Rpm question
 

I have a 71 C10, 350 sb 600 series edelbrock, a cam, 2 1/2 duel exhaust, 275 40/20 tires lowered 4 1/2. I just put a tachometer in and I'm doing 3100 rmp's at 60 mph. It's 4 on the floor granny low. This seems kind of high but it sounds fine. Any thoughts or concerns i should be considering? It's my daily baby and i love to drive it even though the wifey is jealous and doesn't like it

Re: Rpm question
 

If the tach is accurate, it's the gear ratio and tire diameter that's making it run higher RPMs. A factory 4spd is likely to have 4.10s. I doubt it's 4.56 but even could be, but that's rare. I think 3.73s would run lower than what you have. Your tires aren't crazy short, so I think you have 4.10s. I have 4.10s with 32" tall tires (4wd) and I'm probably at around 2500 @ 60mph. I run right about 2800 @ 65. 3.07 gears would bring that RPM down considerably.

Re: Rpm question
 

This might help you figure a real close to what you got for rear gears.
Have you verified the tach for accuracy with a digital timing light? You can determine if the speedo is correct/close also with this chart. With +/- involved Special K is pretty close. Your tire OD should be 28.7" in a perfect world so bump it to 29 on the chart

https://auto.howstuffworks.com/auto-...size-chart.htm

Re: Rpm question
 

Just for giggles you should download a free gps speed app to your phone and check the speedometer against it. Mine reads 45mph at 60mph actual speed. Assuming both the tach and speedo are accurate you have 4.56 gears in the rear according to the TREMEC gear calculator.

Obviously lower rpms will equal less wear on the engine, lower noise, and to a certain extent better fuel economy. Since it’s your daily changing the rear gears may be something you want to look into unless you need those stump pulling gears. Fyi, swapping to 3.73 will drop the rpms to about 2600 @ 60mph, 3.07 will be about 2100 @ 60mph with your tire size.

Re: Rpm question
 

Fill in these blanks:
Truck speedometer reading = ______ (just for comparison to GPS)
GPS speedometer reading = ______ (at same time of above reading)
Tach RPM = ______ (at same time of above reading)
Tire diameter = ______ (because I'm too lazy to look up a 275/40/20 :))

From that, can tell you your rear gear. Truck speedo will not be accurate with a tire change unless that tire just happens to be the same diameter as the original tires, which is not likely.

Re: Rpm question
 

Your tach is right. If it was "off" it would be off by a big factor (ie. configured for a different # of cylinders). As others have suggested, your speed might not be accurate. So you might be going faster than 60mph at that given RPM.

Assuming your 4th gear is a 1:1 ratio we'd be looking at the following:

RPM: 3100
Tires: 28.7" tall
Speed: 60mph

= Rear gears of: 4.41

Since they didn't make that gearset, my guess is you likely have a 4.11, and your MPH is off a bit.

If we set your rear gears at 4.11 instead, then at 3100 RPM you're actually going 64.43 MPH and not 60.

That's my guess anyway :).

Nothing to be concerned about really, but if you want to turn less RPM on the highway you might consider a gear change or an overdrive transmission. My vote is for an overdrive. You'll drop your RPMs ~30% doing that, versus gear changes where you'll go down about 300rpm going to 3.73. A 30% overdrive transmission you could keep your 4.11 gears and shave off over 900RPM at that same 64mph speed.

Hope this helps.

Re: Rpm question
 

I'm not familiar with that tire size. But now that I know the diameter I'll bet you have 4.56s. It was an available option in a 1/2t but you didn't see it much, especially in a V8 truck. But even going by the prior post, 4.56 is a lot closer to 4.41 than 4.10. And going by the C/10s I've had with 4.10s I don't recall the RPMs running that high.

I'd go the simple route and put a 3.08 rear in. You lose the grunt gearing but you still have granny, if having a low gear still matters. You come close to the same numbers from going with O/D as you do going from 4.56 turns to one vs 3.08 turns to one. Divide 3 into 4.56 and you get 1.52. Take 1.52 from 4.56 and you get 3.04, virtually the same as 3.08s.

Re: Rpm question
 

Realize the speedo is likely going to be inaccurate from changing the tire size. Hard to know how inaccurate it is without knowing the original tire size.

Re: Rpm question
 

If by chance you're worried about "wear and tear" on the engine... that 350 was built to run at 3,000 RPM's all day long. Might seem high, but it was just the way we drove them back in the day. We are all now spoiled with fuel injection and overdrive transmissions that are very effecient at lower RPM's.

Gary

Re: Rpm question
 

I thought about it being 4.56 too, as you're right it's much closer to 4.41, but didn't realize it was a factory option so I went with 4.10/4.11 since I figured it was likely to be stock gearing of some kind.

Also, for those that want a quick way to figure out tire size: https://tiresize.com/calculator/

Re: Rpm question
 

The 350 s.b. came out of an 86 chevy blazer. Sorry, forgot to mention. From what i can tell the gear ratio is 3.73. I'm not sure what all that means. The Formula they wanted me to use is.....275×.40×2÷28+20=24.27.857. Now what's next, does mean anything to you?

Re: Rpm question
 

I forgot to mention this 350 sb came out of a 86 blazer. I'm not positive but i think this is a 12 bolt 3.73 gear ratio. They said to use this formula 275×.40×2÷28+20=27.857. Does this mean anything to you?

Re: Rpm question
 

I forgot to mention this 350 sb came out of a 86 blazer. The Formula i calculated....
275×.40×2÷28+20=27.857. Does this mean anthing to you?

Re: Rpm question
 

I forgot to mention this 350 sb came out of a 86 blazer. I believe this is a 1w bolt 3.73 gear ratio. Formula came out to this....275×.40×2÷28+20=27.857. Does this mean anything to you?

Re: Rpm question
 

Not sure where the 28 came from. 275 40 20 tire: Section width of your tire is 275mm which is the width between the sidewalls or approximated tread width. Aspect ratio is 40% of the section width. That is the sidewall height. Rim diameter is 20”.

275mm/25.4mm per inch= 10.83”
40% of 10.83”=4.33” and since you have sidewall at top & bottom multiply by 2.
4.33”x2=8.66” add to rim diameter to get close to actual height.
20”+8.66”=28.66” so figure a 28 and 1/2 inch tall tire.

Re: Rpm question
 

^ This is correct. I came up with the same number.

It is possible you have a 3.73 rear. By my calculations, if you DO have a 3.73 rear, you should be moving about 71 MPH at 3100 RPM.

As others have said, a GPS speedometer will be the best way to verify your actual speed.

Re: Rpm question
 

Before spending any money I’d first verify the speedometer against a gps. Then check your tach to make sure it’s not set for 6 cylinders. If the tach is set for six your 3100 rpms are actually 2325.

Re: Rpm question
 

I checked speed with gps and when i was doing 60 mph gps was at 57 but when i roll across those school road signs that tell if you're going to fast, it reads the same as my speedometer. I timed the truck last night with 12° advanced and I'm still getting 3200 rpm at 60 mph. The engine is under 200° and the carburetor is hardly warm. Gas mileage is about 12 mpg. I'm leaning towards tire size as the culprit.
'

Re: Rpm question
 

You need to know your gear ratio. I still think it's 4.10

Re: Rpm question
 

I'm just gonna have to take the cover off and find out

Re: Rpm question
 

Yeah. Like they say, "Good time for fresh fluid while your at it". That's one place the lube gets neglected. Let us know what you find.

Re: Rpm question
 

That's tbe bummer i just replaced the flui in the spring

Re: Rpm question
 

You can count turns of the driveshaft Vs turns of the tires and get the rear end ratio without dropping the cover.

Re: Rpm question
 

yeah, that usually is accurate enough. You can drain fluid into a clean container, with the area of the housing and cover clean, and reuse it.

Re: Rpm question
 

darn tootin.