bashcraft

When I ran the 4.56's, it was 3300-3400 at 60 mph with an M-20 trans.

Did you have an automatic with a high stall converter?

BKarol

I have 255/60/15s, close ratio 4spd and 3.70 rear.

At 70mph the tach reads @3400

At 60mph the tach reads @3200

If you enjoy watching your fuel needle move while doing highway driving change to 3.70

KevinK

I have 3.08 rear gears with a 4 speed, and at 60mph, I'M at about 2,600rpms......so I'd say he has 3.08s already. Top speed is about 120.

I'm in the same boat....and the only REAL solution is a 5 speed....or 6speed. Someday...

steersdad

Hey Mike,
Sorry, but tire size does directly affect speedometer readings. The speedometer reads the speed of the outpt shaft of your transmission.

Different diameter tires = different rollout. (taller tires roll a greater distance than shorter tires). The speedometer gear doesn't know what size tires you have. This is way speedometer gears have different number of teeth, dependant upon REAR tire diameter.

If you have a tire with 10' or rollout (circumference) and exchange it for a tire with 5' of rollout, your rear tires will have to rotate twice to go the same distance. Your speedometer will read twice as fast with the shorter tire than the taller tire.

kellyr073

I have a 3.36 rear, manual trans, and top speed is 130...so far.

bashcraft

Those numbers work out to a 3.36 rear gear.

jb78L-82

Wrong-Tire overall diameter absolutely effects speedo reading and engine rpm's. Smaller diameter tires than stock overall diameter will raise the rpm's and artificially increase the car's actual speed and vice versa.

77/78/79 with 4 speed and 3.70 gears turn 2860 at 60 mph and 3,400/3,500 at 70 mph. Top speed is 127-130 at 6,000 RPM in 4th gear.-Car and Driver, Dec 1978

kellyr073

Check out to www.wheelsmaster.com, and plug in your tire size and another size to get the comparison on a multitude of things, including the difference in your speedo at 60 MPH. Click on the "wheel size" in the upper right

LancePearson

I was thinking that rather than get into the engine on my l48 which works very well to get a little more punch more like my other vehicle with the LS 2 2008 and 4:10 in it I might go from the 3:36 rear end up to the 3:70 rear end if and when I need to get into the differential for anything at all. Won't be the same but acceleration will definitely be faster for normal driving at the cost of more highway rpm. Not worth doing unless I have to get into the differential anyway but it is a thought. The punch of the LS2 with the 4:10 is undeniable....course it's got 200 more horsepower too...

Lance P.

Red 71

Quote:
Originally Posted by Mike Ward View Post
Tire size has no effect on speedometer readings.This is a old thread but I had to clarify a few errors. I am surprised that Mike did not come back on this.

What Mike said is correct if you think about it.

1) If you are in 4th gear at 1:1 with a 3.08 rear gear and a 225X70X15 tire at 2000 RPM your speedometer should read 52.7MPH IF YOU HAVE THE CORRECT SPEEDOMETER GEAR INSTALLED!

2) Now put the car on jack stands and remove the rear tires. Bring the RPM up to 2000 in high gear at 1:1 and the speedometer will read 52.7MPH How can that be, there is no tire on the car?

3) Now we are going to change the tire diameter and NOTHING ELSE!

4) If you are now in 4th gear at 1:1 with a 3.08 rear gear and a 255X70X15 tire at 2000 RPM your speedometer should read 52.7MPH because it does not know that the tire diameter is now larger!

5) Install the correct speedometer gear and the speedometer will now read 56MPH at 2000RPM at 1:1 in high gear at 2000RPM because now it knows that the tire diameter is different.

So in summary, changing the tire diameter alone does not change the speedometer reading at a given RPM. The speedometer reads what it is calibrated (speedo gear) to read at a given RPM for a particular tire size.

iokepakai

Richmond 6spd trans will cure those hiway blues!
Best upgrade on my C3...You'll need to know your rear gear ratio to get the right first gear & 6th OD ratio's.Example my rear gear is a 3.08 ,the trans has a 3.27 first for great hole shots.My 6th gear is a .76 which still keeps the hiway rpm's low and not lug the engine when cruising.
[IMG][/IMG]
Not cheap but I'm very happy with this trans...Did a swap from a TH350....

SuperBuickGuy

not necessarily. I had 3.73 gears behind a 455 Buick (Skylark), 4 speed, 455.... 3.23s and I was faster 60', 1/8 and 1/4. With 3.73s I could leave in 2nd gear and 3500 rpm. 3.23 I could leave at 3800 without going up in smoke - and not have to use 4th when I went through the lights

Red 71

Somewhat similar to your gearing but mine is a TKO with a 3.27 low and a .68 5th gear, 3.08 rear gear. Recently I had to install a new rear oil seal and decided to use a 1 piece gasket. To do it all correctly I pulled the timing cover and discovered that the timing chain had a little too much slop in it. I installed a new gear set that had the option of 2 degrees advance for better low end torque, 0 degrees and 2 degrees retarded for high RPM torque. I went with the two degrees advance and considering that I had a little slop in the old chain, I probably advanced the timing 4-5 degrees. Now I have plenty of torque at 50-60 MPH in 5th gear. Something to think about for those that need to install a new timing gear set. (350cu, 270HP)