4800 RPM @ 55mph is not fun. Suggestions?

[Pegtop]

According to my VIN, my truck was originally equipped with a 4 speed (manual?) NP 435 transmission and 4.09 rear gears. At some point somebody converted it to a 3 speed C6. My problem is that driving on the highway makes the engine rotate close to 5,000 rpm and makes me think the pistons are gonna blow through the block not to mention the noise and poor gas mileage.
Need some advice on making it a bit more user friendly. Simply swap the rear gears out? Sell the transmission and put something else in? Any thoughts would be appreciated. Thanks

[jzjames]

3.70 or 3.55 would be my suggestion, and a C4 auto transmission.

[JoshT]

I would agree that those would be better gears for highway driving, but I certainly think he's got an underlying problem that needs to be fixed first.

C6 in 3rd gear and NP435 in 4th gear should both be turning at a 1:1 ratio. Speed vs engine rpm should be about the same with both in high gear, which is where you should be cruising on the interstate. Some quick math assuming that Pegtop is running a 31" tire suggests that he should be running about 2500 rpm @ 55mph. Rpm would be a little higher for a shorter tire, or a little lower for a taller tire.

Have verified the engine is actually turning that fast with a (standalone) Tachometer? How about the speedometer accuracy, maybe you aren't going the speed you think? Are you sure that the C6 is going into 3rd gear? Is it possible someone swapped the axle/gears in the past?

Sometimes an engine can sound like it is turning a lot faster than it really is, check it with a tach if you haven't. Tachometers can also be inaccurate for various reasons, if you are relying on a per existing or in dash tach, it might not hurt to verify it ain't another tach.

If tires are taller (or shorter) than stock it can throw off speedometer reading. Also since you stated that transmission was swapped by a previous owner, if they did not swap the speedometer drive gear to match the new transmission it won't read accurately.

If everything else is correct you are running almost twice the RPM you should be. For that to happen you would have to be stuck somewhere between first and second gear on the C6 or running about an 8:1 gear ratio in the rear end, neither seem likely. Your signature says it is a 4wd, I don't know what transfer case you are running, but I think some have about a 1.9:1 ratio for low range. That ratio would effectively double rear gear ratio, making a 4.09:1 act about like a 8:1 ratio, which would match with engine speeds you describe. Maybe your transfer case is stuck in low range?

To me transfer case in low range seems to really fit your problems, but I'd think that would be too obvious. I think some combination of the other swap (tire size, speedometer gear, axle gear) related issues and possibly a slipping transmission are more likely.

Just my 2¢ I may be way off base here.

[sargentrs]

If you have an Android phone you can downliad DigiHUD Speedometer and check your speed against your speedo. It's a GPS speedometer. I've used it when my speedo quit on another car and it's accurate. I'm sure iPhones have a similar app out there.

[DuckRyder]

I’m with Josh T on this one.

[Jacksdad]

I agree. It sounds like it's not shifting into third gear. I have a similar set up, and I cruise at 60mph without issues. I would definitely confirm your speed and RPMs before you do anything else.

[Pegtop]

I would agree that those would be better gears for highway driving, but I certainly think he's got an underlying problem that needs to be fixed first.

C6 in 3rd gear and NP435 in 4th gear should both be turning at a 1:1 ratio. Speed vs engine rpm should be about the same with both in high gear, which is where you should be cruising on the interstate. Some quick math assuming that Pegtop is running a 31" tire suggests that he should be running about 2500 rpm @ 55mph. Rpm would be a little higher for a shorter tire, or a little lower for a taller tire.

Have verified the engine is actually turning that fast with a (standalone) Tachometer? How about the speedometer accuracy, maybe you aren't going the speed you think? Are you sure that the C6 is going into 3rd gear? Is it possible someone swapped the axle/gears in the past?

Sometimes an engine can sound like it is turning a lot faster than it really is, check it with a tach if you haven't. Tachometers can also be inaccurate for various reasons, if you are relying on a per existing or in dash tach, it might not hurt to verify it ain't another tach.

If tires are taller (or shorter) than stock it can throw off speedometer reading. Also since you stated that transmission was swapped by a previous owner, if they did not swap the speedometer drive gear to match the new transmission it won't read accurately.

If everything else is correct you are running almost twice the RPM you should be. For that to happen you would have to be stuck somewhere between first and second gear on the C6 or running about an 8:1 gear ratio in the rear end, neither seem likely. Your signature says it is a 4wd, I don't know what transfer case you are running, but I think some have about a 1.9:1 ratio for low range. That ratio would effectively double rear gear ratio, making a 4.09:1 act about like a 8:1 ratio, which would match with engine speeds you describe. Maybe your transfer case is stuck in low range?

To me transfer case in low range seems to really fit your problems, but I'd think that would be too obvious. I think some combination of the other swap (tire size, speedometer gear, axle gear) related issues and possibly a slipping transmission are more likely.

Just my 2¢ I may be way off base here.

I agree it sounds like there is indeed an underlying issue but I can't put my finger on it. A few details to clarify things: I'm running a 32" tire and have an aftermarket tach installed (granted, it's a cheap one I got from Amazon), the transmission was literally rebuilt two weeks ago and while the shift points don't feel obvious, I'm pretty sure its going into 3rd gear. The transfer case is an NP 208F and it's running in 2wd during my tests. I'm going to take another test drive to make sure it is in fact going into 3rd and I'll also try it in 4wd to rule out that's it not currently in low range. Also, for the speedo, I was relying on an app to tell me my speed as opposed to the factory speedometer. I thought that maybe the C6 just likes to rev high with only 3 gears but you guys seem to think that's not likely. I'm also not convinced of the quality of the rebuild of the transmission. It blew reverse about 50 miles in and they had to pull it again and rebuild it. I can't say how it drove or shifted before as it only recently got back on the road and plated.

[Ranchero50]

While going down the road nudge it up into neutral. If you find another gear you've also found your problem. I assume it doesn't over rev before shifting like if the modulator valve was disconnected?

[Pegtop]

There are definitely 2 shifts while driving meaning it's going into 3rd. The shifts don't feel particularly strong and the tach barely shows any kind of drop in rpm under normal acceleration. Under hard acceleration, the tach climbs all the way to 7,000 rpm and drops between 500-1000 rpms between shifts. Popping into neutral and back didn't change anything but like I said, there are definitely 2 shift points. I tried this in both 2 high and 4 high with the same results. I was starting to think that maybe it's just my tach since its pretty cheap. Maybe it's not registering the correct rpm's. But if this were the case, it would not indicate the right rpm at idle either, would it? It idles right around 1,000.

[cep62]

Tachs have a selector switch for 4, 6 , and 8 cyl engines.

[JoshT]

I agree it sounds like there is indeed an underlying issue but I can't put my finger on it. A few details to clarify things: I'm running a 32" tire and have an aftermarket tach installed (granted, it's a cheap one I got from Amazon), the transmission was literally rebuilt two weeks ago and while the shift points don't feel obvious, I'm pretty sure its going into 3rd gear. The transfer case is an NP 208F and it's running in 2wd during my tests. I'm going to take another test drive to make sure it is in fact going into 3rd and I'll also try it in 4wd to rule out that's it not currently in low range. Also, for the speedo, I was relying on an app to tell me my speed as opposed to the factory speedometer. I thought that maybe the C6 just likes to rev high with only 3 gears but you guys seem to think that's not likely. I'm also not convinced of the quality of the rebuild of the transmission. It blew reverse about 50 miles in and they had to pull it again and rebuild it. I can't say how it drove or shifted before as it only recently got back on the road and plated.

Even with 32s it should be running right around 23-2400 rpm in third at 55mph.

Don't know what to say about the cheap tach. Cheap gauges can be hit or miss, but I still buy them too. cep62 has a point as well, if the tach is set or built for a 4 cylinder it's going to register double rpm on a 8 cylinder.

Transfer case stuck in LR was just a WAG. Being a NP 208 rules that out due to a lower gear ratio. To run 55mph in that scenario, the engine would have to be turning around 5700 rpm.

Even if the C6 "likes to rev high", when crusing at 55mph, the transmission should be in high gear. High gear (3rd) = 1:1 gear ratio which should put the engine running that 2300-2400 RPM range. If its actually running 4800 something ain't right.

Not overly familiar with the C6, does it use a TV or Kickdown cable? Is it possible that cable could be stuck or adjusted wrong and holding the transmission in passing gear?

[basketcase0302]

Another vote on a bad tach setting or a bad tach itself.
My 71' DRW F-350 with 4.10 gears in the D70 ran down the road on it's C-6 at 55 MPH on 2300 RPM all day.

[Pegtop]

Well, I feel a bit dumb now. Got home tonight and the first thing I did was check my tach setting and sure enough it was on “4”, not “8”. Switched it over and went for a drive. Running about 2200 rpm at 55mph. The worst part is I knew about the switch back there and it never occurred to me to check it. Can’t thank you guys enough for the troubleshooting, especially JoshT. Much appreciated.
On a side note, I think my tach could still use an upgrade and plan on swapping it soon. Once I switched it over my idle was now reading around 400 rpm. Does this seem like it makes sense. The number seems low but the truck seems to like it there. Thanks again!

[JoshT]

Thank cep92! I had a feeling a guage was reading wrong, but forgot about that switch until he mentioned it. Glad you got it sorted!

I'd expect 5000 rpm forhard acceleration or passing, but it is way too high for cruising. If it were actually running that high, I don't think you would have driven it. I know if it were screaming like that I'd never have even gotten to 55 mph before I stopped.

As for the 400 rpm idle, it's probably reading low due to inaccuracies of a cheap tach. As long as it's idling good and not trying to die on you I wouldn't worry about it too much. If you do a little research on that tach, there may be a potentiometer that you can use to adjust accuracy. You'd still need an accurate way to judge rpm to adjust it though.

[Pegtop]

yes indeed, thanks cep62 and basketcase0302!