MPH/RPM Calculation

[RedHeartbeat]

I was trying to figure out what gears to get for my truck and had to work out this formula. You can enter any transmission or rear end gear or tire size.


PI of course is 3.1417..... use the button on your calculator or in windows calculator.

Gear ratios of a 4L60E:
1st: 3.06:1
2nd: 1.625:1
3rd: 1:1
4th: .696:1

Calculate tire rollout
Tire width in inches..divide width(275 of 275/60/15) by 25.4 on metric rated tires
Aspect ratio is the 60 in the above example
rim size is the 15 if you didn't know

(2 x (Width X Aspect ratio)) + rim diameter = tire diameter
tire diameter * PI = tire rollout distance

divide this number by 12 to use in the formula so we will be working in feet.

You can figure this manually by placing a mark on the ground and straight up on the tire, then drive the vehicle forward until the mark is straight down again. Take the trusty tape measure and get your measurement in feet for more accuracy.

Now comes the fun part......

Engine RPM / Read End Gear / ratio of Transmission gear you have selected * tire rollout in feet / 5280ft * 60minutes = miles per hour in that gear

I have 32/10.5/15s tires with 3.07 gears

In 2nd gear calculation:
6000rpm / 3.07 / 1.625 * 8.38 / 5280 * 60 = 114.5 MPH at 6000 rpm in 2nd gear

If you found the rollout dimension using a tape measure....
diameter = rollout distance in decimal feet * 12 / PI

Last formula is great for recalibrating the PCM for added accuracy.

If someone wanted to put this excel it would be much easier to use.

[creederado]

I don't know if this will help;

http://www.randysringandpinion.com/calcula...ulatorsind.html

[RedHeartbeat]

Hey, now that's handy. You gotta be sure to check you tire roll out though since most tires are smaller in diameter than advertised.

[blk03]

yeah mine rolled out to just over 30" but if you measure with a tape measure they are almost 31.5

[RedHeartbeat]

Here I put the data into an excel spread sheet. It will calculate tire diameters from metric sizes or diameter from the rollout length. You can change gears in the rear end and transmission. You can set the shift rpm you want with all gear and tire data entered and see what the rpm will be for the next gear at the shift point.

I must have too much time on my hands.

well, shoot, it won't let me upload an excel spread sheet. If you want the file, just pm me.

[GMCwantsLS1]

Try this! http://www.4lo.com/

[RedHeartbeat]

yeah, that one was good too but doesn't show you what rpm you will be at when you shift to the next gear. The excel spreadsheet I did will show you that plus so much more. I just created it for kicks and kept adding every thing I could think of as a variable to the spreadsheet and it is interactive with most fields.

Anyone have an idea why we can't upload a excel *.xls format file?