RPM???

[MississippiSlim]

I have a C-6 and a 351W with 3.55:1 what should the RPM's be at 60-65mph ~ It seems to run just under 3000 on the tach...... Just wondering

 

[Seabronc]

I have a C-6 and a 351W with 3.55:1 what should the RPM's be at 60-65mph ~ It seems to run just under 3000 on the tach...... Just wondering

You need one more thing to calculate the RPM, Tire size that number does sound a bit high for 3.55s are you sure you don't have 4.56s.

 

[MississippiSlim]

I am running stock size 235/75 15

It could be higher gears but nothing else on the truck seems modified and according to the Sticker (although it is hard to read) I believe it has the code for 3:55....could be aftermarket though. I guess i could find a parking lot and check it the old fashoned way. I thought that was a little high and wondered what the deal was. I kinda hope it does have higher gears cause then I won't have to change them later on when I put some bigger shoes on. I have a feeling it had bigger tires before cause when I bought it it had 4 different brand worn out street tires and I know when I sold a 4wd I always stripped the big rubber and put used tires on....but it doesn't have a lift.....oh well something to think about I guess till I get home and check it out next week

 

[MississippiSlim]

BTW, what is the formula for figuirng rpm or gear ratio??

 

[Seabronc]

Based on the tire size and a stock ford 351W with a C6 transmission and 3.55 gears your 3rd gear RPM at 65 should be 2682 (not that you can read the TAC that accurately). If you are getting around 3000 RPM and everything else is stock and accurate, it looks like there may be 4.10 gears in it. With 4.10 gears the RPM at 65 would be 3098 RPM.

Here is how I came up with that: 235/75 tires are approximately 28.9 inches in diameter ( http://www.tirerack.com/tires/Spec.jsp?mak...re1=yes&place=0 ) which will vary a little depending on the rim width of the wheels they are mounted on.

The formula for calculating the expected 1:1 RPM, (the 3rd gear transmission ratio for a C6 with no slippage), is: speed x rearend gear ratio x 336 / Tire diameter. So 65 x 3.55 x 336 / 28.9 = 2682 RPM . The RPM I would expect to see at 65 MPH.

One thing to check is the accuracy of your speedometer since if that is off it throws the whole calculation off. The easiest way to do that is to take it out on a highway and drive 10 miles per the highway mile markers. Mark your milage at the start and the end, (you do not have to maintain any particular speed with this method). If your Odometer says 10.0 after 10.0 by the milage markers, your speedometer is right on. If it says 9.3 you are 7% low, if it says 10.7 you are 7% high. If it is off you may be able to correct it by changing the speedo gear. One tooth change in the speedo gear count corrects by about 7%, ( the reason I gave that in the example /emoticons/[email protected] 2x" width="20" height="20" /> ), (adding to the tooth count will bring the indicated speed down, reducing the count will bring the indicated speeed up).

Hope that helps.

Good luck,

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[Seabronc]

P.S.

Here are acouple of charts from the Broncograveyard 80 and up cagtalog that can help if you decide to regear the rear end or change the size of your tires.

Keep in mind that the gearing chart is calculated for a road cruise speed of 55 so if you cruise at 65 then the RPM numbers on the chart increase. Gearing for low RPMs gives better milage but less power when you step on it. Gearing for the higher RPM numbers gives the opposite result.

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[MississippiSlim]

Thanks for the info...I have been out of the 4wd hobby (addiction) for about ten years and I forgot everything I used to know! Right now I am seeing a couple possibilities: Either I have a 4:10 gear (which was offered and I could not hardly read the sticker on the door jamb) or....they were running bigger tires and the speedo gear was changed or would it make it the other way wait no if that were the case it would show me going faster than I was not slower I think...now I am confused! Have to think a minute /emoticons/[email protected] 2x" width="20" height="20" />

The other option is that my tranny is slipping pretty bad... URGH!

 

[MississippiSlim]

Okay, another question...say I have 4.10 gears and that puts my rpms around 3000-3100 @ 65MPH with my 29 " tires...Is this a problem? Other than suffering in th MPG department (I am getting 14mpg more or less) Is it seriously detrimental to my drivetrain to be constantly running at this higher rpm? Forgive my stupidity but I just bought tires and really don't have the cash to get bigger rubber to bring my rpms down. I have to spend it on other stuff that HAS to be fixed on the truck and hope I am not wearing anything out by taching it so high at normal operating speeds.

 

[Seabronc]

It shouldn't be any problem at all for the drive train. I'd just check the accuracy of the speedometer / odometer. If it is necessary to adjust, a new speedo gear is inexpensive. 4.10s would be a good choice when you go for bigger tires. Off the top of my head 4.10 would work good atleast up to about 36 inch tires.

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