Gas vs PSD
Posting Guru
Joined: Aug 2009
Posts: 1,058
Likes: 0
From: emporium
what math are you using? i used a internet calculator and got 1755 rpm
Elder User
Joined: May 2010
Posts: 650
Likes: 0
From: WV
I figured, but never looked into it. It would make since since most gauges are just plug-it-in today, plus it should be more accurate.
If you want to run at lower RPM's, just shift the face on your tach a little clockwise. I can run at the same RPM's as diesel that way.
Lead Driver
Joined: Jul 2007
Posts: 7,868
Likes: 9
From: Rhode Island
Analog gauges that are not specifically calibrated can loose accuracy depending on where they are. It could be correct 1200 RPM but way off around 2000. The only way to know for sure would be to get an aftermarket calibrated tach, or plug in a programmer and use the digital tach reading.
More Turbo
Joined: Jan 2010
Posts: 680
Likes: 2
From: Las Vegas, Nevada
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
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
10 Most Surprising 2026 Ford Truck Features!
Joe Kucinski
Postmaster
Joined: Apr 2008
Posts: 4,724
Likes: 4
From: Rogersville, TN
Just checked mine. 265/75 16 tires. At 70 mph I was turning 2490 rpm. I was turning 2000 rpm in 2nd at 17 mph and 30 mph in 3rd at 2000 rpm. According to the online calculator I should have been turning 2179 rpm in 2nd at 17 mph, 2106 rpm in 3rd at 30 mph and 2319 at 70 mph. So looks like my tach is off ~170 rpm in two of the gears but only 100 in the other(so not a tire size problem, or it would be off the same amount).
Elder User
Joined: May 2010
Posts: 650
Likes: 0
From: WV
Just checked mine. 265/75 16 tires. At 70 mph I was turning 2490 rpm. I was turning 2000 rpm in 2nd at 17 mph and 30 mph in 3rd at 2000 rpm. According to the online calculator I should have been turning 2179 rpm in 2nd at 17 mph, 2106 rpm in 3rd at 30 mph and 2319 at 70 mph. So looks like my tach is off ~170 rpm in two of the gears but only 100 in the other(so not a tire size problem, or it would be off the same amount).
Thankfully I didn't get written proof, just verbal.When you start nitpicking to that point, tire ware cane even affect things too since most have about 1" diameter that ware down over the life of them (1" diameter = 0.5" of actual ware).
Postmaster
Joined: Apr 2008
Posts: 4,724
Likes: 4
From: Rogersville, TN
Your speedo may also be off. I know when mine reads a hair over 85 (say 86), I'm actually going 82, verified by a cop Thankfully I didn't get written proof, just verbal.
When you start nitpicking to that point, tire ware cane even affect things too since most have about 1" diameter that ware down over the life of them (1" diameter = 0.5" of actual ware).
Thankfully I didn't get written proof, just verbal.When you start nitpicking to that point, tire ware cane even affect things too since most have about 1" diameter that ware down over the life of them (1" diameter = 0.5" of actual ware).
Elder User
Joined: May 2010
Posts: 650
Likes: 0
From: WV
In general speedo are always a couple mph off. It's a percentage, usually 2-3%. My Saab is also off by 2%, and many other Saab owners over at Saabcentral.com say the same thing, a few have even verified it. Manufactures generally add a couple mph to the speedos to prevent people from saying they are to off when they get a ticket. Most people would rather think they were doing 80 and the cop says 78 than the other way around.
I had a Civic that was part of a lawsuit because the odometers read to high. Ends up manufactures added the extra mph for the speedo before the computer took the odometer reading, which makes the odometers read high. Since my car was leased and over mileage, it actually saved me several hundred dollars when I turned it in. I think they were over by 4.5% on average, and the law provides for up to 3% with worn tires if the odometer reading is based on the speedo, which most are.