top speed problem

[outkast4.0]

i need help my car is a 98 model jeep wrangler sport and isnt reaching the top speed of 180 kph it reaches 130 to 140 kph thats around 90 mph in 4th gear at around 3000 rpm. my car is a 4.0 v6 manual gear soft top. recently though i cahnged my tyres and on the 7 inch stock rims i put 265/70/r15 tyres cud this be the reason why my car is slower or not reachin the top speed.

[fredjacksonsan]

First of all, if you want to go fast, get a car. Wranglers are square bricks, and over 70mph(116kph) your wind resistance is mounting rapidly.

Installing larger tires will throw off your speedometer. It will indicate slower than you are actually moving.

Here are a couple of websites where you can put in the tire size difference and figure your speedometer error:

http://www.dakota-truck.net/TIRECALC/tirecalc.html

http://www.1010tires.com/TireSizeCalculator.asp

[fredjacksonsan]

And welcome to AF! There's a lot of good information here.

[fmrmti]

I put 31x10.5s on my 97 Sarhara and it will easily go 90mph. I will occasioally drive it around 80-85mph on the desert freeways, but he's right...it's a box, so wind resistance being the square of the speed...you get the idea. Also, if a crosswind gets up under the Jeep—watch out!