My motor RPMs don't seem right

[Jakes-66-K10]

I have a 350 with 700r4. Cruising down the highway at 60mph I'm running 3000rpm in fourth gear. At 70mph I'm about 3500 rpm. These seem high to me, I thought I had 3.73 gears but maybe I have really high ones or something?

Anyone have a calculator or experience to know if these figures are out of whack? I have 265/75/16 tires, which should lower the rpms too... I don't get it.

[farside847]

here is one I have used:
http://www.angelfire.com/fl/procrastination/rear.html

I also cruze at 60mph at 3000rpm with a 1:1 final gear and 3.73 ratio. I will soon be swapping to 3.08

[Jakes-66-K10]

Hmm, but a 700r4's fourth gear is 0.7:1 isn't it?

[70rs/ss]

pull the cover off of the rear and check the ratio. It might be on the glove box, and one last question, does it go through all 4 gears? It might not be going into OD? I have an 05 Yukon XL with 3:73 and 275/55/20 and I cruise at 85-90 MPH at 2500rpm, so I am guessing you have way high gears (4:56'ish) or it is not going into OD.

[Jakes-66-K10]

Yes, I can feel it shift to all four gear. Fourth gear at like 40-50 rpm is nice, very very quiet ride. The one thing I'm not sure it's doing is locking up the torque converter in fourth, but I have no idea how to test this (using the BowTieOverdrives kit)

Last time I looked in my diff I coulda swore I had 3.73 gears. I remember seeing a 41.11 number, which divides into 3.73. My options tag on my glovebox has been torn off and replaced by drawings of a child practicing their alphabet (previous owners.... roll eyes)

[truckster]

Here's the theoretical:

Tire size 265/75R16 should rotate roughly 650 times in 1 mile.

650 X 3.73 (diff. ratio) X .70 (final drive for 700R4) = 1683.5

At 60 MPH you should be travelling 1 mile in 1 minute, so your RPMs should be 1683.5.

Two possibilities: have you compensated your speedo gearing for oversize tires and for the drive ratios, and is your torque convertor locking?

[70rs/ss]

Originally Posted by truckster

Here's the theoretical:

Tire size 265/75R16 should rotate roughly 650 times in 1 mile.

650 X 3.73 (diff. ratio) X .70 (final drive for 700R4) = 1683.5

At 60 MPH you should be travelling 1 mile in 1 minute, so your RPMs should be 1683.5.

Two possibilities: have you compensated your speedo gearing for oversize tires and for the drive ratios, and is your torque convertor locking?

It's the speedo! I'll bet, but wait, bigger tires should slow down the speedo?! I have 245/75/16 on the Blaze and my speedo is way slow, like it reads 50-60 when I'm doing 70?! Maybe its in the speedo gear in the tranny? I am back to betting the ratio is way high 4:11's or 4:56's.

[67ChevyRedneck]

Those are some low rpm's for 60mph. My truck has a 350/700R4/3.73 and it turns about 2K @ 60mph and about 2500 rpm @ 80 mph. I can't remember my tire size, but 255/70/R15 sounds familiar, but your tires are a good bit taller than mine...... odd.....

[pjmoreland]

Diff. Ratio = Engine RPM x Tire Dia. x .002975 / Speed / Transmission Ratio

Engine RPM = 3500
Tire Dia = (2 x .75 x 265 / 25.4) + 16 = 31.65"
Speed = 70 MPH
Transmission Ratio = .7

Diff. Ratio = 3500 x 31.65" x .002975 / 70MPH / .7

Diff. Ratio = 6.72

There's no way your differential ratio is that high.

I'm guessing one or a combination of things are happening.

1) Speedometer is wrong (check with a stopwatch on the road)
2) Tachometer is wrong (compare against an engine analyzer)
3) Transmission is not shifting into OD
4) Torque converter clutch is not locking
5) Differential gears are not what you think they are

Maybe the torque converter clutch is locking in third gear, making it feel like you are in overdrive.

I would definitely check your speedometer and tachometer first before disassembling anything.

[truckster]

It could still be the speedo gear. Depending on what the 700R4 came out of, it could be as low as 1.94:1, which would put you at about 3000 rpm at an indicated 60 mph. But I would think you would notice that everyone else was driving REALLY slow.

[jlmtrout]

You can check you rear end ratio by jacking up the rear wheels, rotate one tire and count the number of times the pinion yoke (drive shaft) turns during one rotation of the tire. Example: if the drive shaft turns 3 1/2 times, you have a 3:43. If it turns 3 3/4 times, you have a 3:73 , if it turns 4 times, you have a 4:10. if it turns 3 times, you have a 3:08. Good luck and keep on truckin'........Jerry

[Blazer1970]

If you are using a stock tach, there is a 99% chance that it is incorrect. They all read high with age. It is not unusual for them to be off by several hundred rpm.

[Jakes-66-K10]

Originally Posted by truckster

It could still be the speedo gear. Depending on what the 700R4 came out of, it could be as low as 1.94:1, which would put you at about 3000 rpm at an indicated 60 mph. But I would think you would notice that everyone else was driving REALLY slow.

Yeah, I don't think my speedo is that far off. In the last few weeks of driving, my indicated speed matches pretty well with what others are driving. I'll have a friend drive along side of me and check this, but I really don't think this is the problem. It could be contributing though...

How is the best way to check accuracy of a tach? When I had my emissions test done, they hooked up to one of the spark plugs and I guess it matched up pretty good.

[tomatocity]

Originally Posted by farside847

here is one I have used:
http://www.angelfire.com/fl/procrastination/rear.html

I also cruze at 60mph at 3000rpm with a 1:1 final gear and 3.73 ratio. I will soon be swapping to 3.08

If your tires have a 31.65" diameter and you have a 3.73 rear gear ratio and you have a 1:1 final at the transmission: using the above calculator you would be driving 75.7306 MPH at 3,000 RPM's. To make sure check your tire diameter size. Another check is go to the drag strip and get a RPM reading as you go through the trap but be constant as you drive through the trap. This would give you an accurate speed. I doubt the calculator is incorrect but it sounds like you have a speedo gear change to make.

[tomatocity]

Originally Posted by vortexofpain

...How is the best way to check accuracy of a tach? When I had my emissions test done, they hooked up to one of the spark plugs and I guess it matched up pretty good.

Go to a shop and have them check the tach for accuracy. Is the tach an 8 or 6 cylinder tach?

[tomatocity]

Using this calculator, http://www.angelfire.com/fl/procrastination/rear.html , and these specs:

3,000 RPM's, .70 final trans ratio, 3.73 rear gear ratio

28" tires = 95.7101 MPH
29" tires = 99.1283 MPH
30" tires = 102.546 MPH
31" tires = 105.964 MPH
32" tires = 109.383 MPH

[llegos]

oh man, if you can hit these speeds a 3k, then i defintaly need an overdrive.lol

[tomatocity]

Originally Posted by llegos

oh man, if you can hit these speeds a 3k, then i defintaly need an overdrive.lol

The more I think of it these calculations don't seem to be correct. Heck the guy is running 3.73's. I had a 95 Camaro with a 4L60E / 2.73's and I could cruise at 77 MPH and be turning 2,000 RPM's. But that was with 2.73's. I am guessing 100 MPH would have been around 2,500.

[Frank_Zeller]

Is your converter lock up in 4th?

[pjmoreland]

My truck has a 700R4, 3.73 gears and 29" tires. My tach reads a little under 1900RPM when I'm going 60 MPH.

If your overdrive and torque converter clutch are working correctly and you have a vacuum switch in the torque converter clutch wiring, your transmission should feel more like a 5 speed automatic. It will shift like this: first-second-third-overdrive-tcc.

[Jim_PA]

The convertor locking isn't going to drop the RPM's by a thousand, maybe 200. That's really a non-issue here. Either his speedo or tach are way off, or he has 4.56's or something. Do you have a GPS, or a friend with one that you could put it on your dash and see how fast you're really going?

[Frank_Zeller]

Originally Posted by Jim_PA

The convertor locking isn't going to drop the RPM's by a thousand, maybe 200. That's really a non-issue here. Either his speedo or tach are way off, or he has 4.56's or something. Do you have a GPS, or a friend with one that you could put it on your dash and see how fast you're really going?

In OD under load 400 to 500 rpm above lockup rpm.

[powell72]

I have the BOT kit in my truck and right after the Overdrive shift you should be able to watch the tach drop another couple hundred rpm's from the converter locking. I have 4.11's with a 275/60/15 and 70mph is 2500.

[Jakes-66-K10]

*UPDATE* My truck has been in the shop for rear shock mount issues, now that I have it back I took it in today to have the tach tested for accuracy. Here is what they found:

At 1000 RPM Reading on Tach: 780 Actual
At 2000 RPM Reading on Tach: 1340 Actual
At 3000 RPM Reading on Tach: 1860 Actual
At 4700 RPM Reading on Tach: 3000 Actual

So this is a step towards solving my issue. Problem now is how to fix it... I remember seeing a hole in the back of the tachometer which seems to be where you can put a philips screwdriver in to help calibrate the tach. Has anyone ever done this though?

Since my RPMs are only 200 near idle, but nearly 1700 rpms off at high speed I'm wondering if the adjustment screw will even be able to help. What would cause a tach to be off like this? It really sucks because I bought it for a hefty price all nicely restored in a complete cluster from a board member.

[Jakes-66-K10]

Oh, and I also had a tranny shop confirm that all 4 gears work great but lockup is not engaging in any gear.