2nd Gen Non-Engine/Transmission This can't be right?

[EMD-Run8]

Just bought my '98 2500 a few weeks ago. It has Michelin LTX/MS tires, size LT265/75R/16. With a 3. 54 rear, two different ratio calculators I've checked say it is doing 77 at 2100 rpm. The speedo says I'm doing 69 and I'm gettin' passed by 'Ma and 'Pa in their Grand Marquis! Every chart I've checked says 265/75/16 tire is between 31. 6 and 31. 8 inches. Are the "LT" 265's shorter?

 

[blmyachtsales]

Have a 96 3500 with 4:10's and 285's 65 is about 2100rpm.

 

[duaneparks]

Not sure why the calculators say that... I had the same setup and 2100 rpm usually put me at 70 mph. Perhaps the top gear ratio is incorrect in your formula? If you move to 315/75/16's, 2100 is 77 mph. I know this because the local constabulary informed me one day that they "clocked" me at 77 even though I was travelling at an indicated 70.



YMMV of course, but probably not by much!



Duane

 

[JStemann]

Loaded vs. unloaded height?

I may be wrong on this, but aren't the advertised tire heights for an unloaded tire?



I'm running Big O brand Bigfoot A/T's. 285/75/16D. My measured height is about 31. 5" most places list the 285's as around a 33" tire.



I programmed the superchips tuner with 31. 5" and gps checked it as almost dead on over 300+ miles. Maybe off around . 1 miles per hundred.



Oh, and 2000 rpm= ~66mph.



jeff-kc9ftp

 

[EMD-Run8]

Rear end ratio of the truck is 3. 5 (3. 54). NV4500 5th gear ratio is . 73. Some sites I've looked at say . 74 but that makes next to no difference. Tirerack.com shows 265/75/16 tires being anywhere from 31. 5-31. 8 inches. I must be figurin' something wrong. Anybody know the formula? I'll try figurin' it by hand.

 

[Nate]

Have someone drive by you to check your speed... or get a GPS.

 

[RDelissen]

Have someone drive by you to check your speed... or get a GPS.

ditto on the GPS check

 

[Royk]

FWIW, my '02 with 5 speed and BFG LT265/75R/16s does 70 @ 2000 rpm.



Roy

 

[Dkuhlman]

Rear end ratio of the truck is 3. 5 (3. 54). NV4500 5th gear ratio is . 73. Some sites I've looked at say . 74 but that makes next to no difference. Tirerack.com shows 265/75/16 tires being anywhere from 31. 5-31. 8 inches. I must be figurin' something wrong. Anybody know the formula? I'll try figurin' it by hand.

I've run some of your figures and here's what I came up with.



First, my calculations show a 265/75-16 to be 32. 2 inches in diameter but tires vary from brand to brand as well.



Next, with a 3. 54 rearend and the . 73 OD, your effective rearend ratio 2. 58:1.



This puts you at 74. 3 mph at 2000 rpm or if we use 31. 5 inches for the tires, 72. 7 mph, again at 2000 rpm.



The calculation you're looking for is mph=(rpm*TD in inches)/(overall ratio*336)



Don

 

[2-ND TIME]

tires

Check in their catalogue the rpm's this will tell you the % difference per mile. Mine were just over 5 %

 

[EMD-Run8]

This is one of the three I have used so far: www.xse.com/leres/ss/calculator.html. They are all three within a 1/10 mph or two of one another. This one has been the easiest to use. I guess I'm gonna check Michelin's site and see if somethings up. The truck came factory with 245's and based on what I'm looking at my speedo is still off even I had those. I dunno... ... .

 

[DMcPherson]

I have an 02 had the michelins same size same gear and at 2100 is roughly 75-80mph

 

[ceastmen]

I have an 02 had the michelins same size same gear and at 2100 is roughly 75-80mph

My 01 is the same @ 2100 about 78mph.

 

[EMD-Run8]

It must be right then. I did notice after a double-take that the '98 tachs do not read in 100 rpm increments. At least mine doesn't. It's more like 120 or 130. I'm wondering also about the factory tire size. The spare under the truck is a 225/75/16. I'm pretty sure it is not the original (it's an offbrand) but did the stripper trucks like mine come with 225 size tires?

 

[Thomas]

Tachometer increments are 125 RPM.

I think you'll find the OEM tire on you 1998 Ram 2500 was LT245/75R16 Load Range E.

You may find TSB 26-11-99 (at the world famous Dave Fritz site) useful: http://dodgeram. info/tsb/1999/26-11-99.htm

I suspect the "LT245/75R16 LTX M/S" near the bottom of the chart should be LT265... .

Also from the Dave Fritz site, a page with lots of links: http://dodgeram.org/tech/specs/axle/rpm_vs_mph.htm

 

[EMD-Run8]

Thanks. I was wondering because the speedo seems to vary between six and eight mph off. There isn't that much difference between a 245 and a 265. Oddly enough, the rpm's seem about right. I guess fuel just isn't pricey enough for some folks yet. I'm still gettin' passed by folks runnin' 85-90. I just couldn't believe people are still runnin like that at these prices. I dunno...