diameter
------------------------------------------------------------ = Speed
Axle Ratio x Tranny Ratio (1 for direct drive) x 336.13
To get the actual tire diameter you should park on level ground as in your beauty of a pic , and put a level on the top of the tread on the rear wheel. Next measure from the bottom of the level to the ground. Do not use what it says on the sidewall!!
If you want your speedo inaccuracy divide what you read on the speedo by what you calculated and multiply by 100:
actual
----------- x 100
calculated
I might have the actual and calculated backwards, I am not a math wizard.
You can also try calculating what your RPM is at a predetermined speed such as 45 mph or 70 mph. That way you use your tach as the speedo assuming it is accurate itself (it should be close enough). New equation:
speed x tranny ( use 1) x axle ratio x 336.13
--------------------------------------------------- = RPM
tire diametre
Hope this helps
Last edited by RangerMercMan; May 16, 2017 at 09:42 PM. Reason: making the equations legible
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Thanks RangerMercMan. The first calculation was darn close. Looks like I won't be returning the tach anytime soon. I need to recalibrate the ears! May I ask where you reside in the Great White North?
For those that like pics, this is the tach that was added. The dash had a big hole where an aftermarket radio used to be. I decided to fill the hole with gauges. Onto the next job!
For those that like pics, this is the tach that was added. The dash had a big hole where an aftermarket radio used to be. I decided to fill the hole with gauges. Onto the next job!
Fleet Mechanic
Joined: Aug 2016
Posts: 1,480
Likes: 6
From: Great White North
I only have pics of my parts truck available as the good flareside is in BC. Maybe I'm the fall I will create an album
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