96Firechicken

I was reading through this forum and found a lot of people wanting to know how changing the gear in the rear end or putting different size tires will affect MPH. I couldn't find a thread that had a way to determine exact mph ( a few said add or subtract 3-5 mph )for a new gear or tire so I figured I would make one.

This is also useful if you have changed your tire size or gear but have not updated the pcm and want to know how fast you are really going.
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Determining New Speed if you change your tires

Speed * (New Tire Diameter / Current Tire Diameter)

Example: Current Tire 245/50/16 , New Tire 275/40/17 At 60 mph

60 mph * (25.66 / 25.64) = 60.04

* Note: These tires, although different sizes, pretty much have same overall diameter due to the different aspect ratios, and will not cause a change in the speed

The diameter can be in any units, inches, feet, mm, as long as the same units are used for the new and current tire.
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Determining Tire Diameter

First find the tire size data on the tire's sidewall.

Then use this equation to find the tires diameter
Dia. = (2*tire width * aspect ratio)/25.4 + rim diameter (inches) , then divide that by 12 to convert to feet.

Example for tire that is 275/40/17

(2*275*.4)/25.4+17 = 25.66 inches = 2.138 feet
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Determining New Engine RPM for a New Gear

If you know how fast your car goes for a specific engine RPM then you can determine what engine RPM will produce that same speed for the new gear you are putting in.

Example: Assume that at 80 mph your engine is at 2000 rpm with a 3.42 gear and you want to know what the engine rpm will be at 80 mph for a 4.10 gear.

RPM *(New gear / Current gear)

2000 * (4.1/3.42) = 2397, about 2400 rpm.

Or if you want to determine what speed the new gear will produce with the same engine rpm i.e currently 80 mph @ 2000 rpm with 3.42, what will new speed be @ 2000rpm with a 4.10 gear

MPH (Current Gear / New Gear)

80 ( 3.42 / 4.10 ) = 66 mph

If you don't know how fast your car is going at a particular engine rpm keep reading.
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In order to determine how fast (MPH) your car will go at a certain engine rpm you need to know what gear you are in and what it's ratio is, the rear end gear, and the tire size (diameter).

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Transmission Gear (Data from f-body.org technical database)

Model 1st 2nd 3rd 4th
4L60 3.06 1.62 1.00 0.70
4L60-E 3.06 1.62 1.00 0.70


Borg-Warner T5 5-Speed Manual
Year 1st 2nd 3rd 4th 5th
All 2.95 1.94 1.34 1.00 0.74

Borg-Warner/Tremec T56 6-Speed Manual
Year 1st 2nd 3rd 4th 5th 6th
1993 3.36 2.07 1.35 1.00 0.80 0.62
1993 2.97 2.07 1.43 1.00 0.80 0.62
1994+ 2.66 1.78 1.30 1.00 0.74 0.50

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Determining Speed

MPH = Engine RPM / trans. gear ratio / rear end ratio * tire diameter(feet) * .0357

*Note, .0357 takes into account converting from RPM to MPH as well as tire diameter to tire circumference.

Example: 2000 rpm in 6th gear (.5 ratio) in a 94+ T56 with a 3.42 rear end with above tires, 275/40/17

2000 / .5 / 3.42 * 2.138 *.0357 = 89 mph

Sorry that was pretty long. I hope this helps some people who are thinking about putting a new gear in and are concerned about how it will affect cruising speeds/ engine rpms.

alamantia

iTrader: (10)

Algebra for the win!

phantomzer0

iTrader: (17)

Sticky for teh newbs!

JFM-jr

iTrader: (23)

Stalled A4's throw these formulas off correct?