I have a 1990 2500 Getrag 5 spd stock size rubber and wonder what rpms am I at doing 60 mph? Thanks

 

This can be easily figured accurately with a little math.

Everything you need to know to figure it out is fifth-gear ratio, rear-end ratio, and tire diameter.

Most of our tachometers have been sort of guestimated, so most of our answers to your question would be inaccurate.

 

Depends on your gear ratio among other things.

On MY truck with a 3.54 rear gear, 60mph (according to my GPS) comes at about 1850 rpm in 5th gear according to my ISSPRO tach that connects to the factory wiring but I have 245/75R16 tires which throw my speedometer off making it show 64mph. YMMV

 

That's interesting !!
Most of the time when you go to a larger tire the speedo reads - slower --

Since stock tires are 235's how did your speedo GAIN 4 mph ??

 

Because a 245 is a bunch SHORTER than a 235.

The 235s are -85-s; whereas, the 245s are -75-s.

 

I'm running 265-75-16 and at 60 mph with 4:10's it is about 2,000rpm

 

BINGO!! Wider but shorter by almost an inch.

In my case, it actually worked out in my favor because with the 235/85s, my RPMs were too low in 5th if I wanted to run 60 mph when towing. Now the RPMs are up higher in the torque band at 60 mph. Just about right for me.

 

I am stuck with 8 R 17.5 tires and need a taller tire for gearing.
I am turning about 2300 RPM at 60 MPH now.
Does anyone know how much taller 10 R 17.5 tire are if any?
I figure I've got a 4.75:1 rear axel and would like to turn 1800 at 60 in O.D.

 

Thankx you guys I appreciate the info... I'l check my VIN to find out my gear ratio and Healthy Haulin and Happiness to all

 

hmm 17.5 trailer tires ?

Why not just go with 8R 19.5
or 255/85r16

 

Back in the sixties/seventies, it was not un-common for tater-chip trucks, bread-vans, and the like, to have 17.5 wheels.

Some of the OLD motor-homes also came equipped with 17.5s.

Around the shop, unless they have went to the scrap-iron buyers, are several old 8-on-6-1/2 17.5 rims, in both SRW and DRW configuration.

The reason these rims have accumulated is due to people finding out just how costly 17.5 tires were, so they swapped on 16-inchers.

I would have long ago used them for trailer wheels, except that they are not nearly as strongly built as the trailer wheels.

 

VERY CLOSE

I took pen in hand and did the math for MY truck.


235-85-16 tires average 650 revolutions per mile.

Rear-end is 3.55:1

5th-gear in a Getrag is 0.77:1

In 5th-gear, each 148.1 RPM = 5-MPH

1777 RPM = 60-MPH in 5th-gear.

1629 RPM = 55-MPH in 5th-gear.

In 4th-gear, each 192.3 RPM = 5-MPH

2308 RPM = 60-MPH in 4th-gear.

1731 RPM = 45-MPH in 4th-gear.


My guestimated tachometer hovers around 1600 when the speedometer shows 55-MPH in 5th gear.

Drop to 4th-gear at 1600 and speed shows 45-mph.

At 60-MPH in 5th-gear, the tach hovers around 1800 RPM, close enough that I ain't gonna mess with it.

 

Hey bearkiller can you actually see me or are you a good guesser?
I've got $5k in a '72 Commander and can get 8R17.5s for $100 each but I really need to get the O.D. RPM down or get rid of the 4.75:1 Rear axel.
I can't use 16" because of the brake calipers and will need 6 + tires of any rim change. The engine and trans are worth more than the RV so I'm 'economical'. Thanks.

 

I have Getrag 5spd, 3:55 gears and 265/75/16's what do you make of the Rpms at 4th and 5th Bearkiller @ 50 and 60mph respectfully