Rear end gear ratio change or transmission change ?

[squarebiz]

I have 76 c10 fleetside, and have a th350, and recently added a tach and on fwy rpm if 5000, so I been told to change gear ratio ? Then told it won’t fix problem ? And to switch to 700r4, and been told from owners who’ve done so, don’t like 700r4 ? Gears seems to be the best cost wise, but I need some true advice from someone who really could help me out on what’s the best route to take, cuz the shows are coming up soon and I can’t get on the freeway? Thanks

 

[SquareRoot]

5000 huh. E gads. You sure your not cruising in 2nd gear? I had a th400 and 4:56 gears. I suppose I could have hit 5000 rpm if I were on Utah's interstate where the speed limit is 80. . You no likey 700r4, then yes, gears would be cheapest.

 

[legopnuematic]

First is verify tach is set to 8cyl, if it is set to 6cyl or 4cyl it will read incorrectly (set to 4 on an 8 will read double).
Second is make sure that the trans as squareroot said, is hitting all three gears.
Third is to pop the diff cover and verify the ratio (and if you haven’t changed the fluid before it’s probably time anyways to do so).

Then you can measure your tire size and put your information into a calculator like this and see what it should be.

Does it sound like 5000rpm? If the engine is stock more or less, it will probably hit valve float at 4000rpm and stop revving.

 

[RaisedK5]

Fyi. Sounds like either it's not shifting or it's not really spinning 5000. Unless you have serious low rear gears.

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[TotalyHucked]

Well, first thing you need to do is verify what you already have for gear ratio and whether or not the transmission is working properly. You say you're running 5000 at hwy speed, what speed is that? 70? 80? 90?

Easiest way to check your gear ratio is to chock the front tires, jack the rear up and put the axle on jackstands. Then put the truck in neutral (truck not running), make a mark on the driveshaft for reference. Then spin a tire by hand and count how many times the driveshaft spins all the way around for 1 rotation of the tire. Roughly 3 3/4 is 3.73, roughly 2 3/4 is 2.73, 4.5 would be 4.56 and so on.

Once you determine that, go for a drive and pay attention to how many times your transmission shifts. It should be 3 times. If not, you're stuck in 2nd gear and that's why it's buzzing so high. Until we know that information, we can't give you any good advice or recommendations.

 

[WFO]

Well, first thing you need to do is verify what you already have for gear ratio and whether or not the transmission is working properly. You say you're running 5000 at hwy speed, what speed is that? 70? 80? 90?

Easiest way to check your gear ratio is to chock the front tires, jack the rear up and put the axle on jackstands. Then put the truck in neutral (truck not running), make a mark on the driveshaft for reference. Then spin a tire by hand and count how many times the driveshaft spins all the way around for 1 rotation of the tire. Roughly 3 3/4 is 3.73, roughly 2 3/4 is 2.73, 4.5 would be 4.56 and so on.

Once you determine that, go for a drive and pay attention to how many times your transmission shifts. It should be 3 times. If not, you're stuck in 2nd gear and that's why it's buzzing so high. Until we know that information, we can't give you any good advice or recommendations.

I think you meant 2 times for a TH350.

 

[TotalyHucked]

I think you meant 2 times for a TH350.

Oh yeah, you're right. I was thinking 3 gears, not 2 shifts lol

 

[Grit dog]

5000rpms = 90mph and 4.56 gears with the tires in your pic (I assumed 28” tires) and a 1:1 final trans ratio.
You need gears IMO. Assuming what you said is all correct,
Even with a 700r4 if you have 4.56s you’re 3500rpms at 90 mph.

But depends what you want to do with the truck. Lots of high speed highway and long road trips, you really need both. If you like driving 80-90mph cruising speed.
For just sane “highway” rpm’s in the 60-70 mph range and being used as a cruiser, I’d leave the trans if it’s good and drop to 3.42s or 3.73s at the very most.
Use one of the multitude of online rpm gear ratio calculators and punch in all the reasonable scenarios and it will tell you your rpm’s at any speed any gear with any trans ratio. So you can hone in on what’s best.

 

[GTX63]

I have 76 c10 fleetside, and have a th350, and recently added a tach and on fwy rpm if 5000, so I been told to change gear ratio ? Then told it won’t fix problem ? And to switch to 700r4, and been told from owners who’ve done so, don’t like 700r4 ? Gears seems to be the best cost wise, but I need some true advice from someone who really could help me out on what’s the best route to take, cuz the shows are coming up soon and I can’t get on the freeway? Thanks

If you cannot get on the freeway, then without you giving us your current gear ratio, we would have to assume you are spinning 5K @ 65mph?
Probably going to need a few more specifics to understand what is going on. Rear gearing if you know it, tire size and what you consider as highway speed, please.

 

[DoubleDingo]

Must be running an old HO52 with 5.14 gears and driving 80+mph

 

[HotRodPC]

Your tach has to be wrong. It's an opinion thing, but I prefer lower gears always and go with an OD transmission to have the best of both worlds. You can always shut OD off when towing. If you want reliable, spend the money and do a 4L80-E swap. It's costly but worth it if you're keeping the truck forever and drive it frequently. If it's just a toy, then get the tach set right and live with it.

 

[Rickf]

Get taller tires.

 

[gsuburban]

Your information is limited. At what speed and gear is the tach reading 5,000 RPM? You do know its not safe running the stock engine that fast, right?
The 700R4 has a lower 1st gear plus an overdrive or 4th gear vs the TH350. However, that doesn't appear to be a problem solver for the issue you describe.
On the inside door of the glove compartment is a label with several rows of 3 and 4 digit codes known as RPO. Take a picture of it and be sure you get a well focused picture for future reference. In the RPO code it should have the differential the vehicle came with when new. Most of them start with the letter "G" I believe.

 

[peats]

5,000 rpm would sound like it's ready to explode. I think the tach is not accurate.

 

[Ricko1966]

If I was going to do anything, O.D. transmission would be first. Changing gears will reduce highway RPM at the sacrifice of low end pulling power. O.D. transmission would keep the low end but decrease the highway RPM.