cruising rpm's

So I am aware that the my rpm should be at about 2700rpm on the highway (100km/h),But I don't understand why my fuel mileage would improve at the increased rpm? I have 31's with 3:53 with a 89 4.0 auto,I know I need to re-gear. I usually drop to 2nd for hills but back in od for the flat,should I be in drive all the time with the gears I currently run for best fuel mileae? My brothers stock 93 sport with a 5spd and stock tires cruises (100kph) at 2100rpm. Thanks.

2700?!?! That seems pretty freakin' high for a highway cruise...

Mine's more like 1800 at 55mph, or so, and maybe 2300 at 70... I also have 3.55's and 31's... Are you sure that your overdrive is engaging? Or is your torque converter not locking up correctly? That seems a bit odd.

ya..thats wrong cuz mine is the exact same setup.at 65 mph it's at 2000 exactly.

90 XJ 4.0 31x10.5's stock towing package gears. 60 mph (100 kmh) right at 2000 on flats. More on small hills, 3K jammin' up a mountain hiway passing a slow-mo in power mode.

31 inch tires equals 787 cm. Multiply with pi (3.14) gives the length round the tire = 2,47m.
100km/h = 1,6km/minute=1600m/min
1600/2,47 = 647rpm on the wheels.
multiply with the axle ratio 3.53x647 = 2283 rpm on the propeller shaft.

To find the engine rpm multiply with actual gear ratio:
When in overdrive 0,75 = 1712 rpm. no slipping in the converter ie lockup.

The actual calculation may be a little off due to tire not beeing exactly round
and not exactly 31" diameter.

Something is wrong with your rig. Below 2000 rpm is right.

Torfinn

He's not saying what his RPM's *are* at cruising, he's saying what they SHOULD BE.. and he's right. 2000 rpm's is too low, you need to regear.

The 4.0 wants to cruise between 2400 and 2800 rpm's on the highway. Lower than that and your working the engine too hard, therefor dropping your gas mileage.

With 4.56's and 32's, my 5 spd cruises along at about 2700 rpm at 65mph... Right in the sweet spot for the power band....

Yeah, with 4.56's and 32's with a 21 spline aw4 I was getting 19mpg on the highway.

My 4.0 revs about 2900 at 75mph yeilding around 14mpg flat ground. 4.0, aw4, 35's, 5.13's.

Yeah, when I went to 4.88's and 35's my mileage dropped to around 16.

I just leave it in 3rd for most highway driving under 100km/hr and it stays around 2200RPM. I just have stock tires but I find that if I'm in OD it'll usually just cruise at only around 1700RPM and the engine struggles alot more going up hills. I definately have noticed that mid 2000 is the "sweet spot" on this engine and am noticing better gas mileage as well as more power when I'm hitting this area.

torfinn said:

31 inch tires equals 787 cm. Multiply with pi (3.14) gives the length round the tire = 2,47m.
100km/h = 1,6km/minute=1600m/min
1600/2,47 = 647rpm on the wheels.
multiply with the axle ratio 3.53x647 = 2283 rpm on the propeller shaft.

To find the engine rpm multiply with actual gear ratio:
When in overdrive 0,75 = 1712 rpm. no slipping in the converter ie lockup.

The actual calculation may be a little off due to tire not beeing exactly round
and not exactly 31" diameter.

Something is wrong with your rig. Below 2000 rpm is right.

Torfinn

Click to expand...

Rolling diameter for most tyres can be found at the manufacturer's websites (and I plan to collect all the data myself one of these days.)

However, 2600-3000RPM for AMC242 cruise RPM is about right - lower is not better.

One thing I learned way back when I had a private pilot's license - ideal cruise RPM for pretty much any engine is at or near "peak torque RPM." For the AMC242, that's around 2800-2900RPM, but the "torque peak" is so broad and flat that that figure could be reasonably expanded to about 2400-3200RPM.

Will you get better fuel mileage cruising at the higher RPM? Mais oui! Experimentally verified - my 1988 w/5-speed manual and 30's cruised at about 2000rpm in fifth, up around 2600 in 4th. Mileage? About 17 in fifth, and about 20 in fourth.

Result? I quit using fifth gear some time ago. Granted, this is less of a concern if you have the AW4 - the 3.55 gears they used with it are more closely matched to the transmission than the 3.07's they used with the five-speeds. I crunched the numbers some time ago - 3.55's are a drastic improvement, and I think 3.73's turned out to be nearly idea with either the BA-10/5 or the AX-15 and 30" tyres (but don't hold me to it - it's been quite a while, and I should probably do the maths again.) Ths was figured for a cruising speed of 70mph - which is doable pretty much everywhere.

BUT, this still doesn't explain why he's revving at 2700 cruising with 3.55's and 31's.

Either something is going on, or he just simply doesn't shift out of 3rd.

Edit : Bah, i'm the retard, just re-read his post... Disregard my post.

Sorry for the un-clarity, that rpm is in 3rd,on the highway. its about 1500-1600rpm in od. I have noticed improved fuel mileage in drive on most highway driving.

that just does'nt sound right.my 89 stock everything is at 2000 in OD at 65 mph.my 95 which has not been calibrated is the exact same.

My factory manual says to stay under 2000 for best mileage.

Torfinn

What your manual says cannot be helped.. we're talking about practical application.

5-90 said:

R and I think 3.73's turned out to be nearly idea with either the BA-10/5 or the AX-15 and 30" tyres (but don't hold me to it - it's been quite a while, and I should probably do the maths again.) Ths was figured for a cruising speed of 70mph - which is doable pretty much everywhere.

Click to expand...

I ran 3.73's and 31's for a bit.
it was decent, the Mj pulled about 21 highway, in 5th, and would beat most of the imports/6 cyl mustangs stop light to stop light.
with 3.73's and 34's I only use 5th when I'm cruising highway speeds on flat ground, it still pulls about 19.
unfortunately 4th is just too shallow for sustained cruising at 70, and 5th is a litle too deep, but it works. Of course my MPG drop off might be the fact that they're 34" tsls.