31 tires AX15 stock gearing at 70mph

[SBpunk]

Anyone know what my RPM's should be at 70mph? I've been driving down to the city and back a lot recently so I looked up an RPM chart. By its calculations my RPM's are WAAAAY off. Unless I put something in wrong at 3k RPM's in 4th gear I should be going 91 mph. If its true I need to go thru my engine and see what the hell I screwed up on it.

[HOrnbrod]

ASSuming you have 3.55 gearing, 31" tires, and the correct 32-tooth speedometer gear installed :

 

RPM's at 65 MPH/105 KPH w/ 3.55:1 axle gears running 31's: 1,876 RPM. So at 70 MPH you should be turning ~2020 RPM.

[Eagle]

31x10.50-15s, 3.07 gearing:

 

70 MPH is 2436 RPM in 4th gear, 1827 RPM in 5th gear. Chart: https://docs.google.com/spreadsheets/d/1pHFuhGgj6dQDfzyfFJH5z7NCDLW2KX3ABQgAJr3lBvM/pub?hl=en&hl=en&output=html

(The link to the chart is in the DIY forum area)

 

That's based on actual tire manufacturer's revolutions per mile data, not a mathematical calculation based on unloaded tire diameter. Question: If you have an AX15, why are you asking about 4th gear? Don't you use 5th gear at 70 MPH?

[SBpunk]

Great thanks I'll take it out today and check it out. The links weren't working earlier but it might just be this computer. I was just asking because the numbers on 4th gear seemed way off. In 5th gear I'm almost at 3k rpms going 65 so I think something is wrong with my engine.

[SBpunk]

3.07 gearing sorry forgot the other details.

 

4.0 HO AX15 NP231 3.07 gear ratio Dana 30/35 31x10.5 R15 stock rims

[Pete M]

ignoring the rpm calculations, does the engine sound fine?  idle ok?  rev up ok? 

[SBpunk]

Rev is good
Idle is steady on the gauge but the engine isn't nearly as smooth as my 06' (could just be its older and/or I'm being paranoid after upgrading to 703 injectors)

 

Its never not started or died on me but I have this nagging feeling its under powered. I put in a THOR cowl intake (going to the river this Saturday), Bosch 703 injectors, new spark plugs, 15w40L Rotella Oil and something just feels off with it. Like I said it could be in my head.

[Pete M]

assuming nothing is slipping in the transmission, the engine rpms will be directly linked to the tire rpms.  I don't think there's anything wrong with your engine.  might be that your tachometer is off though. 

 

and yes, it WILL feel underpowered if you're pushing 31 inch tires with the stock 3.07 gears.   the cure is a gearing upgrade. :thumbsup:

[Eagle]

Great thanks I'll take it out today and check it out. The links weren't working earlier but it might just be this computer. I was just asking because the numbers on 4th gear seemed way off. In 5th gear I'm almost at 3k rpms going 65 so I think something is wrong with my engine.

The engine wouldn't affect it. Either your speedometer is wrong (which you can check with a GPS) or your tachometer is wrong.

 

Is the tachometer original to the vehicle, or did you swap it in? If it came from a 4-cylinder vehicle, think about this: An electronic tach doesn't really measure engine revolutions, it counts ignition pulses. For a 6-cylinder engine, there are three pulses per revolution (remember, it's a 4-stroke engine). So -- 1000 RPM equals 3000 pulses.

 

Now look at a 4-cylinder engine. One revolution is only two pulses, so three pulses is 1-1/2 revolution. So put a 4-cyl tach in a 6-cyl truck, it sees three pulses, and reports 1-1/2 revolution instead of 1. 1000 actual engine RPM shows up as 1500, and 2000 RPM shows up as 3000 RPM.

[SBpunk]

Everything is stock on it except what I've done. Sat on a farm it's entire life with the same couple. I'll check my GPS and see if my speed is off at 70mph.

 

I have a 29 spline 8.25 sitting in my garage I'm going to regear, lock, alloy shaft and SOA. Insanely excited for that but it'll be awhile.