Is this RPM at speed right?

[trailblazr81]

OK so I just picked up a 1998 Ranger 3.0 auto 4x4 with 144,000 miles. It has axle code 87, (4.10's I believe), and 31X10.5 tires. After looking at the tech library I see my trans has a .75 OD gear ratio. According to the calculators I should be doing 60 MPH at 2000 RPM. Using a GPS I see Im only doing 53 MPH. At 60 MPH Im turning around 2250 RPM and at 65 Im spinning almost 2500 RPM. Is this right? Is my factory tach off, or is my tranny spinning about 200 RPM more than it should? Anyone else have this combo? The previous owner said he never touched the axles so Im guessing Im still running the 4.10 gears.

 

[cammeddrz]

another "my gps says my speedo is wrong thread"..... measure your tires, i'd be that the 31" size could be as short as 29" actually, it is extremely common for tires to be shorter than the numerical size

what else that is also extremely common is that your hand-held device making computations of speed based off of a vehicle traveling across a curved surface (the earth) in relation to several orbiting devices that are several hundreds of thousands of feet away.....has a margin for error its a Global Positioning System, not an extremely precise speed measurement device

 

[trailblazr81]

OK. But it has been pretty accurate in the past. I know my tires arent a true 31" but Im stiff off RPM wise by quite a bit. 250 RPM, I think is a bit off.

 

[Sicktight722]

are the 31's the stock size? that might affect it somehow

 

[BeefStew42791]

(assuming the 98's still had the worm gear in the trans) when you change tire size, you're changing the engine-to-wheel torque ratio, in that the engine's turning power is now a LOT different then the turning power of the wheel, even compared to the stock numbers. Smaller margins of difference between stock & modified don't typically affect the way the truck feels to drive, but what it does have a very noticeable effect on is the measured speed of the vehicle, because remember the truck still thinks its got those different sized tires on it!! what you've gotta do is adjust whatever measures the current speed for the smaller/bigger tires (be it the worm gear in the trans or the actual computer, since more modern vehicles aren't using worm gears anymore).

Basically, if you change your tire size, which it appears that someone did, the speedometer won't read accurately..

 

[cammeddrz]

more modern vehicles aren't using worm gears anymore

i'm pretty sure the 1998 has the electronic vss that sends a varying a/c current which is then measured and translated into speed by the ecm

 

[trailblazr81]

Its not the stock tire size, and I know my speddo is now off by about 3 MPH at freeway speed.

 

[triumphrider-1]

I have a '99 with 265 75 R16 tires (measure out to be 31", calculate out at 31.65") rocking the 3.0L, auto trans, and 4.10 L/S rear.

The stock tire for my truck is 245 75 R16, giving a 3.73% difference in speed and distance as measured by the speed sensor(s).

Cruising at 55mph or 60mph (can't remember offhand, leaning towards 50 though) the tach reads 2000rpm, and when I run 70mph (based on GPS and radar signs) the engine runs at about 2450rpm.

 

[adsm08]

98 will use the rear axle speed sensor for the VSS signal to the speedo. You need to either go to the dealer or get a tuner and have your tire size reset in the computer.