Tesla Semi vs Eisenhower Pass

How will a Tesla Semi behave on the I-70 stretch west of the Eisenhower Tunnels in Colorado. That stretch is one of the steepest in the interstate system, with a 6% grade sustained for 7 miles. The speed limit for trailers downhill is 45mph. There are other stretches like this, and many have tighter turns, too. They require competent drivers and well designed and working equipment. YouTube is full of videos describing the challenges, and some showing what happens if anything goes wrong.

TL;DR: The Semi can keep steady speed down any interstate at any legal speed. It can also slow down the truck on that hill. The brakes will be cool at the bottom.

Profile and Satellite image of the Grade west of Eisenhower tunnel
Profile and Satellite image of the Grade west of Eisenhower tunnel.

Going Downhill

Going down the 6% Eisenhower Grade, an 82,000lbs rig at 45mph produces 440kW of power. (You can enter your own numbers in the calculator). We believe the Semi requires 1.7kWh per mile just to overcome friction (air drag, rolling resistance, internal friction) and heat losses in the battery. If that number is valid for 50mph, that’s another 85kW (calculator). This reduces the power produced to 365kW. Over a distance of 7 miles, it ads up to 12kWh.

Absorbing that much power is not a problem for the Tesla Semi batteries, they can charge at at 1,400kW average for half an hour. It’s also not a problem for the motors (more on that below).

The grade is 7 miles long, which takes 9 minutes 20 seconds at 45mph. Over that time, the Semi generates 55kWh of energy (calculator), less than 6% of the full capacity. However some of this energy will be lost to heat. At 95% efficiency 52kWh charge the battery (calculator). That’s enough to drive the Semi 31 miles flat.

Regeneration doesn’t work well if the battery is full or cold. For this stretch, you need to be below 90% at the tunnel to make full use of all the regen. If you drive up the pass from the other side your batteries are neither full nor cold.

Going Uphill

Going up the same, but in reverse, except that friction works against the Semi. At 45mph you will need 440kW for climbing, and 85kW just for driving (air drag/rolling resistance/losses) for a total of 525kW / 704hp (calculator). 65mph would be 721kW / 967hp – ignoring the increased air resistance at that higher speed.

It’s time to look at the motors. A tri-motor Model S Plaid has 1020hp or 760kW. The Semi pretty much has three of the same motors. For now, we use 746kW/1000hp for the Semi. With that wild guess, the Semi can go up any US interstate at 65mph 721kW/967hp (calculator). There is also power left, which allows the Semi to accelerate on that grade. Now Tesla mentioned “speed limit up at 5% grade”. You could go 75mph on a 5% grade. I don’t know if there are any 5% with a 75mph speed limit in the US.

For comparison, a typical diesel class 8 has about 600hp, which translates to about 35mph on a 6% grade – which is close enough to what trucks do in the reals world. Diesel drive trains will be less efficient, but diesel trucks will also be lighter.

Climbing the grade at 45mph will take 78kWh out of the battery (calculator). This is a significant fraction of the total battery, but remember that you will get 52kWh back on the other end, for a net cost of 26kWh. It would cost you 22kWh just to drive those 14 miles flat, so the hill costs you an extra 4kWh due to losses.

How Steep can the Semi go

So how steep a grade can the Semi handle? We are going with 1000hp / 750kW. That’s enough for the steepest roads in the US (37%) at 10mph. Proper highways like e.g. Sonora Pass in California max out at about 25%. A fully loaded Semi could that at 15mph. Could, because there are typically restrictions on weight and trailer use for any such roads. But basically napkin physics says the Semi can go up and down any reasonable road in the US. The limit will not be the motors or the battery, but other factors.

Small Print on Numbers

Some of the above numbers are pure physics, like the work done by a truck of a given weight dropping a certain number of feet. Others are based on non-verified statements by Tesla, or guestimates or other napkin calculations. Some are documented on this page.

Last Updated on January 23, 2023.