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
Top 6 Best Deals Available on New Fords & Lincolns Right Now
Brett Foote
This Hennessey Takes the Expedition Tremor's Off-Roading Capability to the Next Level
Verdad Gallardo
Top 10 Fords at 2026 Carlisle Ford Nationals
Joe Kucinski
3 Best / 3 Worst Parts of Modern Ford Ownership
Brett Foote
10 Amazing Upgrades That Solve Common Ford Truck Owner Headaches
Pouria Savadkouei
Every 2026 Ford Engine Explained
Brett Foote
10 Ugly Ford Trucks That We Still Kinda Love
Joe Kucinski
10 Things Every Truck Owner NEEDS (2026 Edition)
Michael S. Palmer
Rezvani's Latest Post-Apocalyptic Monster Is a Ford F-150 Raptor Underneath
Verdad Gallardo
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.