My speedo cable is broke and i have been driving aroung with out a speedo. So my question is at what RPM should i be at to run 60 miles an hour and also 45 miles an hour. The car is a 2.8L, 5-spd, with 3.42 rear end gear. Thanks

-Buck

 

In order to answer your question you need two more pieces of information. Rear tire diameter, and the transmission ratio of the gear you're in. I'm guessing you're using 5th gear, so you'll need that ratio. I have the formula around here somewhere, I'll try to post it later.

 

This formula will tell you MPH:

RPM X tire diameter
------------------------------------
trans ratio X axle ratio X 336

 

Thanks, but i do not know what the tire dia is. I have stock tires for a RS on there, exactly what the label on the driver door says and i do not know the trans ratio. Help please. Thanks

-Buck

 

I belive stock tires are 215/65/15, which is 26.00 dia.

I am also "thinking" 5th gear is .63, or .72 in the v6?

 

somethings not coming out right.

at 3000rpm, 26.00 tire, .63 ratio, 3.42 axle, he should be doing over 107mph?


my car, 3000rpm, 27.02 tire, .70 ratio (automatic)?, 3.23 axle, it says I should be doing 106

when at 3150rpm I'm doing 80.

 

The formula above is correct. If you want, do a google search for one of those "calculators," put in your info and you get the same answer.

I think 3rd gen gauges are notorious for being off somewhat. Also, are you *sure* that 3.23 gears are actually what's in your car? Torque converter locking up ok?

 

You're formula is for tire RADIUS, not the diameter. So cut your tire number in half.

 

Not true. Use diameter in the above formula.

 

https://www.thirdgen.org/newdesign/t...ulations.shtml

 

I am quite sure I have 3.23's in the car. As for how the tranny is acting NO. Thats why I ripped the 700r4 out a few days ago and am working on installing a t5

 

Something has got to be off 2500 RPM with a radius of 13 tire with 3.42 gears and a trans ratio of .73 means that i am running 77MPH?
Help please.

-Buck

 

I still cant get the formula to come out right. Anyone else try this one?

-Buck

 

I never heard anything else from this. Any suggestions?

-Buck

 

with mine, what my speedo actually says, not some formula,

3.27 axle
2500rpms
4th gear(auto .70)
26.00 in tire diameter
78-80 mph


if i am doing 3000 rpms i am pretty cose to 100 mph.(i don't see those speeds very often.....lol)

 

sounds like you are in drive and not overdrive.

 

Here is what I figure, tell me if I am wrong but here goes:

For me all these are in 5th gear 35mph = 1500 rpm 45mph=2000 rpm 55mph=2500 rpm

But i mostly travel around anywhere from 2700rpm to 3500 rpm and have had it up to 5200 and it was still climbing (had to get off the exit) anyway, I wish I knew how fast I was rolling. Guess I will have to get myself a new speedo cable. But then If I am hitting over 110 the computer will shut me down, that is why I havent fixed it yet. Thanks for the replies yall!!!!

-Buck

 

The tach in these cars always fails, and it always fails by reading high. The reading gradually eeeks its way upward over time. It can get real high before it gets noticed as such; alot of the "does my transmission slip" type posts you see in here, and the "I must not have the stock 2.73 gears because the calculator says I have 3.42s", are directly because of tach failure. Sometimes they read TWICE the real engine RPM, before the owner realizes something isn't right.

The only one of these posts with numbers that passes the "smell test" is Jeff's. His numbers look about right. My little brother's 84 with a 700 and 3.23s, did about 1850 RPM at 55 mph, for example.

The calculators don't lie. Of course, they can only tell the truth, up to the point of what's been punched into them. Feed them bogus tach readings, they'll give you bogus everything else.

I can tell you from long experience (near 20 years worth) that a T-5 with .73 5th, stock 3.73 gears, and various approximate stock size tires for these cars of 26" -26.5" diameter (13" radius), gives between 2150 and 2250 RPM at 55 in 5th. That's using a tach that's been checked against a frequency counter. Yes, I have one (counter, that is).

 

I also have had dealings with false tach reading. It use to read alot higher then what it does now and bounce all over the place. But with a new coil, cap, wires, pick-up coil and what not, it is suppose to be true. Same here I have had it checked at the emmissons station and it is about 200 rpm high. So i have compensated for this. Would 3.23's vs. 3.42's make that big a difference in reading 1850 at 55 vs 2500 at 55? I hear what your saying, thanks for the reply.

-Buck

 

Well since a T-5 with .73 5th, 3.73 gears, and stock size tires, does 2200ish at 55 in 5th; and a 700 with .71 or whatever it is 4th, 3.23s, and stock tires does 1850 at 55; I'd say that a tach reading of 2500 at 55 with 3.42s = a broken tach.

At what RPM was your reading 200 RPM off? Idle I take it? Keep in mind that if you double the RPM, you also double the error. i.e. if it was 200 RPM off at 800, showing 1000 when it was really doing 800, then it's going to be 400 off at 1600 (will show 2000 when it's really 1600) and will be 500 off at 2000 (will show 2500 when doing 2000). And, in accordance with the numbers above, 2000 at 55 with 3.42s is about right.

Being a numbers kind of guy, that's what I'd guess is really going on.

 

Oh, my bad I understand what you are saying. So pulling 3 grand is actually more like 2600-2700, right? So I can tach this thing up to 7 grand and not have to worry. My redline is 7 grand. That is way better cause my power band is higher than a normal 2.8. Every 800 is 200 off. I think it idles right at.....800 to 1000. Let me know your thoughts. Thanks

-Buck

 

I can pull 7 grand cause it would be more like 5600. I can pull up to 8000 = 6400. Cant wait to get back on the road today!!!!! Thanks

-Buck

 

No, close but not quite....

The tach reads 25% higher than actual (200 is 25% of 800); or, if you turn it the other way around, actual is 80% of the reading (800 is 80% of 1000).

So, if your tach reads 3000, your engine would be doing 2400 (80% of 3000 = 2400).

Meaning, if the tach shows 7000, it's really doing 5600 (80% of 7000 = 5600).

If it was mine, and I had to pay for upkeep and replacement, I wouldn't be doing that to a 6-cyl, since it won't make any power up there anyway. Power in those starts to fall off past 4500 or so from all I've seen (I've never had one), which is to say, the car will go faster if you shift. Not much point in taking a risk of scattering its internals, or even shortening its life, in exchange for no benefit in useable power.

I'd expect your power band is no higher and no lower than any other 2.8. You have the same cam, same exhaust manifolds, same heads, etc. etc. etc. as all the rest of them. I'd expect the reality of your power band, to roughly match the reality of everybody else's. Just, your tach reads higher than most of the rest of them.

 

I did not say that it has all the same internals, I rebuilt it with intentions of dragging it. But thanks for the info anyway. I may install a new tach that way there will be no guessing. Thanks again!!!

-Buck

 

On the home page under Tech Central sub catagory equations. These work very well. You need the radius of the tire. From the center of the wheel to the ground.

 

The derivation of output linear velocity of the car given the input angular velocity of the flywheel, the combined gear reduction ratio of the transmission and differential and the tire diameter is fairly simple from first principles. I'll try and explain just what these 'MPH calculators' actually do.

First off, You need to figure out what the output angular velocity of the wheel is. Simple gear theory comes in handy here. If 'w' is the angular velocity of the system and 'R' is the gear reduction ratio:

w(input)/w(output) = R.

Therefore, w(output--the angular velocity of the rear wheel) = w(input -- flywheel angular velocity)/R

*NOTE: by angular velocity, I mean RPM

Another thing you need to consider is that there are effectively two gear reduction ratios in the drivetrain--the transmission gear and the differential. The effective total gear reduction ratio is the product of the individual ratios:

R(effective) = R(trans) X R(diff)

So, given the gear ratios and the engine RPM, we now have the angular velocity of the rear wheel (in RPM). We need to convert this angular velocity into a linear velocity. To accomplish this, we calculate the circumference of the tire and thus determine the linear distance (circumference of tire) covered per unit time. To do this, we use the relationship (v = linear velocity, d = tire diameter (inches), pi = 3.142)

v = w(output) X pi X d

We now have one final problem--our linear output velocity is in inches/minute. We want miles/per hour. Therefore, we perform the following conversion:

v(inch/min) X 60min/hr X 1mile/63360inch = v(miles/hr.)

So, there is the complete explanation to determine roughly what your MPH is based upon engine RPM. To summarize, I will give you one complete formula that you can use and understand:

v (mph) = [w(engine RPM) X d(tire inch) X pi]/[R(trans) X R(diff) X 1056]

where 1056 is the simplified conversion into MPH, since 60min/63360inch = 1min/1056inch.

 

You can also use that formula to determine your desired RPM for a certain MPH. I cannot, however, tell you how to account for an innaccurate tachometer...you'll have to figure that out yourself

w(engine RPM) = [v(MPH) X R(trans) X R(diff) X 1056]/[d(tire inch) X pi]

To be nice, I've done some RPM calculations for you (assuming 26 inch tire, 3.42 differential gear ratio and .63 5th gear ratio):

At 45 MPH, RPM should be approximately 1250 RPM

At 60 MPH, RPM should be approximately 1670 RPM

These number are also very reasonable...the last time I drove my Iroc-Z (many moons ago!), 130km/h (approx 80 mph) had the engine spinning just above 2000 RPM with 3.42 rear gears and a 700R4. GM has some pretty nice overdrives. My Monte Carlo SS is also around 2000 RPM at excessive highway speed. My daily-driver Mercury does a whopping 3500 RPM for the same speed...therefore, Dale, I think your tach might be off a bit. 107mph at a little over 3000RPM seems reasonable for a thirdgen.

One last thing, after looking above, Section162's formula is pretty much the same, just the contribution of pi is factored into the '336' numerical constant, whereas I've left pi on its own out of habit (since every calculator anymore has a separate pi key anyways). In summary, the first formula give is correct--I guess I lengthily explained why.

 

...and what losses due to friction are going to change the rpm to velocity relationship in an engine powering a manual trans and rear axle that is directly linked (i.e. no clutch slippage or tire spin)?

 

Good question Lo-tec...a question an answer for which I don't have :P. There will be no losses in rotational velocity with a manual gearbox (as you said, everything is directly linked)...so that statement I made is completely useless :P! There will be energy losses of course due to friction between bearing surfaces, but that has no impact on speed whatsoever. With an automatic transmission and an unlocked torque convertor, you may have a different story. I'll fix my post.

 

I am completely lost............LMAO!!!!!! Thanks yall, I think....LOL


-Buck