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Custom Dodge Ram Performance Mods - Engine - 5.7 HEMI V8 Discuss modifying your Dodge Ram with Performance Parts and Accessories!
Factory Spec: 5.7-liter HEMI® V8 engine - 390 horsepower, 407 lb-ft of torque.


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Old 03-16-2011, 12:31 AM
Red05Ram Red05Ram is offline
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Default Speedometer calibration after gear swap?

I have a 93 trans am and when I got higher gears my speedometer got thrown off; will this happen to me when I swap from 3.55 to 4.56? THANKS!
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Old 03-16-2011, 12:47 AM
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Yup, you bet it will!
About 20%. You will be going slower than the speedo says....about 20% slower.
(If speedo shows 60, you will be moving about 48.)

Best part, you can drive way over the speed limit (in your mind) and never get a ticket!
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Old 03-16-2011, 03:57 AM
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20% for a 3rd Gen? Had 2 gear swaps and my RPM changed but not my Speedo.
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Old 03-16-2011, 04:10 AM
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That's what I've heard that it reads it from somewhere else so it won't change, anyone else? Lol
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Old 03-16-2011, 04:32 AM
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For a 3rd gen 1500 9.25 Corp Rear i always thought the Speedo is calculated through the ECM / ABS Sensor and not the Transmission.
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Old 03-16-2011, 09:00 AM
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There are options in most programers to change your gear size so the speed will be set correctly so im assuming the gears effect the speedo.
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Old 03-16-2011, 09:02 AM
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Interesting....
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Old 03-16-2011, 09:16 AM
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To my understanding, the gears will affect the tachometer, but tire size swaps will change the spedometer. Correct me if I'm wrong, but the speed sensor is on the rear axle, so it's independant of the gear ratio, the speed the axle shaft rotates x the diameter of the tire gives you the speed. The gear swap should play merry havoc with the tach though, and this can be fixed using a programmer easily enough.
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Old 03-16-2011, 02:27 PM
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Quote:
Originally Posted by CdnoilRAM View Post
To my understanding, the gears will affect the tachometer, but tire size swaps will change the spedometer. Correct me if I'm wrong, but the speed sensor is on the rear axle, so it's independant of the gear ratio, the speed the axle shaft rotates x the diameter of the tire gives you the speed. The gear swap should play merry havoc with the tach though, and this can be fixed using a programmer easily enough.
But when you change your tire size you essentially change your gear ratio and this messes up the speedo? So if you change the gears it should have the same effect?
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Old 03-16-2011, 04:03 PM
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No, if you change the tire size, you are changing the how far the tire travels for every revolution. So a taller tire will travel faster at the same RPM than a smaller one, this is what messes up a speedometer as it's set at a specific tire diameter. A gear ratio will actually slow down the rotational speed of the tire, making it travel less distance. Example:

A stock 17" rim has a 29.5" diameter tire. The circumferance is pi*D = 92.69 inches. This means that at when your rear axle shafts rotate at 1000 RPM (this is NOT the engine RPM) vehicle travels a total distance of 92689 inches in 1 minute OR 1.462 miles/min OR 87.72 MPH.

Now you upgrade to 33" tires. The circ is pi*D = 103.69 inches, and again at 1000 RPM (rear axle shafts) you will travel 103686 inches in 1 minute OR 1.636 miles/min OR 98.16 MPH.

These speeds are INDEPENDANT of the gear ratio as they are measured by the speed sensor ON the axle shaft, and as such are the absolute rotational speed of the tire being turned.

If your tires are spinning at 1000 RPM, and you have a gear ratio of 4.10, then your transmission running at a 1:1 (3rd gear) ratio for simpliciy's sake, will require the engine to run at 4100 RPM.

Again, if the tires are turning at 1000 RPM and the gear ratio is 3.55, then your transmission running at a 1:1 (3rd gear) ratio for simpliciy's sake, will require the engine to run at 3550 RPM.



To answer you RamTex, you have the right idea, but like most people, you need to look deeper at the relationship between tire size, axle ratio, and engine RPM. A bit of clarification for everyone (I am keeping the transmission's ratio as 1:1 to simplify calculations. Most cruising gears are very close to 1:1, then O/D gears are less ex. 0.45:1)

If you change the TIRE SIZE but keep everything else the same:
For the same RPM of your engine and the same gear ratio; you will travel further on a taller set of tires. The speedometer is messed up because the computer is programmed to take X RPM (axle) and Y diameter tire to output Z speed. If you change the tire size and do not correct (suddenly using M size tire), your speedometer will be off, but your engine RPM will not be affected. Having said that, to travel at the same SPEED, your engine RPM will be higher with the taller tire. This is all with the same gear ratio.

If you change the GEAR RATIO but keep everything else the same:
Keeping the RPM of the engine the same, with the 1:1 transmission ratio (~3rd gear), to get your axle rotational speed: take X RPM (engine) and Y gear ratio to give you Z RPM (axle), then you take Z and multiply it by the tire circumference to get total speed. Again, to keep the same SPEED, if you change the gear ratio, your engine will be required to turn slower with a taller set of gears.




If you change to a larger diameter tire, and want to increase the RPM that your engine needs to run at for a certain speed, then you can change the gear ratio. So, if your prefered speed is 60MPH (for fuel economy), running 32" tires (stock 20" rims), with a 3.55 Gear ratio, your engine will be turning at 2230 RPM. If you upgrade to 35" tires, when cruising at 60MPH, your engine is turning at 2050 RPM, which takes you out of the optimal power band of the motor. In order to correct this, you need to increase your gear ratio to compensate. A 3.71 ratio brings you to 2130 RPM, a 3.92 to 2250 RPM, a 4.10 to 2360 RPM. This allows you to adjust your power band to match your prefered speed.

I hope this helps
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Last edited by CdnoilRAM; 03-16-2011 at 04:12 PM.
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