Ok, I am using way too much fuel, engine tuned very well, trans appears to be working well but I am suspecting torque converter slippage. Getting about 10mpg with conservative driving.

I have a TH350 with what appears to be a 3.55 diff, although I can not confirm that the ratios have never been changed, the wheels are stock alloys with 255, 60,15 tires, overall circumference is about 214cm's. I am only using my dash tacho and speedo, speedo is close to accurate compared to GPS, tacho could be out, dunno.

No matter what calculation I use I seem to come up with about a 4.80 diff ratio..... Is that even possible?

Readings are;

40Mph = 2250rpm
50Mph = 2900rpm
60Mph = 3400rpm

Converter slippage?
4.8 ratio?
Bad Tacho/Speedo?

What gives? Dennis.

 

Probably a 4:11, but why not jack the car up and turn the driveshaft to check? Turn the shaft and count the turns for one rotation of the wheel.

 

or a 4.56.

 

with a 4.56 you'd be doing 3200 rpm's at 60...

This link is for comparing a 200-4R with a th350 or 400.. see the 1:1 column....

http://2004r.com/speedoCalc/rpmcalc.shtml

 

Your '79 has a THM 350 3-speed without overdrive or lockup. If you are running 3400 rpm at 60mph [nearly 4000 at 70mph], you won't get much more than 10 mpg with that combo. Either change the diffy to 3.08 or 2.73 (for mileage) or install an overdrive/lockup tranny (to retain some level of performance).

 

Or a five speed for both performance and economy...and less wear and tear on the engine.

 

In a recent previous post about 79 performance I had a few guys flaming me because I stated that my 79 can burn rubber if I hold it in 1st gear anytime I want, they said it wasnt possible on a stock L48, however, if my diff ratio is crazy high like 4.56 wouldnt that translate to good low end torque but bad mileage? It would explain why I can light the tires when others think I am full of crap, it would also explain 10MPG fuel economy..... I am going to raise the rear end this weekend and turn the shaft and check this out once and for all.

Now if it turns out to be a 3.55 as marked on the housing then I must have slip in the torque converter surely? If so, does anyone know enough about torque converters to explain the fix for it without putting in a whole new tranny that I cant afford.

Thansk, Dennis.

 

Use the Bow-Tie Overdrives rpm calculator to derive your final drive gear. It does look like you are running a very deep gear. One way to detect if your trans is slipping is to accelerate rapidly and then slowly and measure your tach speeds vs. mph as the trans moves through the gears. Once my TH350 started slipping, I knew it was time. It would 'freewheel' when I really put the pedal down and you could tell that it was slipping since, even though the car revved quickly, the car was not moving very fast.

Your overall tire diameter (in inches) is 27.05". I think the Bow-Tie site requires the measurement in inches.

The Bow-Tie page with the speedo/rpm calculator links on it:

http://2004r.com/speedoCalc/speedocalc.shtml

If your trans is not slipping, your calculation is close at 4.8x. You can also test the final drive ratio by observation. With the rear wheels suspended (car on jack stands) and the trans in neutral, you can turn a rear wheel by hand and have somebody underneath watching the driveshaft for complete turns. One easy way to monitor the driveshaft is to mark it with a white chalk mark and then record the number of times the driveshaft turns relative to a full turn of the wheel/tire. My understanding is that if the driveshaft turns 4.8x times for every 1 turn of the tire/wheel, then your final drive ratio measurement is correct.

Some common gear ratios offered in the C3
4.10:1
3.73:1
3.54:1
3.33:1
3.07:1

Also, if you have access to an older engine diagnostic tool, you can verify your rpm. I have one of those 'Sears Multitesters' that has, as one of the tests, an rpm test. If the rpm matches your tach, at least you will know that your tach is reading correctly.

Question: Is your tach filter in place on/near the intake manifold? The tach filter looks like a 'condenser' from the old points type ignitions and has a wire entering one end and one leaving the other. It is normally in-line between the tach side of your HEI distributor (I note you have a '79 and I believe the HEI was first used in '74 vettes). The other wire connects to the wire that leads to the back of the tachometer. If the filter is missing or damaged, you will get an incorrect tach reading.

Also, have you verified your speedometer is reading correctly by having a friend drive ahead of you in a pace vehicle? This is an excellent/inexpensive way to verify your speed at the speedometer. Have them drive steady at 45mph, 55mph, 65mph and record your speed. If you are in Australia, you probably use kilometers but same idea.

Let us know what you find from the above tests.

 

Hi Ted, thanks for some good ideas and advice, I will put the car up on stands this weekend and test the ratio, I have tested the speedo against my GPS which is pretty accurate and it checks out pretty good, about 3% under which slightly improves my ratio calculation, the tacho i have not tested, however I work in a colege that teaches auto mechanics and have at my disposal a full workshop of equiment so I will take the car to work and test the tach against their workshop gear, that will also help the final calculations. The trans is very smooth and it is hard to tell the gear changes sometimes, especially from 2nd to 3rd, I doubt it is slipping but I still suspect it may be. No I dont have the filter you refer to, I will look in to that. Thanks, Dennis.

 

If the tach filter is missing, you will get an excessive (high) rpm reading on your tach. The tach filter is typically located/attached to a metal clip on the intake manifold just forward of the distributor on the right hand side. I've read some articles where you can actually fabricate a tach filter from about $3 in Radio Shack resistors, capacitors, wires and shrink wrap. I built two of these but they kept failing. Some have managed to fabricate pretty reliable filters by encasing the assembly in epoxy (resists the vibrations, underhood temps & corrosives). Also, if you choose to buy a new tach filter, you can actually buy one from the corvette suppliers. I believe Ecklers, Corvette America and Zip offer this device and it has the same appearance as the OEM filter. I believe this item is available for approx $30-40 USD.

From my experience with the TH350 in my '80 L48, the trans does shift smoothly from 2-3. Typically, if not under heavy acceleration, it will shift so easily that you detect only the change in rpm; no hesitation.

Mileage can vary. Depending on the state of engine tune, I've seen anywhere from 10mpg (early when I first bought my L48 in '97) to as much as 17mpg with my 700R4 transmission. I don't recall ever getting better than 15mpg with the TH350 installed (I had the stock 3.07 ring/pinion).

Let us know what you find out.

Here are a few sources for tach filters if you find you need one:

http://www.ecklers.com/product.asp?p...7&dept_id=1135

http://www.parts123.com/parts123/yb....P39.95~~~~~~~F

http://shop.corvettecentral.com/C3-1...-p8238568.html

http://cgi.ebay.com/ebaymotors/ws/eB...d=350326573524

This is the tach filter wiring diagram from a corvette forum poster if you choose to fabricate:

http://forums.corvetteforum.com/phot...05/ppuser/3648

If you have access to eBay Motors, do a quick search on 'tach filter' and you will find a number of used items available. That's how I got mine from a mid-80's Monte Carlo/Buick Regal/Olds Cutlass.

 

when i had a th350/3.55 combo it the engine would rev so high that i wouldent be comftorable leaving the slow lane on the highway, 60 seemed to be about the top speed. it would get about 12 mpg. the tack on my car seems estreemly high for what i know its spinning, i dont know why its high but i'd guess that yours might be off too.

 

That's the beauty of having a 700R4 with the 3.54 gears. It drops the rpm in 4th/OD by about 34% from that of the final/3rd gear in the TH350. Before installing my 700R4 and the new diff with 3.54 gears, I had the TH350/3.07 gears (OEM).

However, even with 3.54 gears and a TH350, you really shouldn't be getting excessive rpm's. I remember pulling about 3000rpm at 55mph with 4.10 gears and a TH400 in a '74 Z28 (tires were the G60/15 bias ply tires of the day). My '71 Nova had TH350/3.54 gears and averaged about 2,500rpm at 55mph (F70/14 bias ply tires). I was easily able to accelerate to over 100mph even with a smaller 283ci engine.

 

I have been meaning to do that as well on mine. I remember there being discussion about if both wheels need to be off the ground to check the ratio, shouldn't it just be one? Or am I thinking backwards (normal for me). Thanks

 

Here's a C5 6-speed with 6th gear at 50% overdrive, no wonder these things get 28 MPG Highway... they came stock with 3.43 which at 50% overdrive is a 1.715 final drive ratio...

 

I've always jacked up both tires, actually I prefer to do it on a lift, so I can easily turn the drive shaft and count the turns. I make a chalk mark on the driveshaft and the tire. With the car in neutral, I turn the driveshaft and count how many times it takes to turn the shaft for one revolution of the tire. I usually make 2 complete turns of the tire and average out the results. I've found this works better for me.

Unless someone has changed the rear in your 81, it will either have a 2.72 gear with a 4 speed, or a 2.87 with an automatic. There was no optional ratios in 81.

 

There have been odd things about this car that some previous owner did and makes me wonder about the rear gears as well. The motor has a slight lope at idle and has been gone into at some time or another and with a proper tune really gets up and scoots without a millisecond of hesitation, so I suspect a slightly larger than stock cam, it has an aluminum intake, not sure if 81's came with an aluminum intake or not. So I am not so sure about the rear end, but might as well do a quick check, thanks!

 

 

Just to compare, my car has a 3.31 ratio, stock tire sizes and I get ~3000 RPM at 60 MPH and ~3500 RPM at 70 MPH... You either have very small tires or a dragstrip-ready rear end ratio!

 

Depending on your tire size, your numbers are more in line with a 3.91 or 4.10 gear.