I have a 78 f150 with an automatic transmission.. A C6 I am assuming.. It is 4x4 with 400 motor. Has 35" tires and everything else is stock from what I know. While on the highway at 65mph my rpm's are around 2700. Seems high to me. Does that sound about right? What rpm's do you guys see at this speed?

 

For a non-OD transmission, that doesn't sound unlikely ... what gear ratio are your differentials?

 

I am not exactly sure but my door has a axle code of H2A. I looked it up and I come up with gear ratio 3.50

 

Im turning 2450 rpm to go 55mph on 35's and 4.56 gears. Sounds pretty normal to me!


You're not going to run into any problems other than drinking gas a those rpms

 

With a 3.50:1 ratio and 35" tall tires, I calculate 2200 rpm at 65 mph (2700 rpm at 80). 2700 at 65 gets me to a ratio of 4.33:1. Could be the PO swapped gears... do you have a tachometer installed, or are you guessing at rpm? Either way, like Steven said, it's not going to hurt anything besides fuel economy.

 

I installed a tach.. We have owned the truck since 1985 or so. Not sure if the gears were swapped before that but since we have owned it I dont think they been swapped. Is there anyway I can tell what the ratio is for sure?

 

If the tach says that and the speedometer is correct, then the ratio is that (by calculation) ... assuming you're making it into 3rd gear (1:1 at the transmission) and not stuck in 2nd.


Vehicle speed (mph) / 60 = miles/min; * 5280 = ft/min; * 12 = in/min; / (35*pi) = rev/min (of the wheel) - at that point, multiply by rear end ratio to get rpm of driveshaft, which if the transmission is in 3rd (1:1), is also the engine rpm (neglecting torque converter slip - this will push the actual engine rpm up a bit - maybe you're really running something like 4.25 gears rather than 4.33).


To really tell the rear end ratio, you could jack up the truck, put it in neutral, make a mark on the driveshaft and one on a wheel, and count how many times it spins around per revolution of a wheel (or how many wheel & driveshaft revolutions, respectively, to get them to line up again), then divide. That, or pull the cover off the diff and count gear teeth.

 

You say you've owned the truck since 1985 ... has this always been the case, or is it a recent change? If something's different, could indicate a problem with the torque converter/transmission. If all the same and it doesn't seem that the transmission is slipping, I'd say it's just the gears/tires you're running.

 

Thanks a lot I will deffinately check that out. I am curious to know now. Yea my dad had owned since then and just recently gave it to me. Its been in and out of storage for many years so has never been running as much as it has now that its mine. It's never had a tach until recently that I installed one. My dad has always had mechanics work on it for what ever reasons so I am not 100% sure that its never been re geared which is why I am interested in finding out now. My dad says its all stock.

 

No transmission doesn't slip. I was just curious since it seemed kind of high to me for rpm's to be that high at 65. Then again I haven't Owned anything this old.

 

Work it out using math and pi, etc and a rolling radius (there is that flat part at bottom .... measure earth to dead center axle) x 2 for tire diameter and you'll be close, the rest being due to TC slip which your and my C-6 have (not lock up TCs). "pi" is 3.1417.

Lock both hubs, jack only left front off ground, rotate only left tire two full turns as you count driveshaft turns ..... that'll tell frt ratio. In rear, might be harder if clutches in posi are tight, but worth a try ..... you might be able to do it ...... but DS has to be free to turn.

It seems "fast" because everything else I have now has OD transmissions, but back when I got it and in the '60s - '70s ..... we thought nothing of 2500-3000 at 60-70 mph.






.

 

H2A:

H2 = Ford 9" Rear Axle / 3.50-1 / Limited Slip / 3,750 lbs. Rear Axle Capacity.

A = Adjustable Steering Column (tilt wheel) with P/S

 

The engine should handle low 3000 RPM range OK so that's fine. My 400 with 4.10's and 31" tires runs at that speed on highway.

With 35's and 3.5 rear it still sounds high. OE tires were ? 28-29". If you have not changed your speedo gear, it will be reading low with the bigger tires. Maybe you are running faster than you think? A GPS would tell you. Or the rear ratio was swapped.

 

I agree hes likely traveling 5-10 MPH faster than the speedo reads.. I usually find a volunteer in a modern vehicle to follow me and assure me of my speeds

 

Timing yourself between mile markers on a rural section of interstate where you can maintain a constant speed without worrying about traffic also works. Or these days, pulling up a mapping app on your smartphone and seeing what GPS says.

If he's actually got 3.50:1 gears and 35" tires, he's doing close to 80 mph at 2700 rpm.