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MD
Dodge Dakota
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3/21/2003
00:43:10
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 Subject: Tire Dia/ratio/mph=RPM Question
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I recently changed gears from 3.55 to 4.10's in my 2000 4.7, automatic Dak. Using the calculator at Randy's Ring & Pinion, and plugging in a 28" tire, 4.10 ratio, 65 mph, and the overdrive ratio for the trans (.69?), I come up with a tad over 2200 RPM. In the real world my tach shows 2500 RPM. Isn't there an electronic, perhaps mechanical, compensating system on these trucks that does compensate for a gear ratio change?
Any thoughts?
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Chris
Dodge Dakota
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3/21/2003
12:13:05
|  RE: Tire Dia/ratio/mph=RPM Question
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Dag Nabbit, Randy's is right. I jsut did these same calcs and got 2207 RPM (rounded up). BUT if I put in the 3.55 gears i get 1910 RPM and 4.56 gears gave me 2454 RPM.... Maybe you have the 4.56's.. I also rememebr someone saying that the newer dakes (2000+) only measure wheel RPM not driveshaft roatations meaning only tire changes affect speedometer accuracy....
BUT at 2500 RPM you should be going 85MPH with the 3.55's and 74 MPH with the 4.10's. Have someone pace you and see if this is right. ALSO remember that with an automatic there are some losses in the convertor, even when locked. Not sure how many hundreds of RPM this could be but I can tell you that Randy's and my calculator will not account for this...
Hope this helps.
Chris
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jaggy
GenIII
3/21/2003
18:18:33
| RE: Tire Dia/ratio/mph=RPM Question
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I made a spread sheet a while back based on the 355 gears and a manual transmition.
the #'s for the 3.55 are acureate.
acording my calculation it would apear that you would have the 4.56 gears if it were a 5 speed.
there is a calculation you can do to find your final gear drive. I can't remember off the top of my head but a little searching on google and you will find it. Hope this helps.
5th gear
3.55 3.92 4.11 4.56
1000 35 32 30 27
2000 70 63 60 54
3000 105 95 91 82
4000 140 127 121 109
4500 158 143 136 123
5000 175 159 152 136
6000 210 190 182 164
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Brian 02 R/T
Dodge Dakota
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3/21/2003
18:21:31
| RE: Tire Dia/ratio/mph=RPM Question
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www.speedworldmotorplex.com/calc.htm
They have a calculator that will tell you mph tire size etc.
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tweaked
Dodge Dakota
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3/22/2003
11:16:28
| RE: Tire Dia/ratio/mph=RPM Question
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hey md what did you do with your 355s? i was wondering if you want to sell them, because i have 3.92s and want better gas millage and longer gears, let me know or anyone else who wants to sell some for that matter if you want to sell them, thanx
also how hard is it to do your self? or should you have a shop do it, thanx
dave
aut0maticai@aol.com
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MD
Dodge Dakota
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3/22/2003
14:55:39
| RE: Tire Dia/ratio/mph=RPM Question
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No, Chris - they came sealed from Dodge - 4.10's - I counted the teeth.
Tweaked - you wouldn't notice enough difference in mileage to make the change worthwile. And, unless you have changed out gears many times and have become proficient at it, there is no way you should attempt the swap yourself.
BTW - tweaked - you decide not to get the OEM swaybar?
MD
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tweaked
Dodge Dakota
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3/22/2003
22:34:19
| RE: Tire Dia/ratio/mph=RPM Question
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yeah it would be too dificult to install, but thanx anyways
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POWER HUNGRY
Dodge Dakota
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3/23/2003
13:09:12
|  RE: Tire Dia/ratio/mph=RPM Question
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MD,
With your set-up, 4.10s and 28"tires you should be turning around 2400rpms at 65 mph. The reason being that your trans has a .75 OD, while the 46re has the .69 OD that you used in you calculations. Krc performance has a box that adjusts for different tire size but I hear that the speedo reading isn't affected by gear changes in the gen 3 trucks.
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MD
Dodge Dakota
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3/23/2003
16:43:29
|  RE: Tire Dia/ratio/mph=RPM Question
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Thank you, PH, for clearing that up for me. I knew there had to be a logical explanation for the RPM levels. I used .69 for the assumed final drive ratio because I had seen that number listed on this site. Is the 46 trans used in the RT - what is the number of the trans I have with the 4.7?
Thanks, MD
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tweaked
Dodge Dakota
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3/23/2003
16:47:02
| RE: Tire Dia/ratio/mph=RPM Question
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hey jaggy how accurate is your numbers up there for higher than 3.55, because i thought i had the stock 3.92s from the dealer because i bought it used there, but when im at 1000 rpms in fifth im only going 30, not a tick more, so according to that i have 4.11s, but i guess i should take it somewhere to have it looked at, because i really want something smaller, i cant stand shifting so quick and first gear only going to like 20, so let me know and thanx in advance
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POWER HUNGRY
Dodge Dakota
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3/24/2003
14:22:22
|  RE: Tire Dia/ratio/mph=RPM Question
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MD,
Trans Model First Second Third Fourth Fifth
45RFE 3.00 1.67 1.50 1.00 0.75
The r/t and 5.2 46re trans is:
2.450-1st, 1.450-2nd, 1.000-3rd, 0.690-fourth
46re and 4.56s= 11.17:1 1st gear and 3.14 OD
45fre and 4.10s= 12.3:1 1st gear and 3.07 OD
As you can see you can use 4.10s where the r/t need 4.56s because of thier trans' lower multiplication.
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MD
Dodge Dakota
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3/25/2003
11:15:01
| RE: Tire Dia/ratio/mph=RPM Question
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Thanks, PH.
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Texas Todd
Dodge Dakota
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3/25/2003
13:03:34
|  RE: Tire Dia/ratio/mph=RPM Question
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jaggy,
Your rpm/gear/speed chart-
I'd have to say, for the 3.55 5spd manual,
I don't quite agree with it.
I have the 4.7, Quad, 5spd
at 2000rpm, in 5th gear, my truck goes about 65mph., not 70. and at 21500, it does 70mph.
Thats with the factory tire and handling pack, which are lower profile/wider tires, I thought had the same overall diameter.
Waht do you think?
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