I'm confused???
Thread Starter
|
Elder User
Joined: Oct 2001
Posts: 974
Likes: 0
I'm confused???
Ok, I'm fairly new to diesels now, but I've been reading a lot about the 6.9 here. I acquired an 87 F-250 6.9 NA 4x4, and it starts and runs beautifully. I did some tests yesterday, and was reading about what people are seeing as far as mileage.
Here's my dilema. I have 3.55's, stock tires (235/85-16), and the C6. According to 4lo(dot)com, I should be turning around 2100 rpm at 55 mph, but I am actually turning 2600 rpm, which should really be putting me at 67.6 mph. I verified I have 3.55's on both the front and rear axles by counting the number of driveshaft turns with 2 rotations of the tire on one side.
So, either my speedo is off, or my C-6 doesn't have a 1:1 3rd gear ratio. Am I missing something here?
Aaron
Here's my dilema. I have 3.55's, stock tires (235/85-16), and the C6. According to 4lo(dot)com, I should be turning around 2100 rpm at 55 mph, but I am actually turning 2600 rpm, which should really be putting me at 67.6 mph. I verified I have 3.55's on both the front and rear axles by counting the number of driveshaft turns with 2 rotations of the tire on one side.
So, either my speedo is off, or my C-6 doesn't have a 1:1 3rd gear ratio. Am I missing something here?
Aaron
Post Fiend
Joined: Nov 2003
Posts: 21,285
Likes: 16
From: Nutter Fort, WV
Have you checked the speedo to see if it is right?
I have an 86 with 3.55's and a 4 speed.
The speedo was always off on it.
When I installed 286/75-16 (33") mud tires on it the speedo was right for the first time ever.
Your tires should give you 54.4 MPH at 2100 RPM with a C6.
I have an 86 with 3.55's and a 4 speed.
The speedo was always off on it.
When I installed 286/75-16 (33") mud tires on it the speedo was right for the first time ever.
Your tires should give you 54.4 MPH at 2100 RPM with a C6.
Thread Starter
|
Elder User
Joined: Oct 2001
Posts: 974
Likes: 0
Thanks for the replies. I'm not sure how the chart could be different for manuals and autos, as both should have a 1:1 top gear drive (unless you have overdrive/5spd). Auto's don't slip at cruising speed, as the torque converter should be locked up by then, and slipping clutches in the tranny mean a short tranny life.
I havn't compared the speedo with my other vehicles yet, but I have found one interesting thing to check. Several other people on the dieselstop forums have indicated that the speedo drive gear in stock form should have 16 teeth, and be maroon in color. Mine's yellow, so I will check that quickly too.
I can understand the speedo being off, as it's never truely accurate, but 13mph off is a big difference.
I havn't compared the speedo with my other vehicles yet, but I have found one interesting thing to check. Several other people on the dieselstop forums have indicated that the speedo drive gear in stock form should have 16 teeth, and be maroon in color. Mine's yellow, so I will check that quickly too.
I can understand the speedo being off, as it's never truely accurate, but 13mph off is a big difference.
Post Fiend
Joined: Nov 2003
Posts: 21,285
Likes: 16
From: Nutter Fort, WV
Deeproots,
Your speeds are lower than they should be as well, although all my tire diameters are for mud tires.
235/85-16 tires are 31.2" diameter, with a C6 your speeds should be.
2000 RPM 51.9 MPH
2500 RPM 64.9 MPH
3300 RPM 85.7 MPH
Radar is the only way to be sure about your speedo.
My speedo drive gear is teal in color, with my 3.55's and 33" tires it is dead on by radar.
Your speeds are lower than they should be as well, although all my tire diameters are for mud tires.
235/85-16 tires are 31.2" diameter, with a C6 your speeds should be.
2000 RPM 51.9 MPH
2500 RPM 64.9 MPH
3300 RPM 85.7 MPH
Radar is the only way to be sure about your speedo.
My speedo drive gear is teal in color, with my 3.55's and 33" tires it is dead on by radar.
Last edited by Dave Sponaugle; Sep 30, 2005 at 09:24 PM.
Hotshot
Joined: Feb 2004
Posts: 11,697
Likes: 3
From: Marengo, Iowa
Originally Posted by adelyser
Thanks for the replies. I'm not sure how the chart could be different for manuals and autos, as both should have a 1:1 top gear drive (unless you have overdrive/5spd). Auto's don't slip at cruising speed, as the torque converter should be locked up by then, and slipping clutches in the tranny mean a short tranny life.
Trending Topics
New User
Joined: Sep 2005
Posts: 3
Likes: 0
Your RPM will be off on the C6 as the converter does not lock up and will be around 300 rpm off. Here is a simple test on a level road check the RPM with the throttle down and the differance with it off.
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
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
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer
Joe Kucinski
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Posting Guru
Joined: Jan 2005
Posts: 1,051
Likes: 0
From: Douglas, Georgia
thanks fellr86/slimerdogs thats what I was trying to say.
*should* be a certain rpms isn't always the case.
but no, dave, I never had my truck checked via radar. I'd be interested to see it tho.
*should* be a certain rpms isn't always the case.
but no, dave, I never had my truck checked via radar. I'd be interested to see it tho.
Senior User
Joined: Nov 2004
Posts: 305
Likes: 0
From: Bothell, WA
Radar is the only way to be sure about your speedo.
You can also make fairly acurate determinations by timing a measured mile.The interstate Hwy's sometimes have speed check sections posted that are supposedly more accurate than mileposts. You can time yourself through these measured miles and check the accuracy of your readings. The hard part is holding a constant speed so you can get an accurate time.
I suppose a person could install a cycle computer. The top quality ones are extremely accurate and give very precise speed (+/- .1 mph) and distance readings (*/- .01 mile) which could then be compared to the speedometer reading.