Butchershop Posted October 21, 2012 Share Posted October 21, 2012 What top end speed should I expect from my MJ? About 65 on a straight flat highway... sound reasonable? Driving these ATL highways, I'm getting blown away.... LOL :fs2: :MJ 2: Link to comment Share on other sites More sharing options...
relyt120 Posted October 21, 2012 Share Posted October 21, 2012 I've gotten mine to about 92 before, measured via gps. About that speed and I was afraid it would begin shaking parts off. Link to comment Share on other sites More sharing options...
mnkyboy Posted October 21, 2012 Share Posted October 21, 2012 My MJ with a 2.5l and AX4 can break 100mph,it easily cruises at 75 on the free way. Link to comment Share on other sites More sharing options...
Pete M Posted October 21, 2012 Share Posted October 21, 2012 What top end speed should I expect from my MJ? About 65 on a straight flat highway... sound reasonable? Driving these ATL highways, I'm getting blown away.... LOL :fs2: :MJ 2: what size tires? Link to comment Share on other sites More sharing options...
AMCJeepMJ Posted October 21, 2012 Share Posted October 21, 2012 There's a lot of factors which will affect your top speed... larger or heavier tires... tuning/upkeep of the rig... added items which create drag [light bar, brush guard, ladder rack, etc.]... with a 2.5, driving into the wind or with the wind... Having a dirty rig will also increase drag and reduce top speed and fuel economy. Keep it clean and waxed smooth. Link to comment Share on other sites More sharing options...
HOrnbrod Posted October 21, 2012 Share Posted October 21, 2012 My MJ with a 2.5l and AX4 can break 100mph,it easily cruises at 75 on the free way. That's with a 100MPH tail wind an traveling down a 45* grade. Just kidding mnkyboy, couldn't resist. :yes: Link to comment Share on other sites More sharing options...
Butchershop Posted October 21, 2012 Author Share Posted October 21, 2012 what size tires? 235 75 r15 Stock wheels. See here : http://comancheclub.com/topic/35135-not-a-real-project-yet-assembling-ideas-right-now/ Prob picking up some 15" canyons this week w 30X9.50X15" TIRES. Better or worse? Truck is clean. Link to comment Share on other sites More sharing options...
Pete M Posted October 21, 2012 Share Posted October 21, 2012 do you live in a county with a sniffer test? Link to comment Share on other sites More sharing options...
Eagle Posted October 21, 2012 Share Posted October 21, 2012 235 75 r15 Stock wheels. See here : http://comancheclub....deas-right-now/ Prob picking up some 15" canyons this week w 30X9.50X15" TIRES. Better or worse? Worse. Actually, maybe the same. 30x9.50s are almost the same diameter as 235/75-15s, but wider, so they offer more drag. But with a 4-cyl 4-speed, your 235s are several sizes larger than stock already, so unless you have changed the speedometer gear, your speedometer is reading slower than your actual speed by somewhere around 5% to 7%. Since it's a 2.5L 4-speed, it must be a base model and you probably don't have a tachometer. FWIW, 65 MPH on the speedo (based on stock tires) is almost 3,000 RPM. The engine will turn faster than that, but remember you're driving a brick on wheels, and both aerodynamic and frictional drag increase exponentially with speed. At an indicated 65, you are probably actually going closer to 70, and that's going to be ruining your gas mileage because that comparatively small engine is having to work a lot harder to push that brick down the highway. You would probably gain more speed (if that's what you want) by going to 215/75-15 tires. Let me offer a real-world experience. When I was in the Army during the Vietnam unpleasantness, for awhile I was stationed in Maryland. At the time I was driving a Rambler American. The engine was the 199 cubic inch version of what ultimately became to MJ 4.0L, and I had a 3-speed manual tranny. Like your 4-speed, top gear was 1:1 -- no overdrive. After a bunch of tinkering, running high-test gas so I could bump the timing, running an open air cleaner and a glasspack muffler, I was able to finally get my top speed up to an indicated 104 MPH (tested late at night on I-95 not far from my post). That was on the stock tires. Then winter arrived. My brother was driving a full-size Rambler Classic V-8, which came with much bigger tires. He had switched over to radials, so he gave me his bias-ply snow tires from the year before. They looked silly on the back of my little American but, more important, my top speed immediately dropped from 104 to 84 MPH (indicated). Even after correcting for the larger tire size, my real top speed dropped by at least 10 MPH. The reason was that the power curve of the engine crossed the line for power required to push the brick -- there just wasn't anything left at that point to make the car go any faster. Link to comment Share on other sites More sharing options...
Butchershop Posted October 21, 2012 Author Share Posted October 21, 2012 Great info. Thanks. New cluster with tach, so I'll check the RPMs and calibrate with another vehicle at same speed. New catalytic converter and muffler. Truck seems to run great. If I'm getting 70mph or so, that's fine... Mostly I drive the country roads here in north Georgia. But I drive this truck to the airport a few times per month. Mpg has been about 24 for the first couple of tanks. 10-20 mph drop with tire size change? Never would have guessed that much. Appreciate the anecdote. Link to comment Share on other sites More sharing options...
mvusse Posted October 21, 2012 Share Posted October 21, 2012 Running 235/75R15 on a 4 cylinder with 3.55 gears I usually cruise 70 on the freeway.75 is no problem, don't know about top speed. Same tires and gears and same ration on a 4.0/automatic I've been over 100 a few times with pedal to spare. Link to comment Share on other sites More sharing options...
Eagle Posted October 21, 2012 Share Posted October 21, 2012 New cluster with tach, so I'll check the RPMs and calibrate with another vehicle at same speed. Calibrate = good. Calibrate to another vehicle = bad. There's very little chance that the speedometer on another vehicle is really going to be accurate. If there's a police department near you that deploys those radar speed sign trailers, run your MJ through that and see what the radar reports compared to your speedo. Or borrow a GPS and calibrate to that. I haven't checked any of my MJs, but I have checked four (4) XJs, and the speedometers on all four read from 2 to 4 MPH faster than actual speed, on stock tires with no changes to gearing. And that's been checked both by radar trailer and (for two of them) GPS. Link to comment Share on other sites More sharing options...
gogmorgo Posted October 21, 2012 Share Posted October 21, 2012 A couple days after I got my 4.0 HO auto I hit 180km/h indicated (~110mph) where it simply stopped accelerating, although I know that at "normal" speed the speedo reads ~10km/h high (thanks to a radar trailer), so likely closer to 105mph. Peak torque's supposedly at 4k, but my tach only showed about 3700rpm. It was a little weird, since it didn't really feel/sound like I'd hit a fuel cutoff or anything, at least not the way my dad's Impala hits the limiter. That was with a slight crosswind and a tarp over the bed, at the time running 3 225/75R15's and 1 235/75R15 (rear passenger side.. stupid PO :thwak: ). Still don't know what gears I've got yet. Link to comment Share on other sites More sharing options...
Eagle Posted October 21, 2012 Share Posted October 21, 2012 10-20 mph drop with tire size change? Never would have guessed that much. Appreciate the anecdote. Going to tires that are several sizes larger than stock is like going the wrong way with an axle gear change, or shifting into overdrive. And the 2.5L doesn't have the Moxie to run an overdrive with that axle ratio. There's a reason the factory used 4.10 gears behind the 2.5L with the 5-speed. Your factory tire size should have been 205/75R15. My chart doesn't even go that small, so I have to look at 215/75s. For that tire size, 60 MPH equates to 2669 RPM, which is below the 3250 RPM torque peak for your year. 70 MPH is 3115 RPM, which is effectively right at the torque peak. Try to go faster, and as the drag increases your torque decreases. With 235s, the 60 MPH engine speed drops to 2556 RPM, which is a loss of about 5 percent. That's about the equivalent of changing your axle ratio to 3.31. It puts you below the torque peak, and WAY down on the horsepower curve. Link to comment Share on other sites More sharing options...
COMJNUT Posted October 21, 2012 Share Posted October 21, 2012 So it sounds like your asking what is the regular top speed of one of our MJ's. The closest I've gotten to the 2.4L is driving the varmint. It got to 65 easily with the 4.10 gears with 235/75r15's at this high altitude, but it had a crazy good upkeep previous owner. Mine hit 105 on the GPS on a closed course with the specs in the sig line. If you really want to play with numbers, go to this page and enter in your specs in the left column and enter whatever you wish on the right. http://www.grimmjeeper.com/gears.html Link to comment Share on other sites More sharing options...
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