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Just wondering because at http://www-5.dodge.com/vehsuite/VehicleCompare.jsp they list the rear axle ratio as 2.82 but at the bottom of the page under the powertrain tab they list the final drive ratio as 3.55 for the hemi.
the rear axle ratio is just that...Stonebreaker said:Just wondering because at http://www-5.dodge.com/vehsuite/VehicleCompare.jsp they list the rear axle ratio as 2.82 but at the bottom of the page under the powertrain tab they list the final drive ratio as 3.55 for the hemi.
What do the tires and the tranny have to do with the final drive ratio?DAYTONA_R/T said:the rear axle ratio is just that...
final drive ratio is the ratio of everything, including the tranny, rear end, and tires....
coming from the "truck world" yes, tire size affects final drive ratio... on cars you normaly dont drastically change the overall diameter of the tires that this would effect any changes you have made to a vehicle...maneval69 said:I don't think tire size is typically included in calculating the final drive ratio. They are not part of the drive train.
taken fromChanging tire size affects the final drive ratio. Switching from a 30" tire to a 35" tire changes the final drive ratio by about 17%. This may drop the engine out of its' "power band" and result in poor performance and fuel economy. To restore performance you must change the gear ratio to compensate for the change in tire size. If you originally had 3.07 gears you need a ratio that is approximately 17% lower, such as 3.55. If you want to increase off road performance you might want a 4.10 or lower ratio.
taken fromFinal Drive Ratio: The final drive ratio will reflect how many times the countershaft will rotate for every rotation of the rear wheel. The greater this number, the lower the gear. As the gear gets lower, ability to accelerate goes up and peak speed goes down.
Yup, exactly right.DAYTONA_R/T said:taken from
http://www.peterverdonedesigns.com/fdrselection.htm
Now keep in mind, a change in tire size will change the "rotation of the rear wheel" which in turn will change the final drive ratio
do I need to post more examples?
if you want to see the revolutions per mile of your tires... you can use this like http://www.miata.net/garage/tirecalc.html (ignore the miata thing... tire sizes are avalible for cars and trucks on it...)
wrong! tire size will effect the distance traviled by one rotation it DOES NOT effect the ratio that the tire turns in relation to the crank therefore it does not affect final drive ratio.Now keep in mind, a change in tire size will change the "rotation of the rear wheel" which in turn will change the final drive ratio
But, using your math, means the engine crank spins 10 times for every one turn of the output shaft- the very definition of "overdriven" - in first gear! wouldn't the reverse be true? I'm so confused!! It would seem that there are many diffrent definitions to "final drive ratio".maneval69 said:Final drive ratio is how many rotations the AXLE makes compared to the crankshaft.
Tire size doesn’t matter. If you have 25 “ tire or a 30” tire, the tire will rotate at the same rpm as the axle. (if you are calculating performance the tire size is always a different entry than final drive ratio)
Chargershed,
You have it backwards. The ratio is always (input)output). The rear gear is 2.82:1, the transmission in 5th gear is .83:1. Overdrive spins the output shaft faster than the input shaft or engine rpm. The .83 ratio means that for every 1 rotation of the output shaft (drive shaft) the input shaft (crank) spins .83 times.
So running a 2.82 rear gear in a .83 overdrive gear would give you the same final drive ratio as running a 2.34 rear gear and our trans in 4th (which is 1:1 gear)
I may have missed the boat on what final drive ratio is referring to but I’ve always referred to it as the ratio between the crank and the axle = drive-train. And you have a final drive ratio in every gear.
Here are our transmission gear ratios; 1st =3.59 2nd =2.19 3rd = 1.41 4th = 1-1 5th =0.83.
You multiply those by the rear gear 2.82 to get each gears final drive ratio.
And here are the r/t’s final drive ratios for each gear.
1st -10.12
2nd -6.18
3rd -3.98
4th -2.82
5th -2.34
Damn, I didnt want to get in the middle of all this, but maneval69, I think you were right in the first place, (input : output). In 1st gear, the crank spins 10.12 times per 1 revolution of the axel. Thats why you are only doing a slow speed at 5500rpm in 1st gear (rear axel is spinning at 592.88 revs). Switch to 2nd gear and you can go faster at the same RPM because the final drive is now 6.18 cranks to 1 axle rev (@5500/6.18 = 898.69 axle revs). Same for the other gears, 3rd (3.98) gets 1381.91 axle revs, 4th (2.82) gets 1950.35 axle revs, and 5th (2.34) gets 2350.43 axle revs.maneval69 said:Good catch,
I miss-typed it should be (output:input).
In first gear the crank spins 10.12 times for every 1 rotation of the tire.
I have edited my earlier post.maybe it make more sense now.
Thanks
They do agree with you as to the meaning of final drive ratio.Changing tire size affects the final drive ratio. Switching from a 30" tire to a 35" tire changes the final drive ratio by about 17%. This may drop the engine out of its' "power band" and result in poor performance and fuel economy. To restore performance you must change the gear ratio to compensate for the change in tire size. If you originally had 3.07 gears you need a ratio that is approximately 17% lower, such as 3.55. If you want to increase off road performance you might want a 4.10 or lower ratio.
From these definitions of final drive ratio, the wheel diameter will change the rotation of the wheel as it relates to speed of the car but does not change the wheels rotation in relation to the crank. An important point.Now keep in mind, a change in tire size will change the "rotation of the rear wheel" which in turn will change the final drive ratio
________Just wondering because at http://www-5.dodge.com/vehsuite/VehicleCompare.jsp they list the rear axle ratio as 2.82 but at the bottom of the page under the powertrain tab they list the final drive ratio as 3.55 for the hemi
Damn-it your right!CO-Charger said:Damn, I didnt want to get in the middle of all this, but maneval69, I think you were right in the first place, (input : output). In 1st gear, the crank spins 10.12 times per 1 revolution of the axel. Thats why you are only doing a slow speed at 5500rpm in 1st gear (rear axel is spinning at 592.88 revs). Switch to 2nd gear and you can go faster at the same RPM because the final drive is now 6.18 cranks to 1 axle rev (@5500/6.18 = 898.69 axle revs). Same for the other gears, 3rd (3.98) gets 1381.91 axle revs, 4th (2.82) gets 1950.35 axle revs, and 5th (2.34) gets 2350.43 axle revs.
Of course this is all theoretical, the actual speed @5500 in 5th the variables of drag, air density, real HP, TQ slip, etc come into play as to how fast the car would be going.