Is my Tach off?

[bduke23]

Took my new truck out for a drive today, and noticed the tach. I was doing 110 km/h (just over 60 mph) and it was showing 1600 rpm. That seems way to low to me. Its a 3.0L with M5OD tranny in 5th gear, and stock 3.73's with 235/75 tires. Is my tach off? If so, how do you fix something like that?

 

[martin]

110 is closer to 70 (68. something) . Anyway not to split hairs it definately sounds off a little, almost half in fact. perhaps the dash has been changed and it has a tach from a v8. Math dosn't quite work out though but that's all I can think of.

 

[James86]

At 55mph with a 4spd auto and 4.10's my rpms are about 2,000rpm. I don't know the gearing in the father in law's 98' 3.0 5spd, but when i drove that it was a fair bit below 2,000rpm in 5th, and both of us have the stock 235/75/R15's

 

[Mac]

I think Martin hit the problem. Someone must have swaped in the 3.0L and didn't get the tach recalibrated. In fifh gear with those tires (15"?) at 60 mph (99 kph) and that gearing you should be turning just over 2200 rpm. Good tire vs gear chart up in the tech library.
Dave

 

[bduke23]

The stock engine is a 3.0L, and it has not been switched nor has the rear end been changed out, I have checked both. I am at a loss as to what it could be, I don't think the tach has been ever been switched either. An it seems to be right on, it reads correctly when idling and seems to go up about the right speed for how I accelerate

 

[GingerKid]

then maybe you don't have 3.73s. My truck with a 3.0, M5OD, and 3.73s is doing 2200 or so at 65 in fifth.

 

[necessaryevil]

maybe gear change in the tranny?

 

[Mac]

The stock engine is a 3.0L, and it has not been switched nor has the rear end been changed out, I have checked both. I am at a loss as to what it could be, I don't think the tach has been ever been switched either. An it seems to be right on, it reads correctly when idling and seems to go up about the right speed for how I accelerate

Just going by the info you had posted under your aviator that showed a 1986 Ranger. Then if everything is stock, yes, your tach is off by about 2/3.
Dave

 

[bduke23]

Sorry, haven't updated my profile yet. This is a different ranger that I just bought last week

Edit: Have now changed profile, vehicle in question is the 93

I also have checked the tag on the axles, both say 3.73. I suppose it is possible that both sets of gears were changed internally, but I suspect not. I don't even think the previous owner changed the oil (it was blacker than black). Don't worry, I have already changed it and will be again in about 1000 km

 

[Mac]

I think you are probably correct in those still being the stock gears then. Not sure but some tachs had a 'pot' in the back to change from 4 to 6 cylinders. Might want to post this in the electrical section or the V-6 section to get a better answer. Good luck anyway!
Dave

 

[pjtoledo]

235/75-15 tires roll between 716 and 721 revolutions per mile according to the TireRack.
718 revs/mile X 3.73 gears gives 2678 revs/mile for the driveshaft.

if your 5th gear is .75 the engine revolutions are 2008
if 5th is .72 1928
for .62 its 1660

since 60 mph = 1 mile per minute those numbers will equal the rpms at 60 mph.

Put it in 5th, jack up your arse end (the trucks), mark the crank pulley and driveshaft, slap a wrench on crank bolt and start cranking to determine the 5th gear ratio.


oh yeah, what's the stock size tire for a 93?

 

[bduke23]

I don't know about stock, but the tires match the sticker on the door that it came with from the factory. I don't think they were that big stock, I'm guessing it was an option.