05 truck with 4.0 wrong rpms

have an 05 Tundra with 4.0 V6 and RA60 6MT. The truck doesn’t have a OEM tachometer. I have an ultra gauge and techstream that reads about 150rpms low from what it should be. For example 65mph should be almost 2600rpms in 6th with OEM 29.5” tires. My gauges will read about 2400rpms instead. Is this normal? I wonder if the gauge cluster affects these rpms because the dash was replaced at one point, I bought the truck used.

I am asking here because there is more info on TW for the 4.0 and what is basically 1st gen Taco technology than on Tundras which is more for the v8s.

 

How is your pickup [for the RPM read] happening. I'd bet it's correct and either your gauge is off or you have another factor. Lets face it, 150 RPM is a single digit percentage ''off'' at 65 mph and even a small difference of tire diameter could cause that.

 

Are you certain of your axle gear ratios?

 

Make sure you're using GPS speed as the speedometer is usually 2-3 mph off.

Make sure you're using your specific tire's "revs per mile" spec when calculating rpms, not the diameter from the nominal size.

Is the discrepancy only apparent in 6th? 6th gear in the RA60 is either 0.80 or 0.85 depending on application: https://www.tacomaworld.com/threads...on-your-6-speed-transmissiom-write-up.338840/

 

Axle ratio is 4.08 for 00-06 Tundra manual V6s.

 

I will check the revs per mile. The tires are Firestone destination LE3 245/70r16s Also the trans is 0.85 for 2wd version of the Tacoma RA60F.

According to tire rack, revs per mile should be 705. Not sure how to calculate the speed.

 

684 revs/mi: https://www.tirerack.com/tires/Spe/...e&tireModel=Destination+LE3&partnum=46HR8DLE3
At 65 true mph, the tire will be doing 684x65/60 = 741 rpm. Multiply that by the 6th gear ratio and axle ratio to get the expected engine speed. On a Tacoma with 3.73 axle it would be 741 x 3.73 x 0.85 = 2349 rpm.

 

The Ultra gauge and techstream connect to the obd port under the steering wheel.

 

Thank you. According to these instructions

705x65/60 = 763.75
763.75 x 4.08 x 0.85 = 2658.6rpms

Not 2450rpms according to ultra gauge.
Even in 5th gear at 55mph it will read about 2450rpms instead of 2638rpms. Strange.

 

Whatever engine speed is reported over obd should be correct as it is actual engine speed that the PCM is reading.
Not sure how the Tundras gearing compares to the 2nd gen Tacoma but mine runs around 2300 @ 65.

 

Most OE tachs read about 200-300 rpms higher than actual if you are getting the "what it should be" from. I know you don't have one.

I dyno'd my '00 Tundra 4.7v8 and the tach was reporting limiter cut at 5500, but dyno was saying it was maybe 5200rpm

Put down a healthy 205hp and 275lbft to the wheels on a Dynojet vs OE ratings of 240hp/315tq with 235,000 miles

If I remember correctly, my '05 Tacoma 4.0 6MT is about 2,500rpms at 70mph with 265/75-16 and OE 3.73 gears.