How Fast Can your 258 Go? -
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post #1 of 28 Old 01-08-2010, 07:36 PM Thread Starter
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1985 CJ7 
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How Fast Can your 258 Go?

Just finished a 900 mile trip with my 82 CJ7 limited. I found that the fastest I could go was 64-67 in 5th and the pedal to the floor board. I am used to my Waggy with a 360 and 727 which can creep up to 90+ if I am not watching my speed.

I know the CJ7 isn't exactly the most aerodynamic vehical out there so I don't expect it to be speed racer in 5th and go 120 mph but what I was getting seemed a little low. My question is: Is this normal for a 258 powered CJ7 with T-5 or should I have a mechanic check it out?

1982 CJ7 Limited, 31" Mud Terrains, GM HEI, Ford Taurus Elec. Fan,Stock Gearing?(not 100 percent on that)

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post #2 of 28 Old 01-08-2010, 08:12 PM
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whats your gear ratio? is your speedo adjusted for the larger tires? I'm willing to bet your going a little faster than 64-67... I could hit well over 75 in 4th gear on my 258/T5 combo with 3.31 gears on 33" tires.

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post #3 of 28 Old 01-08-2010, 08:26 PM
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1978 CJ7 
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well over 80 with a 3 speed road.
Can cruise at 70/75

258, Weber 34, CDI Ignition, TeamRush, Aux Grounds, Borla Header & Exhaust, Modern 2 Barrel Intake from 80's AMC, stock cam and heads with rebuilt long block. Runs very well and pulls from 600 to 4400 rpms and have done 66 @ 4000 rpms in second gear of 3 road gears per calculators. 33x12.5, 3.54 axles, T18 granny

I am not sure how fast mine will go... only have 2500 miles or so on rebuild and have not pushed it. I have taken too long drives one to Central Iowa and one to Northern MN. Both Drives were 4 hrs one way. No problems not a hick up. The Northern MN trip included 4 days of wheeling with a bud and modern jeep. After he said "impressed how well a old CJ can work" and I only used 1 more gallong of gas on the way up there than he did. Does drive nice at 75 and can drive with two fingers at that speed, so the PO did something right his dad owned a semi front end place... but that is all I can say for them they messed up many other things.

I know my 1973 new Jeep Truck could do 90-95 pretty well. Still screeming but could do it. Most health jeep CJs should be able to do 85 or 90 for short time. Way off gears would be a big issue.

I just read what FoggyBottom has and that would be hard for most of us to do in the Mtns. I grew up in mountains and may go back to NY Catskills I am affraid my big tires will not cut it in the hills/mtns. I am changing to 3.73 and lockers maybe this spring.... but the hwy mileage is hard to give up. W/o granny gear I would change to 4.11 most likely.

FoggyBottom.... Very few could follow that hill work at 7000 ft range. Pretty impressive. Next one will go with 4.0 head and FI and 258 base.... what kinda of mileage do you get if you "feather it" as they say and not all the hill suff but hwy cruise????? Foggy if you see this let us know??

I can get 22-24 IF i feather and use a vac gauge to watch what I am doing.... in the hills/mtns sure it would tank to 17 or so. So for 32 yrs old I am pretty happy with it.

Last edited by Fjguercio; 01-09-2010 at 03:56 PM.
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post #4 of 28 Old 01-08-2010, 08:37 PM
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Mine tops out about 70 going down hill.
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post #5 of 28 Old 01-08-2010, 08:37 PM
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100+ twice, average top of 80 to 85MPH, mostly cruised interstate at 75 to 80MPH no problem.
3.31 gears 4.2 with T-5 rolling on 29s....and yes, it is stable at those higher speeds or I wouldn't do it.

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post #6 of 28 Old 01-08-2010, 08:43 PM
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Just did a 400mile round trip on the way there scared to go over 60mph. Tightend the rear left leaf sping, hit 75 on the way back. Fresh 258, t5, 32s. It still sounds like a brick going down the round.
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post #7 of 28 Old 01-08-2010, 10:26 PM
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Gears are everything. If you are too tall, the engine simply won't produce enough power(rpm's) to overcome wind resistance.
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post #8 of 28 Old 01-09-2010, 02:35 AM
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the fastest i've wanted to go was 75, but that is barely half throttle. i have stock 258 weber 34 3.08 gears and 31" at
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post #9 of 28 Old 01-09-2010, 07:16 AM
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My 80 can cruise all day long at 80 to 85 MPH with no problem and has gotten me through any trail or mud pit I've tried Same setup as yours 258/t176/d300/d30 front/AMC20 rear geared at 3.08.

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post #10 of 28 Old 01-09-2010, 08:13 AM
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I can do 80 on level ground but that tapers off to less going up hill on any grade. I probably average about 65 in the mountains on good roads. I have a 258, 4.0 head, HEI dizzy, 33's and 4.30 gearing. And most of my driving is between 5000 and 12,000 feet ASL.
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post #11 of 28 Old 01-10-2010, 08:35 AM
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When I had a 258 it would go 90 with the stock carb (33s, 4.56s & T-176, typically between 1,000 and 11,000 feet, I live around 6,000ft)
After injecting it (junk yard multipoint) it would cruise at 90 part throttle (33s, 4.56s & t-176, typically between 1,000 and 11,000 feet). (Best 6 that I have ever had, 4.0s and Buick v-6s don't come close). I got better performance and usage from that 6 than many of the v-8s that I have owned.


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post #12 of 28 Old 01-10-2010, 09:18 AM
Mike Romain
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Mine will cruise all day at 120 kph or about 70-75 mph.

Here is how mine runs with 91 octane ESSO gas and the timing set at 8-9 BTDC using the stock carb, OEM cap, rotor and wires and an Accel coil with 3.31 gears turning 33's. (while still passing emissions and giving 21 mph highway) Computer disabled.

1st lights up the 33's 'way' too easy off a stop. 2nd hauls fast to 52 mph@4400 rpm. 3rd pulls hard to 75 mph. 4th buries the speedometer and I have 'no' urge to find out how fast 5th will go.... It also will crawl up a sand pit wall or ravine wall at 400 rpm without a chug, get too steep and it just churns the tires in...

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post #13 of 28 Old 01-10-2010, 09:23 AM
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I just did a rough, terminal-velocity calculation and it appears that if I dropped it from an altitude of 2,000 ft., it should do roughly 133 mph. ;-)

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post #14 of 28 Old 01-10-2010, 09:25 AM
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Originally Posted by DirtyDevin View Post
Mine tops out about 70 going down hill.

If I keep my 4 speed in 3rd gear on windy days I top 75. On calm days in 4th I can get her up to 75...

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post #15 of 28 Old 01-10-2010, 09:59 AM
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On a nice dry day on the highway I get my jeep up to 3000 rpm.. which for me calculates out to around 72mph.

I bet it could go faster, but 3k is roaring and I don't see a point in pushing it much farther.

On a windy day I don't feel comfortable taking it that far as it seems to swing from side to side if there's a cross breeze.


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