I'm not sure where to start! I have a 461ci with a stock 10 bolt rearend, gears are still unknown, I drove on this setup with the automatic transmission TH350 for about a year and a half, I converted to an M20 about 8 weeks ago, I have 295's in the rear so we have to take this into consideration. Right now on the freeway it says im at 3k RPM's and going 65mph. From my detective work 65mph is really about 70-75mph, isnt there a different speedo gear I can use? But my main question or curiousity is with the automatic, at whatever speed it may say... may or may not be, the manual is making WAY higher RPM's on the freeway at cruising speeds - I know it will be quite different between the TH350 and an M20, does this sound right? I should really find out what gears im running, isnt there a way to jack up the rear and count the turns on the driveline and wheels, etc??

Yes you can count the turns. Just measure one rotation of the driveline, and see how many times the wheels spin. Or vice a versa.

I believe both trans have a 1:1 ratio. Your freeway rpm should be the same at the same speeds.

TH350 and M20 both have a 1:1 high gear. Your rpms should be near equal and actually a little less with the M20, due to no torque converter slippage to factor in.

This website was very helpful when I had the same issue:

http://chevelleengineer.home.comcast.net/speedo.htm

BTW your tire width (295) has nothing to do with the speedo calibration, your tire diameter does. If it was correct before the manual swap, the difference is probably due to torque converter slippage.

Sounds like 3.08s. Do the math...if it was a 3500 converter, you might have been slipping excessively at speed.

He said the manual is making WAY higher rpms not the other way around.

Yes that sounds like the clutch is slipping. If they both have a 1:1 high gear, the manual should be lower at cruise.

Do you have the same tires on now as you did before?

Same exact tires, same rear end, same gears, same everything except the M20 swap... All the components are brand new, clutch, pedals, pressure plate, I installed it 4 times - so it should be in correctly :) What was it you guys meant before, maybe this is the way it should have been in the first place? I wouldnt be suprised if the TH350 had some problems, it was a stock converter and a pretty tired trans...

Still??, sure it wasn't a 200R4!?

The problem is the speedo gear in your transmission. I replaced the muncie in our Lemans with a Borg Warner Super T10 and now the reading is off.

Isn't the question here why are the rpms way higher????????????
What does a speedo gear have to do with that??
He said freeway speed not a certain MPH is different.
Unless of course everyone on the freeway is driving a lot faster than the last time he was on it.

Yeah the speedo gear is definately off, it was off before though thats an issue but a real concern right now. When I had the automatic the RPM's were WAY lower at the same speeds im running now with the muncie. I hit 3k around 70mph and anything over that feels like it'd be pushing it for anything over a 10 minute freeway trip... I could do 90mph easy in the automatic at the same rpm, or atleast it felt like less - the entire car runs a little meatier with a manual, different feel and sound... if that makes sence.

Yeah exactly unless everyone is driving 20 mph faster in the last 8 weeks... something is alot different. Maybe I can check my gears for starters... if that would help.

Your gears shouldn't matter, what your saying is the rpms are way higher with the manual than with the automatic......correct. Something is definately wrong here!! The manual should cruise at a LOWER rpm. The final ratio in both tran's is 1:1. Either you are not in high gear or the clutch is slipping.
If the trans is the one thing that has been changed and no other changes such as tire size or gearing have been made.

What happens when the clutch is slipping, or what I mean is how is it happening? That sucks, maybe something isnt in right or adjusted incorrectly. You know when the clutch isnt engaged and im at a stand still, light, drive way, etc... it has a slight noise to it, it sounds a little bit like a steering box low on fluid, but not as loud. My clutch pedal isnt up as far as the brake pedal, maybe I should bring it back a little?

hey my 71 chevelle has a M22 4 spd with 26" tall tires and about 3000k at 100 KP/H any guesses on the rear gear i was told 373 in a 12 bolt ive been told that was a lie. thanks

I went to onlineconversion.com cool website, it says 100kp/h is about 62mph - thats almost exactly what im running, except ive got a stock 10 bolt rearend.

Phil, when you're running in 4th gear at speed, if you floor it, does the engine rev up faster than the car speeds up (like a stall?). That would indicate clutch slippage.

mine does not slip like that at hi speeds notasalbe at least altho the cluch ingages right at the top of the pedal throw but i really dont think it sliping onless it sliping the same amount all the time and it ingages enought to allow tire spin.

There is a slight stall, more of a backfire in 2nd if I gun it but im faily certain thats fuel related and timing issues, we wont go there... but in 4th on the freeway or streets if I gun it, it pulls right away - I'll make sure on the way home from work... if there isnt traffic :)

Here's something to kep in mind, with an automatic, the converter will also allow the motor to drop in RPM when there is no load. If you travel on relatively flat roads and let up, the car will not change that much speed, but you can drop a couple hundred RPM. A stick car will maintain consistent rpm with speed. I do agree that with a stick, there is ofen a perception that the motor is running at higher RPM.

What is the rpm difference, you said it is WAY higher. You do have a tach in this thing your looking at right?

Yeah Ive got a tach for sure! Im not crazy =) I never logged exactly what it was running with the automatic, its just very noticable on the tach, sound and feel all different in the fast lane. I just know it was less maybe around 2500, oposed to 3k

What exactly does the pan of the automatic look like, are you sure its a
th350? It could be a 700r4 or a 200 4r, overdrive transmission, they have both been around for 20 yrs. There is no way you are at 500 rpms more in
4th gear after no more than a trans swap. Physically impossible. Either you had an OD trans, or its not being shifted into high gear. Describe the auto trans pan to us.

The same thing happened to me.It just seems like the engine is screaming at 65 but it is the same rpm but the engine just pulls it down when I let off now.you can definitely feel the car more and I think it is just decieving.the auto wont engine brake as much as a stick.I have 3.36 gears and I hate to drive the interstate.I am switching to 2.56 gears since this is my daily cruiser.oh yea my car is a 67 chevelle wagon with a 396 4 speed .Just my 2 cents.............

I agree with blue66 - not possible to have a big drop like that from a TH350 to a stick unless the stick isn't in high gear.

I would vote that the speedo gear is different making the speedo read a lower speed than you're really travelling giving the illusion that the rpms are higher for the same speed.

Peter

Beg/borrow/steal a friend's GPS and get a reliable MPH reading. I agree with the others -- no way you could have a higher RPM in high gear with a M20 than a TH350.

-Dave

Well I dont look at my speedo often its been off ever since I put the meats on the back 2 1/2 years ago. Yeah thats fishy, I just know what you guys tell me here and from what Ive seen and experienced driving... transmissions and gears always throw me off a little bit. Dont they sell different size speedo gears for larger wheels, etc? I cant measure the pan I sold that transmission a few months back, but im without a doubt positive it was a TH350. When you say drop, do you mean increase? It seems higher with the muncie, I think I know what you mean though... Peter, when you say isnt in high gear what do you mean exactly? Just that im not fully in gear? Someone needs to take a test drive with me! :)

You'd have to be driving with the M20 in 3rd gear to have higher rpm's with the M20 than with a TH350. It is a 4-speed you know ;)

There are other speedo gears available to get the right ratio. If you look around you can find tables with wheel diameter and rear gear ratio that will tell you the right gears to use.

Peter