RPM calculators vs. tach reading

[Dusty T.]

Just for kicks I ran some of my truck's numbers through an rpm calculator at: http://www.4lo.com/4LoCalc.htm and http://www.csgnetwork.com/multirpmcalc.html.

Both calculators produced the same numbers for my '91 Ranger. I am running little 27" BFG A/T tires, 3.55 gears, and I'm pretty sure I have the M50D-R1 5-speed manual bolted to the 4.0L (which has a .79 ratio in 5th according to http://www.rangerpowersports.com/tech/?p=71). What I'm uncertain of is the RPM's calculated seem to be on the low side compared to what my tach actually reads.

For example, the calculator shows:
60- 2094 RPM
65- 2269 RPM
70- 2443 RPM
75- 2618 RPM
80- 2792 RPM
85- 2967 RPM

My tach shows:
60- ~2000 RPM
65- ~2300 RPM
70- ~2500 RPM
75- ~2800 RPM
80- ~2950 RPM
85- ~3100 RPM

So I'm wondering if the tach in my truck is not very accurate? I have heard that they aren't It seems like the RPM calculator gives lower numbers than what my tach shows and how the engine sounds. It also seems like the tach shows very little engine speed difference between 78 and 85 MPH, but RPM should increase constantly according to speed, right?

Thanks for any input. My apologies if this topic has been beaten to death.

 

[DeRocha]

What is the exact tire you are running? A BFG AT 215/75 R15 is 27.8" in diameter. If I run the numbers this indicates 60 mph will yield 2033 RPM, which would be hard to differentiate on a stock tach. Of course tire pressure and tread wear are variables which will cause variations in actual MPH readings.

 

[Turdle]

My tach shows just > 1 k when I am idling, but when connected to a
www.scantool.net it reads around 950 rpm's.

My rev limiter hits at what is an indicated just <5500 rpm's, but the scantool reads 5260

I think they can be a bit off, yes. I believe the needle can be pulled off and re inserted, but I haven't bothered.

 

[Dusty T.]

What is the exact tire you are running? A BFG AT 215/75 R15 is 27.8" in diameter. If I run the numbers this indicates 60 mph will yield 2033 RPM, which would be hard to differentiate on a stock tach. Of course tire pressure and tread wear are variables which will cause variations in actual MPH readings.

I am actually running an 8.5" wide X 27" tall BFG TA/KO tire on 14 inch rims. The tires are pretty new, so the tread is in good shape. I've heard these tires are actually a little smaller than 27", so I'll try double-checking the tire's actual size and get back about that. The speedo may read 1 mph too fast, which I did calibrate and check after the regear.

This truck is stock height, 2 wheel drive, but I am thinking of getting new 15" or 16" wheels and slightly taller tires in the future. Do you think 235/75/R15's (28.9") would fit? I'm pretty sure 225/75/R15's (28.3") would.

Thanks!

 

[DeRocha]

I am actually running an 8.5" wide X 27" tall BFG TA/KO tire on 14 inch rims. The tires are pretty new, so the tread is in good shape. I've heard these tires are actually a little smaller than 27", so I'll try double-checking the tire's actual size and get back about that. The speedo may read 1 mph too fast, which I did calibrate and check after the regear.

This truck is stock height, 2 wheel drive, but I am thinking of getting new 15" or 16" wheels and slightly taller tires in the future. Do you think 235/75/R15's (28.9") would fit? I'm pretty sure 225/75/R15's (28.3") would.

Thanks!

You can easily fit 30x10.5x15 on your truck. Many people run 31s and don't have any rubbing issues.. The back will be 100%, its just the front tires that may rub due to steering angle and sometimes sway bar contact.

 

[Dusty T.]

You can easily fit 30x10.5x15 on your truck. Many people run 31s and don't have any rubbing issues.. The back will be 100%, its just the front tires that may rub due to steering angle and sometimes sway bar contact.

Whoa! 30s? 31s? I was thinking 29" tires would be my absolute max. Sure they'll fit on a '91 Ranger (generation II), 2wd stock height?

Thanks for the reply.

 

[4x4junkie]

no, 30s will rub on a pre-'93 2WD Ranger.

The 235s might work, although if no rubbing at all is a top concern, I'd go with the 225s (on stock offset wheels).