rpm at 70 mph

[1ftcr14u3]

i have a 91 with a 3.0L and automatic trans and 3:45 rear axle ratio

and the the tach shows 2800 rpm at 70 mph which seems high to me .

what rpm do other comparable rangers run at 70 mph ??

 

[RonD]

Welcome to TRS

Calculator here: https://www.crawlpedia.com/rpm_gear_calculator.htm

At 70MPH with 3.45 axle, 28" tires and transmission in 3rd(1.00 ratio) engine would be at 2,898 rpms
In OD, 4th(.75 ratio) it would be 2,174 rpms

Correct tires diameter matters so change that to match your vehicle

A4LD automatic has 3rd at 1:1 ratio, OD/4th at .75:1

So your transmission is probably not be shifting into OverDrive/4th gear

 

[1ftcr14u3]

thank you , that must be it .

 

[1ftcr14u3]

i did a road test today . at 70 ( around 2700 rpm ) i manually shifted from OD to 3rd


tach went to 3500 rpm . Wouldn't that indicate i was in OD ??

 

[85_Ranger4x4]

With 3.73's, 31" tires and a 3 speed automatic (third is 1:1 like your third) I ran 2700ish at 60mph.

Smaller tires and 10mph more I could see being over 3k even with the slightly higher gear.

With stock 2wd sized tires I am going to say that is the nature of the beast. Keep in mind the national speed limit was 55mph when your truck was built too.

 

[RonD]

i did a road test today . at 70 ( around 2700 rpm ) i manually shifted from OD to 3rd


tach went to 3500 rpm . Wouldn't that indicate i was in OD ??

Yes, but it would also indicate you have a 4.10 rear axle ratio, not 3.45

Use the calculator, its just math, but calculator makes it easier

I used 27" tires size
70mph
.75:1 ratio OD
Then use 1:1 ratio 3rd

If rear axle was 3.45 then
in OD RPMs should be 2,254
in 3rd RPMs should be 3,005

If rear axle is actually 4.10
OD RPMs are 2,679
3rd RPMs are 3,572
Which reads like what you have

You are there we are not, so only you know the variables like tire size and rear axle ratio
SPEED is also a variable if it has not be tested, if tire size was changed then speedometer is not correct
Use Mileposts to check if odometer is correct, or use GPS APP on smart phone to check speed

Constants are .75 OD ratio and 1.00 3rd gear ratio in an A4LD, these don't change