Engine/Transmission (1994 - 1998) RPM at Cruise

[drrhoades]

I have a 3. 55 rear end and a NV4500 with 265/75 R16 tires.

Running at 70 mph I am at 2100 or 2150 RPM on the tach. This doesnt seem right. What are other people running at at this speed.



2000 RPM is 67 or 68 mph.



Is something messed up with the tach?



Doug

 

[JJW_ND]

Pretty close!

More likely your speedo is off a bit. Your truck came stock with 245/75R16 which are a bit shorter then you are running.



I would expect 68 - 69 maybe 70 at 2000 rpms with your setup. Use a gps to check speed.



jjw

ND

 

[rjones]

At 70 Im at 2200 w/4:10 axles..... I use 315/75r16 tires and the speedo reads exactly 10% slow... ... Rick

 

[Blakers]

My 95 with automatic, 3. 54 gears and 285-75-16 tires ran 1800 rpm at 70 mph. This is with using a GPS for speed measurement. With 265-75-16 tires, it would run about 1900 rpm.



Blake

 

[Bill Lins]

My '98 12 valve with NV4500, 3. 54 axle, & 245 tires runs @ 2000 RPM at 65 mph.

 

[drrhoades]

Either I am missing something or something is wrong.

If the tire size was increased to 265 from 245 then the rpm should drop at 70 mph. In my case the rpm at 70mph is 2125 or so.

I dont understand the problem unless someone changed the speedo gear before I bought the truck.



I guess I will have to check with a GPS.



Thanks

Doug

 

[Blakers]

I think a lot of these speedometers indicate way too high. My 95's speedometer didn't get close to being right until I put on the 285s - and then it still indicated about 2 mph faster than actual speed.

 

[zack_76401]

my 98 12v nv4500 reads right about 2200 at 75-77 mph

 

[OnlyCummins]

Here's a formula that relates MPH, RGR (Rear Gear Ratio), ODR (OverDrive Ratio), TD (Tire Diameter) & RPM:



RPM = (336 x MPH x RGR x ODR) / TD



Formula requires 'actual' MPH not 'indicated' so since your 265's are ~3. 87% larger than stock. multiply indicated speed of 70MPH by the TD difference to get approximate actual speed. So ... 70MPH x 1. 0387 ~ 72. 709MPH. Taking that # & inputting it into the formula:



RPM = (336 x 72. 709MPH x 3. 54 x 0. 75) / ~31. 65"



yields ~2049RPM. Value assumes that measured TD is in fact true & that static (sitting still) unloaded TD is the same as dynamic (rolling) loaded TD. Lopping off ~1/2" for rolling loaded TD yields ~2082RPM. This last # isn't too far off from your stated RPM at your indicated MPH. I think you should be able to get a more accurate # by measuring the radius of one of your truck's rear tires from center to road surface. Don't forget to double that # before inputting it into the formula.

 

[drrhoades]

I still havent tested the speedo with a GPS. But I was doing some calculating on 4lo.com and it comes out that if everything is working correctly I am running a 3. 73 rearend. I got the truck a month ago and I dont know what was done to it before this so I guess it could be possible that the person before me change the diffs out to a 3. 73. The orignal window sticker says 3. 55 so I guess I will have to pull the cover and see what the deal is.



Doug

 

[OnlyCummins]

You don't need to pull the cover to determine the rear (or front) gear ratio. Easiest thing to do is to look for a 'tag' or identifying info on the rear axle & then decipher it. The other option is a technique I'm not too familiar with but I think it involves lifting the rear tires (or tire) off the ground then rotating the tire one revolution while counting how many revolutions the driveshaft makes -- the difference btwn the two would be your truck's rear gear ratio. Also, I think, technique is best done in direct drive, 1:1, not OD. More calculations would be involved in anything but DD.



Good luck.

 

[drrhoades]

I know about the rear tag but I think I looked at it and it was hard to read. I know about the spinnin wheels way but with the ratios being so close it might be hard to tell. Anyway I was going to change the diff lube sometime so I guess now is a good time.

Thanks for the input.



Doug