3.73 question

[Jay 02 TA ws6]

What is the MPH per 1000 RPMs in 4th gear with a 3.73 rear and the stock T-56?

[tabahr]

4th gear on a T56 is 1:1

so... 1000 engine RPMs would = 1000 driveshaft RPMs.

1000 driveshaft RPMs divided by 3.73 = 268.1 wheel RPM.

The MPH will depend on what size tire you are running.

[Jay 02 TA ws6]

I am using the stock wheel, 17", and a 40 mm sidewall tire.

Double the sidewall since it is on both sides - 80 mm = 3.15 inches... so total rotating diameter is 20.15 inches. Multiplied by pi and i get a circumference of 63.3 inches... multiplied by the RPM of 268.1 = 16,791.5446 inches per minute. Going to feet per minute, divide by twelve, and it becomes 141.295. Divide by 5280 to go to feet per mile and it becomes 0.26785897 miles per minute. Multiplied by 60 to get MPH and I get 16.1 MPH.

16.1 MPH per 1000 RPM???

That can't be right considering that 4th gear with the stock 3.42 gets about 22 MPH per 1000 RPM if I remember correctly. There has got to be something I am missing, there can't really be this much of a difference. That would mean 4th gear would top out at 96 MPH, and I would be shifting to 5th during a 1/4 mile run. With the 3.42 it tops out around 130... there has to be something I missed...

[Jay 02 TA ws6]

It's definitely flawed because when I plugged a 3.42 into the equation where you got the wheel rotations per 1000 driveshaft rotations, i got 292 RPM... when I put that through the rest of the process I got 17 mph per 1000 RPM with the 3.42 rear end, and I know that isn't right.

[KnightFire]

You can use this site:

http://www.f-body.org/gears/





KnightFire

[Jeff_in_Atl]

275/40/17
Diam. 25.7"
Revs Per Mile 811

[Joe 1320]

Jay, I don't know where you are coming up with a 40mm sidewall. A 275/40/17 has a sidewall that is 40% of the treadwidth, not 40 mm.

[Joe K. 99 Zeee!!]

About the calculator on the link....I think it's a bit off. It's saying that with my 26" tires (255/50 16's) and 3.42 rear gears my auto should be doing 81mph at 2500. In fact it's closer to 75mph.

What gives?

[quantum]

joe K.

what is your automatics gear that you put in. And what is the stall on your converter?

or by auto do you mean your car?

[Injuneer]

Tire diameter for 245/50-16, 275/40-17, 315/35-17 is approx. 25.65" to 25.68".

Example:

315mm / 25.4 mm/inch = 12.40" section width

12.40" section X 0.35 aspect ratio = 4.34" sidewall height

2 x 4.34" = 8.68"

8.68" + 17" = 25.68"

Joe K.

Did you make the minor correction to your speedo? That would cause it to read approx 1.5mph low due to your oversize tires. My personal spreadsheet, which seems to be accurate, agrees with the results of the link, for O/D on an automatic.

Jay 02:

My spreadsheet shows the following, for MPH/1000rpm:

Gear..... 3.42 ....... 3.73
1.......... 8.2 ........... 7.6
2......... 12.3 ......... 11.3
3......... 16.9 ......... 15.5
4......... 21.9 .........20.1
5......... 29.6 .........27.2
6......... 43.8 .........40.2

Quantum:

Say what?????????

[quantum]

He said "and 3.42 rear gears my auto should be doing 81mph".

I wasn't sure if he ment a auotmatic transmission, or if he ment automobile. And if he ment automatic transmission I was wondering what gears were in it and what the stall on the converter was. I may be wrong (and most likely am); but if the stall on the converter is 3500 and his engine is at 2500rpms he is loosing some rpms thought the converter.

[Jay 02 TA ws6]

Originally posted by Joe 1320
Jay, I don't know where you are coming up with a 40mm sidewall. A 275/40/17 has a sidewall that is 40% of the treadwidth, not 40 mm.

Hahahahaha... I thought the 40 was the height of the sidewall... okay... I feel embarassed Thanks.

[Jay 02 TA ws6]

Originally posted by Injuneer

Jay 02:

My spreadsheet shows the following, for MPH/1000rpm:

Gear..... 3.42 ....... 3.73
1.......... 8.2 ........... 7.6
2......... 12.3 ......... 11.3
3......... 16.9 ......... 15.5
4......... 21.9 .........20.1
5......... 29.6 .........27.2
6......... 43.8 .........40.2

Thanks a lot.

[Joe K. 99 Zeee!!]

Originally posted by Injuneer

Joe K.

Did you make the minor correction to your speedo? That would cause it to read approx 1.5mph low due to your oversize tires. My personal spreadsheet, which seems to be accurate, agrees with the results of the link, for O/D on an automatic.

My speedo is a little off since Superchips messed up my programer (whole nuther story). I'm using a GPS as a backup right now. So my measurement should be on.

On the web link I used the 93-97 Automatic selection with 26" tires (255/50R16=26.08") and 3.42 rear gears. That's my setup.

Just wondering why the inconsistancy 81 vs 75 is over 7% off. Is it because of the lack of a rigid connection with the Torque converter. I thought the stock converter locked above 2200RPM.