top speed for a 200 inline 6: can I get more?



Hello. New to this formum and new to inline 6 mustangs.

I have a 68 200 with a 3 speed. It still has the 1bbl carb (autolite).
I recently bought it off the original owner. It has 417,000 miles on it. I just put in new front and rear suspension kits and have installed pertronic ignitor, flamethrower wires, new yada yada yada...

I was wondering the top speed of a inline 6...with a 3 speed.

I had it on I-95 the otherday and it didn't go much above 75. Is it the three speed? Is it that it is a 6 cyl? Would getting a 2bbl carb & a spacer help?

any info would help before I spend my money on something I don't need...cause there is plenty I do need...


What are your rear gears? I'd suggest they and the gearbox (no OD) are holding you back in speed.

Regards, Adam.
8) gears and top end breathing are needed here. like addo said o/d will help alot. the 2bbl will allow the motor to breath easier at the higher rpm.

The 3rd gear should be at least 1:1 usually. so at speed there isnt alot more you can do without modding the motor.

IIRC the 3speed has either a 2.52 or a 2.73:1 gears so it is basically the inability of the engine to breath and the 4 cylinder like stock power levels that are limiting the top end.

Alignment and properly inflated tires, and a good clean car with wax can also improve top end.
My 71 Maverick, with a 3 speed, and 3.00 rears, would go around 114mph. But this wimpy 250 may only go 80mph, no high rpm, about 4,000 is all it will go, in any gear, maybe even in neutral? Of course, I really can't say how fast it goes anymore, since I only drive about 10mph over the limit! (getting old sucks!)
How many miles on the engine? I'm assuming it has been rebuild at least once inthe 417K miles.

Is your vacuum advance working? The '68s may have had dual advance distributors, I'm not sure but I had a '65 Mustang FB with the 200 (and AT) and it's top speed was 95. I slapped a Jacob's Mileage Master ignition computer and variablemagnetic core coil and opened up the plug gap to 0.045 and it's top speed increased to 103. Picked up 2 more MPG (in normal freeway driving). Not bad for single point igniton! This was in the mid '80s.

I could always tell when the vacuum diaphram on the distibutor went out as the MPG would drop to 13.


Dean T
To work out top speed, you need to do three things.

1.)Aerodynamic info

2.)an understanding of what drive train losses and rolling resistance is present.

3.) An appreciation that maximum speed isn't related to vehicle weight as much as having the power peak right for the required top speed. (No use in having a 500 horse 426 Hemi if your 5.1:1 Dana gears cause the engine to red line at 100 mph. With the right ratio diff or an overdrive of the right ratio, it could do 170 mph!)

If you go to the tech section by clicking on the mortif, you have the option of hitting on Bowlings formulaes. Lots of fun if you want to go that far.

I first came across this formula in NZ after a ricer (Honda CRX) driver did 150 mph to take the NZ 97 cube land speed record 80 miles north of my town. If you've got the right info you can work out the exact hp, gearing to give you the required top speed.

Rather than hijack another fruit-full talk, I'm off to rework my metric formulas back to proper American Units, and I'll drop some info into the "Hardcore" forum a little later!
I have a lot of good advice here.

I had just gotten 4 brand new tires. I figured they were good with pressure. Turns out I had some old cracked valve stems put on and a few of the tires were low,low,lowon air.

I will check the vaccum advance though. I am getting around 13 mph.

To answer KBNLINE, my engine was rebuilt about 165,000 ago. It seems to run pretty strong, with not a lot of oil burning. I do have a leaky valve something or other on #4. Fouls plugs every so often.

I need to find a GOOD mechanic.

next up: Vaccum and maybe a 2bbl Autolite...anyone have one for sale?

Thanks for you suggestions guys!
8) i owuld start by getting the engine in good shape first. freshen up the short block, rebuild the head, etc... at 165k the motor is getting tired and that will limit your top speed in and of itself.
I'd also reccomend thinking about a drivetrain swap; a T5 and 3.75:1 rear gears should run really well
Have you checked your speedometer calibration? :unsure:: Those tires look pretty big to me. :shock: If your tire diameter is larger than the stock 6.95-14 tires, you are actually going faster and getting better MPG than indicated. :) Check your odometer readings against the mile marker on the highway for a 10 mile stretch. That should tell you if speedo/odometer are reading accurately. ;)
tires are 215/70/14. That's not too big is it?

Looks like I have a lot to check...first I am checking my vacuum advance.

Then look into a T5 or C4. I will at someitme get a donor car and get a 9 inch rear and disk all aorund.

Engine rebuild first tho.

I still have to take care of that healdiner that is falling...11 months now..haha
The closest tire diameters would be 185/75-14 or 195/70-14. Your tires are a larger diameter, so your speedometer is reading low. You'll know exactly when you hit a radar trap! :oops:
Most tire stores have charts giving the dimensions of the various tires. Compare the difference in circumferences and that should tell you how far you're off.
One effect of a larger diameter tire is going to a taller over gearing, so the car feels more sluggish, like trying to start in second gear. :(
Depending on your power, a car can sometimes go faster in a lower gear. 8)
Those tires are about 4% too large. That's about a 3mph difference at 65MPH.

Not a huge difference if you keep it in mind, but there's some performance and weight disadvantages to having bigger tires. I tried to run the correct size out back and slighty smaller diameter up front.
hmm... so I guess my top speedis 78 mph. I had no idea that the tires were too big. That size was on the car when I got it and when I replaced them, I just kept the same. They look nice and they have a nice ride quality...since I am not racing it.

today I am off to get prices on vacuum advance diaphram replacement. I have a line on an autolite 2100 from ebay (plus some other parts).

someday soon I will make a restoration page on the web to document all this. I have been taking pictures

Thanks again guys
CC, you might want to check those % figures. 195/70 should be close, 205/70 about 4-5%, and 215/70 about 8-9% too tall. :shock:
I run 185/70-14 on my 66 convert, and they help it scoot. My speedo reads about 5% high, but it helps keep me from getting too many tickets. ;)
I probably dont have much to offer about the subject, but when I drive the car, I have always noticed that my car isnt gooing as fast as the speedometer is showing, it would go faster. For instance when I go like 30 mph and one of those construction or chp signs would show 37, and when I went like near 60 mph on the highway it would show 74. It still does this, but a ratio that I have figured out and it what ever the gauge would say I would multiply that by 20 percent and that would give me my real speed. I think that that has something to do with the rear axle that was installed before I bought the car. It is a v-8 rear axle using 14 in.

When I first got the car I didnt know this or anything about cars, but I looked at the gauge and it said 84 mph. Still rear axle and I figured that I went over 100 mph. Stock engine and everthing else. Today, I dont drive that fast but when I go like 72 mph, real speed, my tach shows 2700 rpm and 66/65 mph it shows 2500 rpm. And the most that I have pushed it recently was at about 85 mph, real speed, at 3050 mph. Stock and everything, but takes a while to get there though.

When I get money and dump it into my car, I am hoping that all of that will change.
using the formula given,my new top speed (feeling comfortable without it's engine screaming) is 90 actual. Hopefully with the new leaf springs and soon a will be faster

I dont have the luxury of a tach to tell rpm's...

I am trying to findone small enough to go in the clock delete...any suggestions?
My 66 stang with 205/70/14's will do 93 mph and that is 4400 rpm's.