Tire Size = Rpm

[tragicwagon]

What is the formula to find out the rpm with diffrent tire sizes,and gear ratios?





Most of my driving is highway miles,pulling a trailer,GCVW 26,000,the truck pulls hills very well with this combination,but when empty at



75mph-2200rpm = 16mpg,



68mph-1900rpm = 19mpg.



What I would like to do is run a taller tire so that when Im at 75mph the rpm is at 1900 so that maybe it will get 19mpg, is this possible or would the tire have to be huge to achive this ratio? Also would it have a drastic effect on pulling? And finally how do you get the speedometer recalibrated? TIA.

 

[Chipstien]

What is your current gear ratio and tire size? As far as the formula... Somebody will post it as I can't remember the web site.

 

[Cummins4Life]

This might help:

http://www.quiknet.com/~ke6vut/rpmcalc.html I dont know the formula but this does it for ya.

 

[tragicwagon]

Thanks for the link,but I dont know what 5th gear ratio is in a five speed,rear gear is 354 and tire size is 235-85-16. sorry about the lack of info I forgot to click (show sig).

 

[Cummins4Life]

www.dodgeram.org

5th is . 75 check out:http://www.dodgeram.org/tech/transmission/nv4500_spec.htm

 

[HEMI®Dart]

More good info from Fritz's web page:



http://www.dodgeram.org/tech/specs/axle/rpm_vs_mph.htm

 

[Double 6]

Here is another web site for figuring out tire size, RPM's, and a whole bunch of other calculators, is real easy to use.

www.4lo.com

 

[Loren]

Tragicwagon,



I think you will find that trying to change your gear ratios significantly via the tires won't work well. I have about the biggest tires you can put on our trucks without worrying about something rubbing. Here are the numbers I got with my truck:



Stock Aluminum Wheels:

Michelin LTX A/S 265/75R16 - Load Range E



MPH RPM

50 1375

55 1500

60 1650

65 1800

70 1950

75 2125





Rickson Wheels:

Toyo M-608Z 245/70R19. 5 - Load Range G



MPH RPM

50 1300

55 1450

60 1600

65 1750

70 1900

75 2050



As you can see, the RPM's at 75 mph only changed from 2125 rpm to 2050 rpm. I did not see any change in my fuel mileage. My odometer/speedometer was corrected for the new tire size.



If you want a substantial change in your gear ratios, you'll need to add an overdrive transmission. I hope to do that someday, but not soon.



Here are the transmission ratios, in case you need them:



47RE automatic

1st 2. 45

2nd 1. 45

3rd 1. 00

4th 0. 69

Rev 2. 21



NV4500 HD (5-speed)

1st 5. 61

2nd 3. 04

3rd 1. 67

4th 1. 00

5th 0. 74

Rev 5. 61



NV5600 (6-speed)

1st 5. 63

2nd 3. 38

3rd 2. 04

4th 1. 39

5th 1. 00

6th 0. 73

Rev 5. 63





Good luck with it all.



Loren

 

[Big_Daddy_T]

fuel mileage

I have an auto with 355s and 34. 4 in tires. at 65 im spinning 1600 rpms. I think that you will find your mileage isnt dictated by rpm but by the amount of energy required to overcome the wind resistance at speeds above 60 mph. in my opinion speed equals mileage not rpm

 

[tragicwagon]

Thanks for all the help fellas,but after reading Fritzs ram page I guess Ill leave well enough alone. It says you cant put any thing larger than 235-85-16 without adding spacers between the rear wheels,the way tires are sized to get taller you must go wider. Im not sure about all this so if any one has first hand experience fell free to chime in.