4.88 w/35" question...

[Imdone]

For those that run this I am curious of your RPMs at Speeds.

I took mine for innitial runs today and checked speed and rpms against GPS for accuracy.

At 50MPH I am turning 2500 RPM in 3rd gear.

At 70MPH I am turning 2600 RPM in O/D.

Is this the same for everyone else?

[Imdone]

Ted...

Joe...

Bueller...
Anyone confirm that I am not wacked out of my skull???

[dyogim]

This past weekend I noticed when my speedo says 70 I'm at about 2100-2200rpms. OD on and I do not have my speedo recalibrated.

[Imdone]

Originally Posted by dyogim

This past weekend I noticed when my speedo says 70 I'm at about 2100-2200rpms. OD on and I do not have my speedo recalibrated.

Do you know Truley how fast you are going? I put my Tru-speed on this sunday so I now know how fast I am going.

I think I am about 100rpm above stock. If I remember right I was at 2400 rpm at 70. Now I am around 2500 - 2600.

This is also to say that I am by far from stock on the rest of the engine components as well which is likely causing a difference.

Thanks for checking though, gotta make sure I am not goin' loopy.

[dyogim]

When I passed through one of those speed limit detectors on the freeway, my speedo said 70 and the radar board kept jumping from 63-66.

[rakent]

Maybe this will help:

RPM at 60 mph = (Tire Revolutions Per Mile)*(Axle Ratio)*(Transfer-case Ratio)*(Transmission Ratio)

From the Tirerack.com - BFG 315/75-16's are 34.7" outside diameter & roll 599 times in one mile. You have 4.88 gears in the axles, you're running in 2wd high, and the overdrive gear ratio in your tranny is .70 - so:

RPM at 60 mph in OD = 599*4.88*1*.70

RPM at 60 mph in OD = 2046

RPM at 70 mph in OD = 2387

If you were in third gear (1:1 ratio) at 60 mph, your RPM should be 2923, at 50 mph about 2435.

A stock truck with 265/70-16's (nominally 31") and a 4.10 rear would be:

RPM at 60 mph in OD = 678*4.10*1*.70

RPM at 60 mph in OD = 1945

What's really interesting is how the gear swap brought the truck back into its power band, if you had left the 4.10's (or 3.91's or whatever you had) in there your RPM at 60 would have been around 1720 or so, and the truck would be a dog.

All of this is assuming your torque converter is locking up and you don't have any slippage at the transmission.

Remember as well that your speedo accuracy comes into play, if you're indicating 70 but actually running 77 (a 10% error) because of larger tires, then your RPM's for the indicated speed will be higher than before. If you actually slowed down to a true 70 (in this instance an indicated speed of around 63 or so) your RPMs would be lower than before the tire change.

I'm sure this is probably way more than you wanted.

Hope it helps.

[Imdone]

Stock for a Tundra is 3.91 ratio.

So I was 1855 with stock tires at 70 and now I am at 2300 at 70 with 4.88s and 35s. interesting. That is kind of nice though because with the S&S headers the engine really perked up around 2500 so I don't have to go far to get to where I need to pull. Sweet.

[TMS2U]

I cant remember what it was, but yours sounds about right. w/37's at 65 its right around 1800 in OD. I usually keep the OD off to keep the rpms near the power band when I'm on the freeway for those rapid lane changes that are often required in traffic

[Imdone]

Thanks T.